CHEMISTRY NOTES FOR FORM FOUR – ALL TOPICS

TOPIC 1 – NON METALS AND THEIR COMPOUNDS


TOPIC 2 – ORGANIC CHEMISTRY


TOPIC 3 – SOIL CHEMISTRY


TOPIC 4 – POLLUTION


TOPIC 5 – QUALITATIVE ANALYSIS

IMPORTANCE OF CHEMISTRY IN OUR DAILY LIFE – PART 4 

17. Studying Chemistry Enables You to
Perform Fun Experiments, Even at Home

While experiments are usually conducted in
labs, you can also perform some fun experiments at home. And you do not even
need special chemicals for that. As many of the household products you already
own are made of certain chemicals, they can act as reagents for the reactions.
There are plenty of kid-friendly science experiments that you can
perform using the materials you already have and, trust me, you will not regret
it.

18. Studying Chemistry Makes You Aware of
How Safe or Dangerous Household Products Are

You may not even realize how many
chemicals you have in your home. And believe it or not, many of these household
products can be extremely toxic. Luckily, chemistry lets you determine how
safe or dangerous chemicals are, allowing you to properly use them whenever
needed.

19. Chemistry is important in
agriculture

Organic chemistry is essential for
developing methods for food production and farming. Agricultural chemists seek
to understand the causes and effects of chemical reactions. That
includes studying soil fertility and water and air quality as it
affects the growth of crops. Chemists also study pests, weeds, and fungi and
develop ways to control them. Chemists play a big role in creating and testing
herbicides, fertilizers, and insecticides.

20. Chemistry is important in
agriculture

Organic chemistry is essential for
developing methods for food production and farming. Agricultural chemists seek
to understand the causes and effects of chemical reactions. That
includes studying soil fertility and water and air quality as it
affects the growth of crops. Chemists also study pests, weeds,
and fungi and develop ways to control them. Chemists play a big role
in creating and testing herbicides, fertilizers, and insecticides.

21. Helps you to understand
current events, including news about petroleum, product recalls, pollution,
the environment and technological advances.

…………..

The Concept of Volumetric Analysis

Volumetric analysis is a quantitative analysis involving the measurement of different solutions. These solutions are made to react completely and the completion of the reaction is indicated by certain substances called indicators. The quantitative composition of the solution is then determined.

Important steps of volumetric analysis include:

• Weighing;
• Preparation of the solution;
• Titration; and
• Calculation

In volumetric analysis, we deal with volumes of solutions. That is why this quantitative determination of solutions of substances is called volumetric analysis.

The amount of a substance present in a solution is given in terms of its volume and its concentration. The volume of a solution is usually given in litres (dm³). The concentration of a solution is given in moles per litre (mol/dm³) or grams per litre (g/dm³).

Volumetric analysis is a means of finding the concentration of an unknown solution. For example, the concentration of an unknown solution of an acid can be found if it is reacted with a standard solution of an alkali. A standard solution is one whose concentration is well known and does not change with time.

In volumetric analysis, the reaction is carried out in a carefully controlled way. The volumes are measured accurately using a pipette and burette. The method is to add a solution of one reactant to the solution of another reactant until the reaction is complete. When the reaction is complete, we say the end-point has been reached. If the reactants are acids and bases, completion (end-point) is determined by the change in colour of an acid-base indicator. The method is called titration. In other reactions, completion is determined by a colour change of reactant(s). The concentration of one of the reactant solutions must be known in order to be able to find the concentration of unknown solution.

Significance of Volumetric Analysis

1. Volumetric analysis is used to quantify the amount of substances present in solutions by analytical procedure, which involves precise measurements of volumes of solutions and masses of solids.
2. Volumetric analysis helps in the determination of the accurate volumes and concentrations of the reacting substances, often solutions.
3. Volumetric analysis (titration) helps in the preparations of standard solutions.
4. Volumetric analysis knowledge helps in the standardization of acids and bases.

Volumetric Apparatus

Use volumetric apparatus

We have seen that volumetric analysis involves determinations of quantities of substances, usually acids and alkalis, present in volumes of solutions. This is usually done by using measuring apparatus.

Apparatus used in volumetric analysis is based on volume measurements and since the analysis demands high accuracy, the apparatus has to be calibrated with the highest possible accuracy. It is for this reason that all apparatus for volumetric analysis are specifically for this and not other purposes.

Apparatus used for volumetric analysis include, burette, pipette, burette stand, white tile, conical flask, filter funnel, reagent bottle, watch glass, beaker, measuring cylinder and measuring flask (or volumetric flask). For approximate measurements, measuring cylinders may be used. For accurate measurements of volumes, volumetric flasks are used.

Burette

This is a long glass tube with a narrow lower part, which is fitted with a tap that controls the amount of solution let out of the burette. This instrument is calibrated from 0 to 50 cm³.Before measuring the solution, rinse the burette with distilled water, then with the solution it is going to hold. It has to be filled to the tip and all gas bubbles removed. Thus, the burette is an apparatus used for transferring the solution to the titration vessel (normally a flask).

