Question Answers For All Chapters – General Science Class 7
Materials we Use
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 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.
2. Write answers to the following questions:
(a) How does the use of a detergent help to clean soiled clothes?
→ Detergent molecules have two ends—one attracts water, and the other attracts oil and dirt. This helps in removing grease and stains from clothes.
(b) How will you check with the help of soap powder whether water is hard?
→ If soap forms less lather and more scum in water, it means the water is hard. If the soap foams easily, the water is soft.
(c) What are the important ingredients of toothpaste, and what is the function of each?
→ Calcium carbonate & calcium hydrogen phosphate remove dirt and polish teeth.
→ Fluoride prevents cavities and strengthens teeth.
→ Flavors & sweeteners improve taste.
(d) What are the ingredients of cement?
→ Cement is made of lime (CaO), silica (SiO₂), alumina (Al₂O₃), iron oxide (Fe₂O₃), gypsum (CaSO₄), and magnesia (MgO).
(e) What will happen if cement is not used in making concrete?
→ Without cement, concrete will not be strong and durable, and structures will crack and break easily.
(f) Make a list of detergents that you use.
→ Examples: Soap, liquid soap, washing powder, shampoo, dishwashing liquid, and hand wash.
(g) What should be expected from a detergent for delicate garments?
→ It should be gentle, non-damaging, free from harsh chemicals, and should not fade colors.
(h) What is meant by ‘surface activity’? Name three chemicals responsible for the surface activity of various detergents.
→ Surface activity refers to the ability of detergents to reduce surface tension and help in cleaning.
→ Chemicals responsible: Sodium lauryl sulfate, alkyl benzene sulfonate, and sodium stearate.
3. Similarities and Differences:
(a) Natural detergents and man-made detergents
| Natural Detergents | Man-Made Detergents |
|---|---|
| Derived from plants like soap nut (ritha). | Made from chemicals in factories. |
| Eco-friendly, does not harm skin. | Can cause skin irritation and pollution. |
| Example: Soap pod (shikekai), soap nut (ritha). | Example: Synthetic liquid detergents, washing powders. |
(b) Soap and Synthetic Detergents
| Soap | Synthetic Detergent |
|---|---|
| Made from natural oils and fats. | Made from petrochemicals. |
| Works well in soft water but not in hard water. | Works in both soft and hard water. |
| Example: Bath soap, laundry soap. | Example: Liquid detergents, powders. |
(c) Bath soap and soap for washing clothes
| Bath Soap | Soap for Washing Clothes |
|---|---|
| Contains moisturizers and perfumes for skin. | Contains strong cleansing agents for removing dirt. |
| Made using potassium salts (soft soap). | Made using sodium salts (hard soap). |
| Mild and gentle on skin. | Harsh on clothes, removes grease. |
(d) Modern cement and ancient cement
| Modern Cement | Ancient Cement |
|---|---|
| Made from lime, silica, alumina, iron oxide, and gypsum. | Made by mixing volcanic ash with lime. |
| Used in concrete structures (buildings, bridges). | Used by Romans for strong constructions. |
| Example: Portland cement. | Example: Roman cement. |
4. Explain why:
(a) Soap cannot be used in hard water.
→ Hard water contains calcium and magnesium salts that react with soap to form scum, reducing its cleaning ability.
(b) Oil does not mix in water. However, oil and water become homogeneous if a sufficient quantity of detergent is added.
→ Detergents have hydrophilic (water-loving) and hydrophobic (oil-loving) ends, which help mix oil with water.
(c) Synthetic detergents are superior to soap.
→ Synthetic detergents work in both hard and soft water, clean better, and produce more foam than soap.
(d) Often colored spots are formed on clothes during washing.
→ If detergents do not dissolve properly, they leave chemical residues, causing colored spots.
(e) Tobacco masheri should not be used for cleaning teeth.
→ Tobacco masheri contains harmful chemicals that damage tooth enamel and cause gum diseases.
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