HARDNESS OF WATER

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 hydrogencarbonates. 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. Thenature 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.

CAUSES OF WATER HARDNESS

Do you live in an area with hard water? If you do, you will be used to the scum that forms when you use soap in the water. In this unit you can find out what hard water is, how it forms, its effects, and how we can remove hardness from 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 hydrogencarbonates. 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 hydrogencarbonate 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), anhydrite (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.

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 WATER HARDNESS

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

DIFFERENCE BETWEEN SOFT AND HARD WATER

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.

ADVANTAGES AND DISADVANTAGE OF HARD WATER
There are advantages and disadvantages for people who live in hard water areas. Look at the table below:

Disadvantages of hard water Advantages of hard water It is more difficult to form a lather with soap. Some people prefer the taste. Scum may form in a reaction with soap, wasting the soap. Calcium ions in the water are good for children's teeth and bones. Limescale (a hard crust) forms inside kettles. This wastes energy whenever you boil a kettle. It helps to reduce heart disease. Hot water pipes 'fur up'. Limescale starts to coat the inside of the pipes which can eventually get blocked up. Some brewers prefer using hard water for making beer. A coating of limescale inside copper pipes, or especially old lead pipes, stops poisonous salts dissolving into water.

  

CONCEPT OF SOFT WATER

Basicallly we have two types of water namely: hard and soft water. The concept of hard water has been covered, what I want to do here is to give you a brief concept of soft water.

SOFT WATER is that water which can easily form leathers with soap. In short soft water have no chemical components like magnessium and calcium that may cause water hardness

ADVANTAGES AND DISADVANTAGES OF SOFT WATER
Soft water have both advantages and disadvantages, there is one saying "huwezi chinja mbuzi akakosa mifupa" 

The advantages of soft water

1. Since soft water lathers easily with soap, it helps in saving a lot of soap when used in washing. It is therefore economical to use soft water in washing.
2. It is compatible with dyeing, which is why the dyeing industry prefers using it in their business.
3. It does not stain white clothes when used in laundry, unlike hard water does.
4. Unlike the hard water, soft water does not form scales in kettles or pipes when it stays long in these containers.

The disadvantages of soft water

1. Soft water can easily lead to lead poisoning if it  is transferred through lead pipes or kept in lead containers. This is possible because it can quickly dissolve lead.
2. Soft water does not help in strengthening our bones and teeth since it contains no calcium.
3. Soft water has a taste which is not pleasant in the mouth. Hard water, on the other hand, has a very pleasant taste.
4. Soft water is not very safe for drinking as compared to hard water.