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Chemistry in Everyday Life — Exercise Solutions

Complete solutions for Samacheer Kalvi Class 6 Science, Term 3, Chapter 3: Chemistry in Everyday Life. Solve every question on your own first, then check your answers here.

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I. Multiple Choice Questions

Q1. Soaps were originally made from \_\_\_\_\_\_

• (a) proteins
• (b) animal fats and vegetable oils
• (c) chemicals extracted from the soil
• (d) foam booster

Traditional soaps were made by heating animal fats or vegetable oils with alkali (saponification). The fats react with the alkali to produce soap and glycerol.

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Q2. The saponification of a fat or oil is done using \_\_\_\_\_\_ solution for hot process.

• (a) potassium hydroxide
• (b) sodium hydroxide
• (c) hydrochloric acid
• (d) sodium chloride

In the hot process of saponification, concentrated sodium hydroxide (NaOH) solution is used. The NaOH reacts with the fat or oil to produce soap.

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Q3. Gypsum is added to the cement for \_\_\_\_\_\_

• (a) fast setting
• (b) delayed setting
• (c) hardening
• (d) making paste

Gypsum (CaSO₄·2H₂O) is added during the final grinding of cement to delay (slow down) the setting process. Without gypsum, cement would set too quickly, making it difficult to use.

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Q4. Phenol is \_\_\_\_\_\_

• (a) carbolic acid
• (b) acetic acid
• (c) benzoic acid
• (d) hydrochloric acid

Phenol is chemically known as carbolic acid. Its molecular formula is C₆H₅OH. It was one of the first antiseptics used in surgery.

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Q5. Natural adhesives are made from \_\_\_\_\_\_

• (a) protein
• (b) fat
• (c) starch
• (d) vitamins

Starch dissolved in water is an example of a natural adhesive. Other natural adhesives include gum arabic and tree resins.

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Additional MCQs

Q6. \_\_\_\_\_\_ change results in the change of the substance.

• (a) Physical
• (b) Chemical
• (c) Reversible
• (d) Temporary

A chemical change produces a new substance with different properties. Burning paper, cooking food, and rusting are examples.

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Q7. Percentage composition of Nitrogen in Urea is \_\_\_\_\_\_

• (a) 23%
• (b) 34%
• (c) 40%
• (d) 46%

Urea has the formula CO(NH₂)₂. Nitrogen constitutes 46% of urea by mass, making it a nitrogen-rich fertilizer.

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Q8. \_\_\_\_\_\_ is the important material in the construction industry.

• (a) Plastic
• (b) Rubber
• (c) Cement
• (d) Glass

Cement is a key binding material used in construction. When mixed with water, sand, and gravel, it forms concrete — the backbone of modern construction.

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Q9. Who invented cement?

• (a) John Dalton
• (b) Joseph Aspdin
• (c) Marie Curie
• (d) Dmitri Mendeleev

Joseph Aspdin invented Portland cement in 1824. It was named after the high-quality building stones found in Portland, England.

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Q10. \_\_\_\_\_\_ concrete is a composite material made by mixing iron mesh with cement.

• (a) Plain cement
• (b) Prestressed
• (c) Reinforced cement
• (d) Quick-setting

Reinforced Cement Concrete (RCC) is made by embedding iron rods or mesh within concrete. The iron provides tensile strength while concrete provides compressive strength.

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Q11. Molecular formula of Plaster of Paris is \_\_\_\_\_\_

• (a) CaSO₄·½H₂O
• (b) CaSO₄·2H₂O
• (c) CaSO₄
• (d) Ca(OH)₂

Plaster of Paris is calcium sulphate hemihydrate (half a water molecule per formula unit). When mixed with water, it re-forms gypsum (CaSO₄·2H₂O) and hardens.

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Q12. Chemical name of gypsum is \_\_\_\_\_\_

• (a) Calcium oxide
• (b) Calcium hydroxide
• (c) Calcium carbonate
• (d) Calcium sulphate dihydrate

Gypsum is CaSO₄·2H₂O — calcium sulphate with two molecules of water of crystallisation.