Pipette

This apparatus has a wider middle part with narrow parts at either ends. The upper narrow part has a mark which marks the volume of all the space below it. If, say, the pipette is one that is marked 25 cm³, we can say that a solution, when filled in the pipette up to this mark, will have a volume of 25 cm³.

The pipette is used in transferring a standard solution to the titration flask. There are many types of pipettes depending on their volume capacity. The common ones are the 25-cm³ and 20-cm³ capacity pipettes. Less common ones are the 10-cm³ capacity.Before measuring the solution, rinse the pipette several times with distilled water and then with the solution to be measured; suck the rinsing solution above the graduated mark, then discard the rinsing.

The pipette is commonly filled by mouth suction but the use of pipette fillers is highly recommended. When using a pipette, never blow out the last drop.

(a) A pipette (b) A pipette and pipette filler (used to fill and empty pipettes)

Measuring (Volumetric) flask

The flask is made of glass and has a mark at the upper part of the narrow tube. The space in the flask up to this mark represents a certain volume. If a solution is filled up to this mark, the volume of the solution is equal to the volume indicated by inscriptions on the flask e.g. 50 cm³, 100 cm³, 150 cm³, 250 cm³, 500 cm³, etc.

Filter funnel

A filter funnel is required for effective transfer of the weighed solid, liquid or solution into the volumetric flask or burette.

A filter funnel

Wash bottle

Wash bottle contains water and when squeezed, water squarts out. This is used in washing down the remains of the weighed solid into the volumetric flask.

A wash bottle

A weighing bottle

This is used in weighing the solute. It is a stoppered bottle. A watch glass can also be used to serve the same purpose.

Retort stand

A burette stand is used for holding the burette in place while carrying out volumetric analysis experiments.

A burette stand

Dropper

A dropper is used to add the indicator dropwise into the solution.

White tile or paper

A white tile or piece of paper is placed under the flask to give a clear background for accurate observation of the colour change at the end of the reaction (end point).

Standard Solutions

The Steps for Preparation of Standard Solutions of Common Acids

Explain the steps for preparation of standard solutions of common acids

A standard solution is a solution of known concentration. For example, a solution containing 15g of sulphuric acid in 1 dm³ of solution is a standard solution.

It has now been approved that volumetric work should be based upon the molar (M) solution. A 1 molar (1M) solution of a compound is a solution which contains one mole of that compound in 1 dm³ of the solution. For example, 58.5g of sodium chloride (NaCl) dissolved in 1 dm³ of the solution makes a molar solution of sodium chloride (1M NaCl). Likewise, 106g of sodium carbonate (Na₂CO₃) in 1 dm³ of the solution gives a molar solution of sodium carbonate. Therefore, a 1 molar sodium carbonate solution contains 106g of the salt in 1 dm³ of the solution.

1 molar solution of some compounds commonly used in titration contain the following masses of the compounds in 1 dm³ of solution:

Derivative concentrations are also used e.g. 0.1M, 0.5M. 2M, etc.

Preparation of standard solutions

A standard solution is required as a starting point for volumetric analysis. We learned early that in order to find the unknown concentration of a substance in volumetric analysis, the concentration of one of the solutions must be known.

A small range of substances are suitable for direct preparation of accurately standard solutions. Substances that cannot be used for direct preparation of standard solutions include sodium hydroxide, potassium hydroxide and concentrated sulphuric acid. These substances absorb water vapour from the air and hence cannot be weighed out precisely without taking extra precautions. Apart from absorbing water vapour from the air, sodium and potassium hydroxides react with carbon dioxide of the air to form respective carbonates.

2NaOH_(s) + CO_2(g)→ Na₂CO_3(s) + H₂O_(l)

2KOH_(s) + CO_2(g)→ K₂CO_3(s) + H₂O_(l)

Some solutions are volatile in nature and so are likely to change slowly in concentration during ordinary use. These include concentrated hydrochloric acid and ammonia.

A compound commonly used for preparation of a precisely standard solution is anhydrous sodium carbonate. It is best prepared from highly pure sodium carbonate. This is achieved by heating sodium bicarbonate to constant mass to make sure the compound is fully decomposed.

2NaHCO _3(s) →Na₂CO_3(s) + H₂O_(g) + CO_2(g)

The sodium carbonate so formed is suitable for preparation of a standard solution and can be weighed without undergoing any appreciable change in composition.

Precautions to be observed while preparing a standard solution

• Transference of the substance from the weighing bottle to the beaker or flasks should be done with outmost care so that not a single particle of the substance is lost.
• Undissolved substance should not be transferred to the measuring flask. Make sure all the solid dissolves into solution before transferring the solution to flask.
• During making up of the volume, the last drop of the water should be added carefully. Do not blow out the final drop.

Standard Solutions of Bases

Prepare standard solutions of bases

Preparation of 0.1M sodium carbonate solution

The molecular mass of sodium carbonate (Na₂CO₃) is 106g. Therefore, a molar (1M) solution of sodium carbonate contains 106g in 1 dm³ (1000 cm³) of solution.

