15. Materials We Use
Complete solutions to the Exercises. Maharashtra State Board (Balbharati), Grade Seven General Science.
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Exercises
(a) The substance that helps water to remove dirt from the surface of materials is called detergent.
(b) Fluoride is used in toothpaste to prevent tooth decay.
(c) Soap is a salt of fatty acid and sodium hydroxide.
(d) Synthetic detergents can be used in hard water as well.
(e) For construction purposes Portland cement is the most commonly used cement.
(a) How does the use of a detergent help to clean soiled clothes?
Dirt sticks to clothes because of an oily layer. Water alone cannot remove oil. A detergent molecule has two ends. One end holds water. The other end holds oil. So the detergent removes the oily dirt from the cloth. The dirt goes away with water while rinsing.
(b) How will you check with the help of soap powder whether water is hard?
- Take some water in a bottle.
- Add a little soap powder and shake well.
- If plenty of lather forms, the water is soft. If little lather and white scum form, the water is hard.
(c) What are the important ingredients of a toothpaste, and what is the function of each?
- Calcium carbonate and calcium hydrogen phosphate: They remove dirt and polish the teeth.
- Fluoride: It prevents tooth decay.
- Detergent: It forms foam and helps in cleaning.
- Flavour and sweetener: They give a pleasant taste.
(d) What are the ingredients of cement?
Cement is made from lime, silica, alumina, iron oxide and magnesia. Portland cement contains about 60% lime, 25% silica and 5% alumina. The rest is iron oxide and gypsum.
(e) What will happen if cement is not used in making concrete?
Concrete is made of cement, sand, gravel and water. Cement binds them together. Without cement, sand and gravel will not stick together. The concrete will not become strong. It will break easily. So buildings made with it will not be strong.
(f) Make a list of detergents that you use.
Bath soap, washing soap, washing powder, liquid soap, dishwashing bar, shampoo, hand wash, soap nut (ritha) and soap pod (shikekai).
(g) What should be expected from a detergent for delicate garments?
- It should be mild.
- It should not damage silk and woollen fibres.
- It should not fade the colours.
- It should clean well in cold water.
- It should rinse out easily.
(h) What is meant by ‘surface activity’? Name three chemicals responsible for the surface activity of various detergents.
The property of a substance to spread on a surface is called surface activity. Formation of lather is an effect of surface activity.
Chemicals responsible for surface activity:
- Saponin (in soap nut and soap pod)
- Sodium and potassium salts of fatty acids (in soaps)
- Sulphonates (in synthetic detergents)
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(a) Natural detergents and man-made detergents
Similarities: Both remove dirt. Both form lather.
| Natural detergents | Man-made detergents |
|---|---|
| Obtained from plants, e.g. ritha, shikekai. | Made by chemical processes, e.g. soap, washing powder. |
| They contain saponin. | They contain salts of fatty acids or chemicals. |
| They are very mild. | Some are strong. |
(b) Soap and synthetic detergent
Similarities: Both are man-made. Both remove oily dirt and form lather.
| Soap | Synthetic detergent |
|---|---|
| Made from fat or oil and an alkali. | Made from fats or kerosene by chemical processes. |
| Does not form lather in hard water. | Works in hard water also. |
| Used since ancient times. | Developed recently. |
(c) Bath soap and soap for washing clothes
Similarities: Both are soaps. Both remove dirt and form lather.
| Bath soap | Soap for washing clothes |
|---|---|
| It is a soft soap (potassium salt). | It is a hard soap (sodium salt). |
| It is mild for the skin. | It may harm the skin. |
| It contains perfumes and moisturizers. | It has fewer such substances. |
(d) Modern cement and ancient cement
Similarities: Both contain lime. Both harden when mixed with water. Both are used in construction.
| Modern cement | Ancient cement |
|---|---|
| Made from lime, silica, alumina, iron oxide and gypsum. | Made by mixing volcanic ash in wet lime. |
| Made in factories. | Made by the Romans by hand. |
| Used all over the world today. | Its use stopped after the Roman empire ended. |
(a) Soap cannot be used in hard water.
Hard water contains salts of calcium and magnesium. These salts react with soap. They form a sticky substance called scum. So soap does not form lather in hard water. It does not clean well.
(b) Oil does not mix in water. However, oil and water become homogeneous if a sufficient quantity of detergent is added.
Oil and water do not mix. Oil floats on water as a separate layer. A detergent molecule holds water at one end and oil at the other end. So it links oil with water. The oil spreads evenly in the water. The mixture becomes homogeneous.
(c) Synthetic detergents are superior to soap.
- They work well in hard water also.
- They form lather easily even in cold water.
- They are made from kerosene. So edible oils are saved.
- Useful substances like perfumes can be added to them.
(d) Often coloured spots are formed on clothes during washing.
The colour of some clothes is not fast. It dissolves in water during washing. This colour sticks to other clothes. So coloured spots are formed. Coloured and white clothes should be washed separately.
(e) Tobacco masheri should not be used for cleaning teeth.
Masheri is made from tobacco. Tobacco contains nicotine. It is addictive. It damages the teeth, gums and mouth. It can cause cancer of the mouth. So masheri should not be used.
- Limestone and clay are dug out from quarries.
- They are crushed and mixed in proper proportion.
- The mixture is ground into a fine powder.
- The powder is heated at a very high temperature in a kiln. Hard lumps called clinker are formed.
- The clinker is cooled. Gypsum is added and it is ground into fine powder.
- The cement is packed in bags.
Cement House: I am made of cement, bricks and steel. I am very strong. I can have many floors.
Mud House: I am made of mud, straw and cow-dung. I am cheap. I stay cool in summer and warm in winter.
Wattle-and-daub House: I am made of bamboo and sticks plastered with mud. I am light and easy to build.
Cement House: Heavy rain does not harm me. What about you?
Mud House: My walls need plastering every year. But I do not cause pollution.
Wattle-and-daub House: Termites and fire are my enemies. But I let fresh air pass through.
Cement House: I become very hot in summer. Making cement also causes pollution.
Mud House: So each of us has good and bad points.