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Q13. \_\_\_\_\_\_ is used in making blackboard chalks.

• (a) Plaster of Paris
• (b) Gypsum
• (c) Cement
• (d) Phenol

Plaster of Paris is widely used for making blackboard chalks, statues, toys, and casts for setting fractured bones.

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Q14. \_\_\_\_\_\_ is used as a surgical antiseptic.

• (a) Cement
• (b) Phenol
• (c) Gypsum
• (d) Urea

Phenol (carbolic acid) was first used as a surgical antiseptic by Joseph Lister. In low concentrations, it kills germs and prevents infection.

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II. Fill in the Blanks

1. Propanethial S-oxide gas causes tears in our eyes while cutting onions. 2. Water, coconut oil and animal fat are necessary for soap preparation. 3. Earthworm is called the farmer's best friend. 4. Organic fertilizer is eco-friendly. 5. Starch dissolved in water is an example of a natural adhesive. 6. Burning of paper is the best example of a chemical change. 7. Chemistry is the branch of science which deals with the study of particles around us. 8. Salt is a combination of sodium and chlorine. 9. We could prepare soft idli as a result of a chemical change named fermentation. 10. We use wash powder (detergent) to remove strong stains on clothes. 11. Organic fertilizers help plant growth and restore soil fertility. 12. Compost is an organic fertilizer. 13. Joseph Aspdin invented Portland cement. 14. Epsom is a medicine for skin problems. 15. Calcium sulphate hemihydrate is called Plaster of Paris. 16. Phenol is used in mouthwash in low concentrations. 17. The adhesive used in puncture shops is an artificial adhesive.

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III. True or False (with corrections)

Q1. Concentrated phenol is used as a disinfectant.

False ✗ — Low concentrated phenol is used as a disinfectant. Concentrated phenol is corrosive and harmful to skin and tissues.

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Q2. Gypsum is largely used in medical industries.

False ✗ — Gypsum is largely used in cement preparation and construction, not primarily in medical industries. It is added to cement to delay its setting time.

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Q3. Plaster of Paris is obtained from heating gypsum.

True ✓ — When gypsum (CaSO₄·2H₂O) is heated to about 150°C, it loses three-quarters of its water of crystallisation and becomes Plaster of Paris (CaSO₄·½H₂O).

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Q4. Adhesives are the substances used to separate the components.

False ✗ — Adhesives are substances used to join or bind components together. They are bonding agents, not separating agents.

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Q5. NPK are the primary nutrients for plants.

True ✓ — NPK stands for Nitrogen (N), Phosphorus (P), and Potassium (K) — the three primary macronutrients essential for plant growth.

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Q6. Water boiling into water vapour is an example of chemical change.

False ✗ — Boiling water into vapour is a physical change. No new substance is formed; only the state changes from liquid to gas. The process is reversible.

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Q7. The water we drink is a combination of hydrogen and oxygen.

True ✓ — Water (H₂O) is a compound of hydrogen and oxygen in a 2:1 ratio by number of atoms.

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Q8. When we soak onion in water, the irritation is increased.

False ✗ — Soaking onion in water reduces the irritation. The propanethial S-oxide gas dissolves in water, reducing its concentration and the tearing effect.

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Q9. Mortar is a paste of cement and sand mixed with water.

True ✓ — Mortar is a mixture of cement, sand, and water used in bricklaying and plastering. It binds bricks and stones together.

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Q10. Portland cement resembled the high-quality building stones found in Portland, England.

True ✓ — Joseph Aspdin named his invention "Portland cement" because the hardened cement resembled the famous limestone quarried on the Isle of Portland in England.

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Q11. Reinforced Cement Concrete is a mixture of cement, sand, and gravel.

False ✗ — Concrete is a mixture of cement, sand, and gravel. Reinforced Cement Concrete (RCC) is concrete with iron rods or mesh embedded inside for added tensile strength.