In order to prepare 0.1M solution of the carbonate, we have to weigh 10.6g of the carbonate and put it into a volumetric flask, which has a capacity of 1000 cm³.

However, normally 250 cm³ flasks are used. This means, in a 250 cm³ flask we have to add 10.6/4= 2.65g calcium of sodium carbonate.

Thus, 1000 cm³ ≡ 10.6g

250 cm³ ≡ X g

X = 250/1000×10.6 = 10.6/4 = 2.65g

The same procedure can be followed when preparing 0.25M, 0.5M, 2M, etc. of the solutions.

Procedure

• Weigh exactly 2.65g of sodium carbonate using a common balance and put it onto a watch glass.
• Transfer it slowly into a beaker of 500-cm³capacity containing about 50 cm³ of hot distilled water.
• Wash down the watch glass with a jet of hot distilled water from a wash bottle and allow the washings to fall into the beaker (figure 5.7). Make sure all the sodium carbonate is washed into the beaker.
• Stir with a glass rod until all the solid is completely dissolved, and then cool the solution to room temperature. Leave the rod standing in the solution.
• Pour the solution carefully down the glass rod into a 250 cm³measuring flask.
• Wash the beaker out at least twice with jets of cold distilled water directed round the slides and pour the washings down the glass rod into the measuring flask (figure 5.8).
• Shake the flask gently and fill it up with cold distilled water almost to the mark.
• Add more distilled water drop by drop from a pipette until the meniscus is on the graduation mark (figure 5.9).
• Stopper the measuring flask and shake well. The liquid should then be exactly 0.1M sodium carbonate solution.

Preparation of standard solutions of other bases of different molarities e.g. 0.2M, 0.5M, 1.0M, 2.0M, etc. can be achieved by using the above procedures. The only variable will be the weight of the solids and volume of water as stated early.

Washing the watch glass

Filling the washings into the flask

Correct reading of the liquid volume

Acid-base Titration Experiments

Carry out acid-base titration experiments

Preparation of 0.1M sulphuric acid solution

A standard solution of sulphuric acid cannot be prepared directly because concentrated sulphuric acid is hygroscopic in nature (it tends to absorb water vapour from the air diluting itself) and is never reliably pure. A solution a little above 0.1M is prepared and then standardized and diluted with distilled water to exactly 0.1M.A molar solution of H₂SO₄ contains 98g of pure acid in 1 dm³. Therefore, the 0.1M acid contains 9.8g of the acid in 1 dm³ of the solution. The pure concentrated acid has a density (concentration) of about 1.8g/cm³. So, 9.8g of it occupy about 9.8/1.8 = 5.5cm³

The preparation of a standard solution of sulphuric acid involves two stages:

1. Diluting a concentrated solution of the acid to an approximate molarity.
2. Finding the exact concentration of the acid (standardizing it) by titrating it against a standard solution of a base (previously prepared).

Dilution of concentrated sulphuric acid

Caution: Make sure you wear safety goggles and gloves before carrying out this experiment.

Procedure

• Cautiously, because the acid is very corrosive, take 5.5 – 6.0 cm³of concentrated sulphuric acid in a small measuring cylinder.
• Pour the acid carefully, with stirring, into a 250-cm³volumetric flask containing about 100 cm³ of cold distilled water.
• Pour this solution into, say, 700cm³of cold distilled water in a measuring flask of capacity 1000cm³.
• Wash out the acid solution remaining in the measuring cylinder with cold distilled water twice and add the washings into the measuring flask.
• Then add distilled water approximately to the mark on the measuring flask, stopper it, and shake well.

This should give sulphuric acid of concentration a little above 0.1M. The diluted acid is now standardized with the 0.1M sodium carbonate solution prepared above.

Determination of the molarity (standardization) of sulphuric acid solution by titrating it against 0.1M sodium carbonate solution

The estimation of the concentration of a solution of an acid by reacting the acid with a standard alkali solution is known as titration. The end-point of an acid-base reaction is commonly determined by using a substance known as an indicator.

Procedure

• Measure 25 cm³of 0.1M sodium carbonate solution and transfer it into a conical flask, using a pipette. Add a few drops of methyl orange This will turn the sodium carbonate solution yellow.
• Set up the apparatus as shown in figure 5.10
• Pour the acid into a 50-cm³ Read and note the level of the acid in the burette.

Titration setup

• By means of a tap at the base of the burette, drip the acid slowly into the conical flask, swirling the flask continuously until the colour of the liquid in the flask turns orange. This is the end-pointof titration. Record the new level of acid in the burette.
• Repeat the titration three to four times, noting the initial and final reading of the burette each time.
• Find the volume of the acid as shown below:

Specimen readings

Neglecting the first (rough) trial run, the average titration is 23.60 cm³

Calculation

The first step in calculating the molarity of any solution from the results of an acid-base titration is to write the equation for the reaction. From the equation, find the number of reacting moles of the acid and base.

Na₂CO_3(aq)1 mole+ H₂SO_4(aq)1 mole→Na₂SO_4(aq) + CO_2(aq) + H₂O_(l)

Now we have the following data.

