Maharashtra State Board
Standard VII – Science
Chapter 15 – Materials We Use
Textbook Solutions
1. Fill appropriate terms in the blanks.
(white cement, soap, detergent, wearing of bones, tooth decay, hard, soft, Portland, fatty acid)
(a) The substance that helps water to remove dirt from the surface of materials is called ………..
Ans: detergent
(b) Fluoride is used in toothpaste to prevent ………..
Ans: tooth decay
(c) Soap is a salt of ……….. and sodium hydroxide.
Ans: fatty acid
(d) Synthetic detergents can be used in ……….. water as well.
Ans: har
(e) For construction purposes ……….. cement is the most commonly used cement.
Ans: Portland
2. Write answers to the following questions.
(a) How does the use of a detergent help to clean soiled clothes ?
Ans: The molecules of a detergent are long and the properties of their two ends are different. One end of a detergent molecule attracts water while the other end attracts oil or grease. When clothes are washed, the detergent molecules attach themselves to the dirt and oil particles. These particles then mix with water and are washed away, thus cleaning the clothes.
(b) How will you check with the help of soap powder whether water is hard ?
Ans: If soap powder is added to water and it does not form lather but instead forms a sticky layer called scum, the water is hard. Thus, by observing whether lather forms or not, we can determine whether the water is hard.
(c) What are the important ingredients of a toothpaste, and what is the function of each?
Ans: The principal ingredients of toothpaste are calcium carbonate and calcium hydrogen phosphate. These substances help to remove dirt from the teeth and polish them. Toothpaste also contains a small amount of fluoride, which helps prevent tooth decay and strengthens the enamel of the teeth.
(d) What are the ingredients of cement ?
Ans: Cement is made from silica (sand), alumina (aluminium oxide), lime, iron oxide, and magnesia (magnesium oxide). The most commonly used cement is Portland cement, which is prepared mainly from raw materials containing lime, silica, and alumina.
(e) What will happen if cement is not used in making concrete ?
Ans: Concrete is prepared by mixing cement, water, sand, and gravel. Cement acts as a binding material that holds the mixture together. If cement is not used, the mixture will not bind properly. As a result, the slabs and pillars will not be strong, leakage may occur, and the building may collapse.
(f) Make a list of detergents that you use.
Ans: Examples of detergents used in daily life include:
- For washing hair: Head & Shoulders shampoo, Dove shampoo
- For bathing: Lifebuoy soap, Dettol soap, Dove soap
- For washing clothes: Rin, Tide, Surf Excel
(g) What should be expected from a detergent for delicate garments ?
Ans: A detergent used for delicate garments should clean the clothes without damaging the fabric. Natural detergents such as soap nut (reetha) and soap pod (shikakai) are suitable because they are gentle on the skin and safe for delicate fabrics like silk and wool.
(h) What is meant by ‘surface activity’ ? Name three chemicals responsible for the surface activity of various detergents.
Ans: The property of a substance to spread over the surface of water and help remove dirt is called surface activity. Substances that show this property are called surfactants. Chemicals responsible for surface activity in detergents include petroleum products, alcohols, and sulphur compounds.
3. What are the similarities and differences between –
(a) Natural detergents and man-made detergents
Similarities:
Both are used for cleaning clothes and other materials.
Differences:
| Natural Detergents | Man-made Detergents |
|---|---|
| They contain naturally occurring substances. | They are produced artificially by humans. |
| They contain a chemical called saponin. | They contain additives such as perfumes, dyes, germicides, alcohol, anti-foaming agents, etc. |
| They usually do not harm clothes or skin. | They may sometimes harm skin and clothes. |
| They do not pollute water bodies. | They may cause pollution of water bodies. |
| Examples: Soap nut, soap pod | Examples: Synthetic detergents |
(b) Soap and synthetic detergent
Similarities:
Both are used for cleaning.
Differences:
| Soap | Synthetic Detergent |
|---|---|
| Made from oils or fats and sodium or potassium hydroxide. | Made from petroleum products and other chemicals. |
| Does not produce lather in hard water. | Produces lather even in hard water. |
| Cleaning action may be less effective in hard water. | Cleans clothes effectively in both hard and soft water. |
| Mild soaps are safe for the skin. | Some detergents may irritate the skin. |
(c) Bath soap and soap for washing clothes
Similarities:
Both contain fatty acids and are used for cleansing.
Differences:
| Bath Soap | Soap for Washing Clothes |
|---|---|
| Used for personal cleanliness. | Used for washing clothes. |
| It is mild and gentle on the skin. | It is harder and stronger. |
| It is usually a potassium salt of fatty acids. | It is usually a sodium salt of fatty acids. |
(d) Modern cement and ancient cement
Similarities:
Both are used for construction work.
Differences:
| Modern Cement | Ancient Cement |
|---|---|
| Developed by the British scientist John Smeaton. | Used by the Romans during the Roman Empire. |
| Made from silica, alumina, lime, iron oxide, and magnesium oxide. | Made by mixing volcanic ash with moist lime. |
4. Explain why –
(a) Soap cannot be used in hard water.
Ans: In hard water, soap reacts with minerals such as calcium and magnesium and forms a sticky substance called scum instead of lather. Because of this, soap loses its cleansing property and cannot clean effectively.
(b) Oil does not mix in water. However, oil and water become homogeneous if a sufficient quantity of detergent is added.
Ans: Oil and water do not mix because they are immiscible liquids. Detergent molecules have two different ends. One end attracts water and the other end attracts oil. When detergent is added, the oil particles attach to the detergent molecules and mix with water, making the mixture appear milky.
(c) Synthetic detergents are superior to soap.
Ans: Synthetic detergents are considered superior to soap because they work effectively in both soft water and hard water. The salts formed by detergents remain soluble in water, so they continue to produce lather and clean clothes efficiently.
(d) Often coloured spots are formed on clothes during washing.
Ans: Soaps are basic in nature. When substances like turmeric come in contact with soap solution during washing, they may react and form red or coloured spots on the clothes. These spots usually disappear after washing the clothes with plenty of water.
(e) Tobacco masheri should not be used for cleaning teeth.
Ans: Tobacco masheri contains tobacco, which is harmful to the body. Using it for cleaning teeth can damage the gums and teeth. It may also cause serious health problems such as oral cancer, gum disease, tooth decay, and receding gums. Therefore, tobacco masheri should not be used for cleaning teeth.