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Q12. Gypsum is used in improving plant growth in agriculture.

False ✗ — It is Epsom salt (magnesium sulphate) that is used for plant growth and as a skin medicine. Gypsum is primarily used in cement production and making Plaster of Paris.

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IV. Match the Following

Explanations:

• Soap — Sodium hydroxide (NaOH) is used in the saponification process to make soap from fats/oils.
• Cement — Used to make Reinforced Cement Concrete (RCC) by mixing with iron mesh, sand, and gravel.
• Fertilizers — NPK stands for Nitrogen, Phosphorus, and Potassium, the primary nutrients in fertilizers.
• Gypsum — Calcium sulphate dihydrate (CaSO₄·2H₂O) is the chemical name of gypsum.
• Phenol — Phenol has the molecular formula C₆H₅OH and is also known as carbolic acid.

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V. Arrange in Correct Sequence

Arrange the steps of soap preparation in correct order:

Given statements: 1. Pour that solution into an empty matchbox; soap can be obtained after drying. 2. Take necessary quantity of water in a jar. 3. Then add coconut oil drop by drop and stir it well. 4. Add concentrated sodium hydroxide in the jar and allow it to cool. 5. Try this soap to wash your handkerchief. 6. Cover your work area with old newspaper.

Correct sequence:

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VI. Analogy

1. Urea : Inorganic fertilizer :: Vermi compost : Organic fertilizer

Urea is a chemical (inorganic) fertilizer. Vermi compost is made from organic waste by earthworms — it is an organic fertilizer.

2. Starch dissolved in water : Natural adhesives :: Cello tape : Artificial adhesives

Starch dissolved in water is a natural adhesive. Cello tape is an artificial (synthetic) adhesive.

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VII. Very Short Answers

Q1. What are the three main constituents of soap?

The three main constituents are:

1. Water (H₂O) 2. Sodium hydroxide (NaOH) 3. Coconut oil (or other vegetable oils / animal fats)

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Q2. What are the two different types of molecules found in soap?

Soap molecules have two distinct parts:

1. Hydrophilic (water-loving) head — the ionic end that dissolves in water 2. Hydrophobic (water-hating/oil-loving) tail — the hydrocarbon chain that dissolves in oil/dirt

This dual nature allows soap to remove grease — the hydrophobic end attaches to oil while the hydrophilic end attaches to water, pulling the dirt away.

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Q3. Give an example of an inorganic fertilizer.

Examples of inorganic (chemical) fertilizers:

• Urea
• Super phosphate
• Ammonium sulphate
• Potassium nitrate

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Q4. Mention any three physical properties of phenol.

1. Weak acid — phenol is mildly acidic in nature 2. Highly volatile — it evaporates easily 3. White crystalline powder — in its pure form, phenol appears as white crystals

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Q5. Explain the uses of Plaster of Paris.

1. Making blackboard chalks 2. In surgery for setting fractured bones (orthopaedic casts) 3. Making casts for statues and toys 4. In the construction industry for decorative work and smooth finishes

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Q6. What are the ingredients of cement?

The three main ingredients of cement are:

1. Limestone (calcium carbonate) 2. Clay (provides silica, alumina, and iron oxide) 3. Gypsum (added during final grinding to delay setting)

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Q7. Why is gypsum used in cement production?

Gypsum is added to cement during the final grinding stage to delay the setting of cement. Without gypsum, cement would set (harden) too quickly, making it impossible to transport and use at construction sites. Gypsum slows down the reaction so workers have enough time to pour and shape the concrete.

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VIII. Short Answers

Q1. Why is the earthworm called a farmer's friend?

Earthworms are called "farmer's best friend" because:

• They consume organic waste and convert it into nutrient-rich compost (vermi castings)
• Vermi castings improve soil structure, texture, and aeration
• They enhance water retention capacity of the soil
• They help prevent soil erosion
• The castings contain valuable plant hormones like auxins and gibberellins
• They enrich the soil with essential nutrients naturally, reducing the need for chemical fertilizers

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Q2. Explain the process of manufacturing cement.