• Volume of acid, Va = 23.60 cm³
• Volume of base, Vb = 25.00 cm³(this is the average amount of the base that was added to the flask in titration)
• Molarity of acid, Ma = ?
• Molarity of base, Mb = 0.1M
• Number of moles of acid, Na = 1
• Number of moles of base, Nb = 1

The molarity of the acid can be calculated from the following general formula:

The concentration of sulphuric acid is 0.106M.

To make the molar concentration of the sulphuric acid solution exactly equal to that of the sodium carbonate solution (0.1M), 23.6 cm³of the acid must be diluted to 25 cm³, that is, 1.4 cm³of distilled water must be added to 23.6 cm³of the acid.

Remember it was stated early that in order to prepare 0.1M sulphuric acid solution, you need to dissolve 9.8g of the acid in 1000 cm³ (1 dm³) of distilled water. Assume that some of the acid was wasted through spillage and mishandling and that only 920 cm³ of the acid was left. If, say, 920 cm³ of the acid was left, it can be made exactly 0.1M by the addition of 920×1.4/23.6 = 55cm³of distilled water. This gives exactly 0.1M of the acid. This is the same as saying that, if 23.6 cm³ of the acid were diluted with 1.4cm³ of distilled water, then 920cm³ of the acid would be diluted with

If, for instance, the volume of acid left was, say, 850 cm³, the amount of distilled water to be added would be = 850×1.4/23.6 = 50.4cm³. This, also, would give exactly 0.1M of the acid.

In principle, the amount of distilled water to be added is always calculated based on the amount of the acid left as exemplified above.

These two standard alkaline and acidic solutions can be used to standardize other solutions, e.g. sodium hydroxide, potassium hydroxide, hydrochloric acid, nitric acid, etc. You may dilute any base or commercial acid to some required concentration e.g. 0.2M, 0.5M, 0.25M, etc and then standardize it by similar procedures.

Choice of indicators in acid-base titration

We learned early that the estimation of the concentration of a solution of an acid or base by reaction with a standard alkali or acid solution respectively, is known as titration. The end-point of an acid-base titration is commonly determined using substances known as indicators, which usually portray certain characteristic colours when in alkaline or acid solutions.

The indicators in acid-base titrations must be chosen carefully because the choice of an inappropriate indicator would lead to an incorrect result. The choice of an indicator is based on the strength of an acid or base involved in the reaction.

There are three common indicators which are used in titration experiments involving acids and bases namely, methyl orange, litmus and phenolphthalein. The other indicators in less common use are as included in the table below. The table shows the colours which each of these indicators take up in acid or alkaline solution.

Colour of indicators in acid and alkaline solutions

Indicators suitable for particular types of acid-base reactions are as given in the table below:

Indicators suitable for different acid-base reactions

Volumteric Calculations

Common Mineral Acids

Standardize common mineral acids

Data for calculations of volumetric analysis problems are obtained from volumetric analysis experiments. For any volumetric analysis problem, at least one standard solution is required. A correctly balanced reaction equation (from which moles ratios can be derived) is a prerequisite for all these calculations. This is because the mole ratio is an integral part of the general expressionused for all volumetric analysis calculations. The general expression is given by:

where;

• V_A= Volume of acid
• M_A= Molarity of acid
• V_B= Volume of base
• M_B= Molarity of base
• N_A= Number of moles of acid
• N_B= Number of moles of base

The Relative Atomic Mass of Unknown Element in an Acid or Alkali

Find the relative atomic mass of unknown element in an acid or alkali

Example 1

12.5 cm³ of 0.5M sulphuric acid neutralized 50 cm³ of a given solution of sodium hydroxide. What is the molarity of the alkali?

Solution

Reaction equation is:

H₂SO_4(aq) + 2NaOH_(aq) →Na₂SO_4(aq) + 2H₂O_(l)

From the equation, 1 mole of sulphuric acid solution reacts with 2 moles of sodium hydroxide solution. So, the number of moles of the acid, N_A = 1 and the number of moles of base, N_B = 2.The other data are as follows:

V_A = 12.5 cm³, M_A = 0.5M and V_B = 50 cm³

So, the molarity of the acid = 0.25M

Example 2

20 cm³ of a solution containing 7g/dm³ of a metal hydroxide, XOH, were exactly neutralized with 25 cm³ of 0.10M hydrochloric acid.

1. Write a balanced chemical equation for the neutralization of the metal hydroxide, XOH, with hydrochloric acid.
2. Calculate the concentration of the metal hydroxide in moles per dm³.
3. (i) Calculate the molar mass of XOH (ii) Identify element X

Solution

<!–[endif]–>HCl_(aq) + XOH_(aq)→ XCl_(aq) + H₂O_(l)

From the reaction equation, N_A = 1 and N_B = 1.

V_A = 25 cm³, V_B = 20 cm³, M_A = 0.1M, M_B = ?

X = 39g

Therefore, element X is potassium (K)

The Percentage Purity of an Acid or an Alkali

Calculate the percentage purity of an acid or an alkali

Example 3

5.1g of impure sodium carbonate solution was dissolved in water to make 500 cm³ of solution. 20 cm³ of this solution was titrated against 20.45 cm³ of 0.04M hydrochloric acid. Calculate the percentage purity of the sodium carbonate solid.

Solution

To calculate the concentration of impure Na₂CO₃ solution:5.1g per 500 cm³ = 10.2g dm^-3Reaction equation is:

Na₂CO_3(aq) + 2HCl_(aq) → 2NaCl_(aq) + H₂O_(l) + CO_2(g)

From the equation, N_A =2 and N_B = 1.

= 0.02045M

Therefore, molarity of the base = 0.02045M

To calculate the concentration of pure base (Na₂CO₃ solution) in g dm^-3:

Concentration = Molarity × Molar mass

But molar mass of Na₂CO₃ = 106.

So, concentration (g/dm³) = 0.02045 × 106 = 2.1677g dm^-3

The percentage purity of sodium carbonate = 21.25%

The Number of Molecules of Water of Crystallization of a Substance

Find the number of molecules of water of crystallization of a substance

Example 4

0.465g of a hydrated form of sodium carbonate exactly reacts with 75 cm³ of 0.10M hydrochloric acid. Calculate the number of molecules of water of crystallization present in one mole of the hydrated salt.

Solution

The balanced reaction equation for the anhydrous salt is:

Na₂CO_3(s) + 2HCl_(aq) → 2NaCl_(aq) + CO_2(g) + H₂O_(l)

Number of moles of HCl present in 75 cm³ = 75/1000×0.1 = 0.0075M

Mass of 0.00375 moles of Na₂CO₃ = 0.00375 × 106 = 0.3975g (= mass of anhydrous salt)

Molar mass of anhydrous salt = 106g

Mass of water contained in the hydrated salt = mass of hydrated salt – mass of anhydrous salt

= 0.465 – 0.3975

= 0.0675g

Therefore, the mass of water of crystallization = 0.0675g

But, remember that mass of the anhydrous salt = 0.3975g

Now, let the formula of the hydrated salt be Na₂CO₃.XH₂O, where X is the number of moles of water crystallization

Therefore, the formula of the salt is Na₂CO₃.H₂O

Application of Volumetric Analysis

The Application of Volumetric Analysis in Real Life Situations

Explain the application of volumetric analysis in real life situations

Volumetric analysis has a variety of laboratory and industrial applications in everyday life. The following are just a few of the applications (uses) of volumetric analysis in daily life:

• Use in preparation of standard solutions: Standard solutions are prepared by applying the knowledge of volumetric analysis. Volumetric analysis is used in school, college and university chemistry laboratories to determine concentrations of unknown substances. The titrant (the known solution) is added to a known quantity of analyte (unknown solution) and a reaction takes place. Knowing the volume of the titrant allows one to determine the concentration of the unknown substance.
• Use in environmental and water safety: Titration is important in environmental chemistry, where scientists can use it to analyze acid rain or contaminants in surface water samples. Environmental studies usually involve an analysis of precipitation and its response to pollution. To quantify the degree of contamination in natural rainwater or snow, titration is used. The process is quick and results are reliable. Since most titration processes do not require expensive or specialized equipment, the test can be performed often and in different areas with relatively little effort.The safety of water is based on its chemical ingredients. By analyzing wastewater, the extent of contamination and the requirements for filtering and cleaning can be determined. Titration is a key mechanism in this analysis. Often, more specialized titration equipment is used in this application, which measures ammonia levels in combination with other reactants to quantify other chemicals present.
• Use in food and beverage industry: In the food and beverage industry, manufacturers must ensure their products meet certain quality criteria or contain standard concentrations of specific additives, so titration is often used to analyze the products before sale. Wine is often affected by its degree of acidity. It is possible to improve wine production by measuring acidity using titration. Simple, inexpensive titration kits are available to winemakers for this purpose. The results of a titration test on wine can suggest if additional ingredients are necessary to maintain its quality.In general, all brewing industries and distilleries apply the knowledge of volumetric analysis (titration) to determine the acidity and alcohol contents of their beers and other alcoholic beverages.The process also finds ample use in food industry. The compounds which make up food products help determine their nutritional implications. Titration is one technique that assists in these studies. The acidity of orange juice, for example, is easily determined using a standard titration process. In this process, an electrode is added to a solution made up of orange juice and deionized water. The titrant catalyst then measures the acidity of the juice. Manufacturers can use the technique to vary this quality to satisfy customers or those with special nutritional needs.
• Use in agriculture: Volumetric analysis technique is used to determine the soil pH. This is important because, if the pH of a certain soil is found to be extremely low or high, corrective measures are taken by adding the correct quantity of agricultural limes or other chemicals to make the soil suitable for plant growth.The method is also used by agronomists and farmers to analyse the kind and amount of plant nutrient elements present in a particular sample of soil, the knowledge of which helps determine soil fertility.