Cement is manufactured by the following process:

1. Crushing — Naturally occurring minerals like limestone and clay are crushed into a fine powder 2. Mixing — The powdered limestone and clay are mixed in correct proportions 3. Heating — The mixture is heated in a rotary kiln at about 1500°C to form clinker (hard nodules) 4. Grinding — The clinker is cooled and ground into a fine powder 5. Adding gypsum — About 2-3% gypsum is added during the final grinding to delay the setting time

The resulting grey powder is Portland cement.

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Q3. What are the uses of gypsum?

Gypsum (CaSO₄·2H₂O) has the following uses:

1. In cement production — Added to delay the setting of cement 2. In making Plaster of Paris — Heating gypsum produces Plaster of Paris, used for casts and chalks 3. As a fertilizer — Gypsum provides calcium and sulphur to the soil (though Epsom salt is more commonly used for direct plant growth)

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IX. Detailed Answers

Q1. How is soap manufactured? Describe the saponification process.

Saponification is the chemical process of making soap by reacting fats or oils with an alkali (NaOH or KOH).

Materials required:

Process (Hot Process):

1. Cover your work area with old newspaper for safety. 2. Take 35 ml of water in a glass jar. 3. Add 10 ml of concentrated NaOH solution to the water and allow it to cool (the reaction generates heat). 4. Add 60 ml of coconut oil drop by drop while stirring continuously. 5. Keep stirring until the mixture thickens and forms soap. 6. Pour the thick solution into an empty matchbox (as a mould). 7. Allow it to dry for 1-2 days. 8. The solidified product is soap.

Chemical reaction:

Fat/Oil + NaOH → Soap + Glycerol

How soap cleans: Soap molecules have a hydrophilic (water-loving) head and a hydrophobic (oil-loving) tail. When soap is applied to a greasy surface, the hydrophobic tails attach to the oil/grease while the hydrophilic heads remain in water. This forms spherical structures called micelles that trap the dirt. When water is poured, the micelles are washed away, carrying the dirt with them.

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Q2. Describe vermicomposting. How can a farmer convert biowaste into compost?

Vermicomposting is the process of using earthworms to convert organic waste into nutrient-rich compost (vermi castings).

Steps for vermicomposting:

1. Construct a pit — Build a cement tub about 2½ feet high and 3 feet broad, or dig a compost pit in the ground. 2. Add bio-waste layer — Put agricultural biowaste (crop residue, animal waste) in the pit to a height of about 5 cm. 3. Add earthworms — Introduce a few earthworms (red wigglers or compost worms) into the bio-waste layer. 4. Add dry material — Add sawdust, coir waste, or husk on top of the bio-waste layer. 5. Add sand layer — Spread a thin layer of sand (about 3 cm) over the dry material. 6. Add green waste — Add garden waste (leaves, grass clippings) on top of the sand layer. 7. Moisten — Spray water to keep all layers moist. The contents must remain damp but not waterlogged. 8. Wait — After 10 to 15 days, the earthworms convert the waste into vermicompost manure.

Benefits of vermi castings:

• Rich in all essential plant nutrients (N, P, K)
• Improves soil structure, texture, and aeration
• Prevents soil erosion
• Contains valuable plant hormones (auxins, gibberellins)
• Neutralizes soil pH
• Eco-friendly alternative to chemical fertilizers
• Enhances water retention capacity of the soil

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Q3. Compare organic and inorganic fertilizers.

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X. Higher Order Thinking Skills (HOTS)

Q1. Ravi is a farmer who rears many cattle. His field has many biowastes. Advise Ravi on how to change this biowaste to compost using vermicomposting. Explain the benefits of vermi castings.