Industrial and Laboratory Skills of Volumetric Analysis

Compare industrial and laboratory skills of volumetric analysis

The knowledge of volumetric analysis (titration) is used in hospitals and medical laboratories to carry out such duties as preparation of solutions and suspensions, blood analysis, and diagnosis of certain diseases and health problems. For example, when dissolving a solid drug to make a solution for injection, utmost precision is required to measure the correct volume of liquid to be used to dissolve a correct amount of solid drug to prepare the solution of a given concentration to inject to a patient.

Also titration is very important in the pharmaceutical industry, where precise measurements of quantities and concentrations are essential throughout the manufacturing process. Titration is thus an important part of the pharmaceutical industry to ensure quality control. Many variations of the titration technique are used, and specialized equipment for pharmaceutical titration is often developed to make the process more efficient.

……………

The Concept of Hardness of Water

Explain the concept of hardness of water

As water flows over the land, it dissolves many mineral substances. The dissolved minerals are deposited together with water in rivers, lakes and oceans. Water is said to be hard if it contains some specific type of dissolved minerals. It is important to note that not all dissolved salts make water hard.

As you learned early, water is treated in water purification plants before being piped to your home. The treatment removes only the insoluble particles and kills bacteria. So the water still is not pure. It contains natural compounds dissolved from rocks and soil. It may also contain traces of chemicals dumped from homes, farms and factories.

Water obtained from an area where the rocks contains chalk, limestone, dolomite or gypsum, contains dissolved calcium and magnesium sulphates and hydrogen carbonates. These salts make the water hard.

One can distinguish between hard and soft water when washing with soap. Hard water does not form lather easily. Instead, it forms a precipitate or scum. It requires much soap to react with all the dissolved minerals before enough lather is formed. Therefore, hard water wastes soap during washing.

When soap is used with hard water a “scum” forms on the surface. This is a result of a precipitation reaction between calcium and/or magnesium ions and soap. Soaps are the sodium or potassium salts of long-chain organic acids. Soaps are made from animal fats by treatment with alkali (NaOH or KOH). Ordinary washing soap is a compound of stearic acid, C₁₇H₃₅COOH. The nature of such soaps is the salt, sodium stearate, C₁₇H₃₅COONa⁺. Sodium stearate is soluble in water but calcium stearate is not.

When soap is mixed with hard water, the calcium or magnesium salts in the hard water react with soap and precipitates as scum. The nature of scum is either calcium stearate or magnesium stearate:

Soap will not form any lather with water until all the calcium and magnesium ions have been precipitated. Hard water, therefore, wastes soap. This means that more soap may be used for an efficient washing. The amount of soap needed to just produce froth can be used to estimate the hardness of water.

The problem of scum formation only occurs with soaps. Soapless detergents do not produce scum. The trade names for some soapy detergents sold in Tanzania include Komoa, Kuku, Taifa, Mbuni, Mshindi, Changu, Jamaa and several other bar soaps. The trade names for some soapless detergents include Omo, Foma, Tesa, Toss, Dynamo, Swan, etc.

Causes of Permanent and Temporary Hardness in Water

State causes of permanent and temporary hardness in water

Water is generally said to be hard if it contains soluble salts of calcium and magnesium. The salts are calcium and magnesium sulphates and hydrogen carbonates. Hardness of water is caused by higher than usual levels of calcium (Ca²⁺) and magnesium (Mg²⁺) ions in water.

As rain water passes through the atmosphere, it dissolves carbondioxide to form a weak carbonic acid.

As this solution passes over and through rocks containing chalk, limestone or dolomite, the rainwater very slowly dissolves them:

H₂CO_3(aq) + CaCO_3(s) → Ca(HCO₃)_{2 (aq)}

The calcium hydrogen carbonate formed is soluble in water and is responsible for the presence of calcium (Ca²⁺) ions in water.

Some of the rocks may contain gypsum (CaSO₄.2H₂O), an hydrite (CaSO₄), Kieserite (MgSO₄.H₂O) or dolomite (CaCO₃. MgCO₃) which can dissolve to a limited extent in water. The presence of these dissolved substances also causes the water to be hard. These substances dissolve sparingly in water to form Ca²⁺ and Mg²⁺ ions which are responsible for water hardness as stated early.

Activity 1

Investigation of the causes of water hardness

Materials:

• Test tube rack-
• Five clean test tubes
• Measuring cylinder (100cm³)
• Calcium sulphate solution (1 mol dm^-3)
• Soap solution
• Magnesium sulphate solution (1 mol dm^-3)
• Sodium sulphate solution (1 mol dm^-3)
• Potassium sulphate solution (1 mol dm^-3)
• Distilled water

Procedure:

• Label five clean and dry test tubes as A, B, C, D and E. 2
• Add 10 cm³of 1.0M calcium, magnesium, sodium and potassium sulphate solutions and distilled water in each of the test tubes respectively.
• Add 5 cm³of soap in each test tube
• Shake the test tubes well and place them in a test tube rack
• Observe the amount of lather formed in each test tube, and if there is any precipitate (scum) formed.

Results:

Results of experiment showing minerals which cause water hardness

Interpretation of the results

From the result of experiment, we can conclude that scum is produced when either calcium or magnesium salt is present in water. So, high levels of calcium or magnesium ions in water are responsible for water hardness.

When the concentration of either of these minerals is over 150 milligrams per cubic decimeter (150 mg/dm³), water is considered to be hard. The upper limit allowed is 300 mg/dm³

Types of Hardness of Water

Types of Hardness of Water

Identify types of hardness of water

The hard water in some areas can be softened simply by boiling the water, but this is not true in all cases. This means that the hardness in water can be divided into two types – temporary and permanent hardness.

Temporary hardness

Temporary hardness in water is caused by dissolved calcium or magnesium hydrogencarbonates. The most important characteristic of temporarily hard water is that it can be softened by simply boiling. When the water is boiled, the soluble sodium hydrogencarbonate is decomposed to form the insoluble calcium carbonate.

The decomposition causes the “furring” of kettles, hot water pipes and shower heads. This means that the inside of kettles, pipes and shower heads become coated with a layer of calcium carbonate (limescale) caused by the decomposition of the hydrogen carbonate according to the equation above.

In many supermarkets, it is possible to buy a limescale remover. This is often a solution of methanoic acid (formic acid). This weak acid is strong enough to react with limescale but not with the metal. The insoluble limescale (carbonate) is probably dissolved to a soluble compound, calcium methanoate that can be flushed away with water.

2COOH_(aq) + CaCO_3(s)(insoluble)→ Ca(HCOO)_2(aq) + H₂O_(l) + CO_2(g)calcium methanoate (soluble)

Permanent hardness

Permanent hardness in water is caused by soluble sulphates and chlorides of calcium and magnesium (CaSO₄, MgSO₄, CaCl₂ and MgCl₂). This type of hardness cannot be removed by boiling the water. This is because boiling does not decompose the chlorides of calcium or magnesium. Such water may only be softened by chemical treatment or ion exchange methods

The Difference between Soft and Hard Water

Differentiate soft from hard water

Activity 2

Distinction between temporarily and permanently hard water

Materials:

• Calcium carbonate
• Dilute hydrochloric acid
• 4 test tubes
• Test tube rack
• 1 litre of distilled water
• Calcium chloride
• Soap solution
• Beaker
• Heat source

Method:

1. Prepare carbon dioxide gas in the laboratory by mixing calcium carbonate with hydrochloric acid in a gas generator.
2. Bubble the gas through a suspension of calcium carbonate in water. Shake well as you bubble the gas until most of calcium carbonate has dissolved. Filter and divide the filtrate into two test tubes M and N.
3. Prepare a 0.5M solution of calcium chloride and divide it in two test tubes P and Q.
4. Prepare soap solution in a large beaker.
5. Arrange the four test tubes in a rack
6. Heat the solutions in test tubes M and P to boiling. Allow them to cool.7. Add 10cm³of the soap solution to each of the four test tubes, M, N, P and Q. Shake well and allow to rest. Observe lathering and scum formation.
   
   

Note:

When calcium carbonate is reacted with dilute hydrochloric acid, carbon dioxide gas is produced.

CaCO_3(s) + 2HCl_(aq) → CaCl_2(aq) + H₂O_(l) + CO_2(g)

When carbon dioxide gas is bubbled through a suspension of calcium carbonate in water for a long time, the insoluble calcium carbonate dissolves to give the soluble bicarbonate, the presence of which makes the water hard.

CO_2(g) + CaCO_3(s) + H₂O_(l) → Ca(HCO₃)_2(aq)

The purpose of heating solutions in test tubes M and P was to try to remove water hardness. However, only the hardness in test tube M was merely removed by boiling because it contains the temporarily hard water.

The hardness in test tube P could not be removed by just boiling because it contained the hard water. Calcium chloride cannot be decomposed by heat. So, no change is expected after heating.

Results:

• Scum was formed in test tubes N, P and Q but P and Q contained more scum than N.
• Lather was formed in test tube M only.
• Test tube M contained temporarily hard water and test tube P contained permanently hard water. The hardness in test tube M was removed by boiling while that in test tube P was not.
• Test tubes P and Q contained permanently hard water. The hardness in this water could not be removed by mere boiling.

Treatment and Purification of Hard Water

Process of Hard Water Treatment and Purification

Examine process of hard water treatment and purification

Because of the problem it causes, hard water is often softened for use in factories, industries and homes. That means removing the dissolved calcium and magnesium ions. Described below are the methods of treating and purifying hard water.

Boiling

Boiling removes temporary hardness in water, as you saw early. Boiling causes calcium carbonate to precipitate. The hydrogencarbonate in water are decomposed to carbonates, which are insoluble in water

In this way, the calcium is removed, since the calcium carbonates being insoluble, takes no further part in the reaction. An insoluble calcium salt cannot cause hardness. However, this method uses a lot of fuel, which makes it expensive to do on a large scale.

Distillation

Distillation removes all impurities from water. This gets rid of both temporary and permanent hardness. In distillation, the water is boiled and the steam collected, cooled and condensed. Distilled water is pure and softest water. All the dissolved substances have been removed. Like boiling, it is an expensive option in terms of fuel used. But it is essential for some purposes, for example for laboratory experiments and for making drugs.