Step-by-step vermicomposting for Ravi's farm:

1. Build a cement tub (2½ feet high, 3 feet broad) near the cattle shed. 2. Collect all cattle waste (dung, crop residue, leftover fodder) and place it in the tub to a height of 5 cm. 3. Introduce earthworms into the bio-waste. 4. Add sawdust, coir waste, or husk on top of the waste. 5. Add a 3 cm layer of sand over that. 6. Add green garden waste on top of the sand. 7. Spray with water to keep everything moist. 8. After 10-15 days, harvest the vermicompost manure.

Benefits of vermi castings for Ravi:

• Converts waste that would otherwise pollute into valuable fertilizer
• Saves money on buying chemical fertilizers
• Enriches soil naturally with all essential nutrients
• Contains plant growth hormones (auxins, gibberellins)
• Improves soil structure and prevents erosion
• Neutralizes soil pH for better crop yield
• Produces chemical-free, organic produce

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Q2. Why does cutting an onion make us cry? How can we reduce the irritation?

When we cut an onion, its cells break and release an enzyme called allinase. This enzyme converts amino acid sulphoxides into sulphenic acids, which rearrange to form a volatile gas called propanethial S-oxide. This gas reaches our eyes and dissolves in the tear fluid, forming a mild sulphuric acid that irritates the eyes, causing reflex tearing.

Ways to reduce irritation:

• Soak the onion in water before cutting — this dissolves the gas before it reaches your eyes
• Chill the onion in the refrigerator — cold temperatures slow down the enzyme reaction
• Cut under running water — water washes away the gas before it reaches your eyes
• Use a sharp knife — causes less cell damage and releases less gas

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Common Mistakes

1. Saying "concentrated phenol is used as disinfectant" — Concentrated phenol is corrosive and burns skin. Only dilute (low concentration) phenol is used as a disinfectant and antiseptic.

2. Confusing soap and detergent — Soaps are made from natural fats/oils with NaOH/KOH and do not work well in hard water. Detergents are synthetic and work in both hard and soft water.

3. Saying "gypsum is used in medicine" — Gypsum is mainly used in cement production and making Plaster of Paris. For skin problems, Epsom salt (magnesium sulphate) is used, not gypsum.

4. Confusing Plaster of Paris with gypsum — Gypsum (CaSO₄·2H₂O) is the raw material. When gypsum is heated, it becomes Plaster of Paris (CaSO₄·½H₂O). They are different compounds with different uses.

5. Saying "adhesives separate components" — Adhesives join or bind components together. They are bonding agents, not separating agents.

6. Confusing concrete with RCC — Concrete is a mixture of cement, sand, gravel, and water. RCC (Reinforced Cement Concrete) additionally has iron rods or mesh embedded inside for tensile strength.

7. Saying "boiling water is a chemical change" — Boiling water is a physical change. No new substance is formed; water simply changes state from liquid to gas. The process is easily reversible by cooling.

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Exam Checklist

• [ ] Define saponification and list the three main constituents of soap
• [ ] Explain the two types of molecules in soap (hydrophilic and hydrophobic) and how they clean
• [ ] Differentiate between soap and detergent
• [ ] List uses of phenol (disinfectant, antiseptic, mouthwash) and its physical properties
• [ ] Explain the manufacturing process of cement (ingredients: limestone, clay, gypsum)
• [ ] State why gypsum is added to cement (delays setting)
• [ ] Describe Plaster of Paris — its formula (CaSO₄·½H₂O), how it is made from gypsum, and its uses
• [ ] Differentiate between organic and inorganic fertilizers with examples
• [ ] Explain vermicomposting and why earthworms are called the farmer's friend
• [ ] Define NPK and explain its importance as primary plant nutrients
• [ ] Differentiate between natural adhesives (starch) and artificial adhesives (cello tape)
• [ ] Explain why cutting an onion causes tears (propanethial S-oxide gas)
• [ ] Distinguish between mortar, concrete, and RCC
• [ ] Know the inventor of Portland cement (Joseph Aspdin, 1824)
• [ ] Identify Epsom salt as a medicine for skin problems