Addition of calcium hydroxide

Addition of calculated amounts of calcium hydroxide can remove temporary hardness. The quantity to be added should be properly calculated because excess would cause hardness on its own account. The amount of calcium hydroxide to be added is calculated based on knowledge of the hardness of water and the capacity of the reservoir (Clark’s method). The calcium hydroxide reacts with the hydrogen carbonates dissolved in water and precipitates as the insoluble calcium carbonates.

Ca(OH)_2(s) slightly soluble + Ca(HCO₃)_2(aq) → 2CaCO_3(s) + 2H₂O_(l) insoluble

Addition of sodium carbonate (washing soda)

Washing soda removes both temporary and permanent hardness by precipitating calcium carbonate. It reacts with calcium hydrogen-carbonate (which causes temporary hardness) to form sodium hydrogen-carbonate like this:

Na₂CO_3(aq)+ Ca(HCO₃)_2(aq)→ 2NaHCO_3(aq))+ CaCO_3(s)

It also reacts with calcium sulphate (which causes permanent hardness) to form sodium sulphate thus:Na₂CO_3(aq)+ CaSO_4(aq)→ CaCO_3(s)+ Na₂SO_4(aq)

These sodium salts are soluble, but do not cause the water to be hard. The calcium and magnesium ions are precipitated as insoluble calcium and magnesium carbonates. Ionically, the situation is like this:

Ca²⁺_(aq)+ CO₃^2-_(aq)→ CaCO_3(s)

Mg²⁺_(aq)+ CO₃^2-_(aq)→ MgCO_3(s)

Ion Exchange

Another method that removes both temporary and permanent hardness in water is the use of ion exchange resin. A typical ion exchanger is a container full of small beads. These beads are made of special plastic called ion exchange resin. The resin beads are porous and contain sodium ions. When hard water flows through the resin, the calcium and magnesium ions in the water are exchanged for the sodium ions and attach themselves to the resin. This process, therefore, removes calcium and magnesium ions from the water. They are replaced by sodium ions, which do not make the water hard.

An ion exchange column removes ca²⁺ and mg²⁺ ions from the water and replaces them with Na⁺ ions

When all sodium ions have been removed from the resin, it is regenerated by pouring a concentrated solution of sodium chloride through it. The sodium ions remove the calcium and/or magnesium ions off the resin and the ion exchanger is ready for the use again. Other ions could also be used instead of sodium for the resin. But sodium chloride is normally used to supply the sodium ions because salt is cheap.

Use of softeners

Many modern washing powders now have softeners added to them. The softeners are often phosphates. The phosphates ions react with calcium ions to form calcium phosphate and remove the hardness. 3Ca²⁺_(aq)+ 2PO₄^3-_(aq)→ Ca₃(PO₄)_2(s)

The Importance of Hard Water Treatment and Purification

Describe the importance of hard water treatment and purification

The significance of water in daily life is well known to everyone. The water we obtain from natural sources is never pure. It contains dissolved minerals which render the water unfit for direct uses. The water from some sources contains calcium and magnesium compounds dissolved in it. These compounds are responsible for water hardness. To make the water fit for various uses, it is imperative to remove the hardness. The following points state why it is important to treat and purify hard water:

Hard water wastes soap. To get enough lather with hard water, it requires more soap than it does with soft water. So it is important to soften the water in order to save the soap and hence reduce the cost of washing. Laundry uses less soap and can be done at lower temperatures.

Treating and purifying hard water eliminates the possibility of forming limescale deposits in water boilers, kettles, washing machines, water heaters, shower heads and dish washers. The scale formed around the heating elements can cause the element to overheat and fail.

Treated and purified water leaves no scum on clothes during washing. Scum spoils the finishing of some fabrics. It forms nasty deposits (marks) on clothing that has been washed.

Softened water has the advantage of not blocking the water pipes. In industry, deposits of scales can block the pipes in boilers. This is a safety hazard as it could cause pressure to build up until there is an explosion. A similar coating can occur in hot water pipes at home and in central heating systems.

The Importance of Hard Water in Daily Life

State the importance of hard water in daily life

Hard water is not always disadvantageous. The following points explain the importance of hard water:

1. The dissolved calcium and magnesium salts improve the taste of water. Distilled water is tasteless and quite unpleasant to drink. This is why water-processing plants add some salts in the distilled water to make it tasteful.
2. Calcium dissolved in hard water is an essential mineral for growth of bones and teeth. It makes our teeth and bones hard, strong and resistant to shear and pressure.
3. In some places, old lead pipes are used for water supply. Lead is very poisonous, and a little of it can dissolve in soft water. But the carbonate (CO₃^2-) or sulphate (SO₄^2-) ions present in hard water reacts with lead to form a coating of lead carbonate or lead sulphate that prevents lead from dissolving. This prevents lead poisoning.
4. A coating of calcium carbonate inside pipes, boilers and radiators helps to prevent corrosion.
5. In recent years, it has been suggested that drinking hard water helps to prevent heart diseases. 6. It has also been found that hard water is good for brewing beer.