CBSE Class 10 Science Carbon and its Compounds VBQs Set 06

This is Set 06 of value-based and real-life questions for CBSE Class 10 Science Chapter 4, Carbon and Its Compounds, with step-by-step answers and a Teacher's Note for every question. The questions connect chemistry to daily life, covering fuels and LPG, cooking oils, vinegar and pickles, the dangers of methanol and alcohol, fermentation, soaps and detergents, hard water and water pollution.

Very Short Answer Type Value Based Questions

Question 1. Which oils should be chosen for cooking to remain healthy?
Answer:
Step 1: Oils that contain unsaturated fatty acids (such as groundnut, mustard, sunflower, soyabean and olive oil) should be chosen for cooking.
Step 2: Animal fats and hydrogenated (saturated) fats, such as ghee, butter and vanaspati, contain long saturated carbon chains. They raise cholesterol and are harmful to the heart when taken in large amounts.
Value: Healthy food habits come from choosing unsaturated oils and using them in moderation.
In simple words: Choose oils made of unsaturated fatty acids, not saturated animal fats.

Teacher's Note:
1. Unsaturated means the carbon chains have double bonds.
2. Hydrogenation of oils makes them saturated and solid.

Question 2. LPG (liquefied petroleum gas) is used as a fuel for cooking in homes. Why is LPG stored as a liquid?
Answer:
Step 1: LPG (mainly butane and propane) is a gas at room temperature, and a gas occupies a very large volume.
Step 2: Under high pressure it turns into a liquid, which occupies a much smaller volume, so a large quantity of fuel can be filled in a small, portable cylinder.
Step 3: When the valve of the regulator is opened, the pressure falls and the liquid changes back into gas, which is burnt in the stove.
Safety value: A strong smelling substance (ethanethiol) is added so that a leak is noticed quickly.
In simple words: Liquid LPG takes much less space, so more fuel fits in the cylinder.

Teacher's Note:
1. Increasing pressure converts a gas into a liquid.
2. Always keep the gas cylinder upright and ventilate the kitchen.

Question 3. Why are carbon and its compounds used as fuels for most applications?
Answer:
Most carbon compounds burn in air (oxygen) and give a large amount of heat and light, so they have high calorific values.
Saturated hydrocarbons burn with a clean flame and produce little or no smoke.
Fuels such as LPG, petrol, diesel and CNG are easy to handle, store and transport, and they are easily available.
Value: Because burning also releases CO₂ and may release CO and soot, fuels should be used carefully with proper ventilation.
In simple words: Carbon compounds burn to give a lot of heat and light, so they make good fuels.

Teacher's Note:
1. Combustion is an exothermic reaction.
2. Complete combustion of a hydrocarbon gives CO₂, water, heat and light.

Question 4. A mixture of oxygen and ethyne is burnt for welding. Can you tell why a mixture of ethyne and air is not used?
Answer:
Step 1: Ethyne burnt in air gives a sooty flame because of incomplete combustion, and the flame is not hot enough for welding.
Step 2: When ethyne is burnt in pure oxygen, combustion is complete and a clean flame at about 3000 °C is obtained.
2C₂H₂ + 5O₂ → 4CO₂ + 2H₂O + heat
Step 3: Air contains nearly 80% nitrogen, which absorbs heat and does not help burning, so such a high temperature cannot be reached with air.
In simple words: Ethyne in air burns sootily and not hot enough; in pure oxygen it burns cleanly and very hot.

Teacher's Note:
1. The oxy-acetylene flame is used to weld and cut metals.
2. Complete combustion gives a hotter flame than incomplete combustion.

Question 5. Give reasons, why (a) The element carbon forms a very large number of compounds. (b) Air holes of a gas burner have to be adjusted when the heated vessels get blackened by the flame.
Answer:
(a) Carbon has two special properties. Catenation lets carbon atoms join to each other in long straight chains, branched chains and rings. Tetravalency lets each carbon atom form four bonds with other atoms (H, O, N, S, halogens). Together they give millions of compounds.
(b) A blackened vessel means that the fuel is burning incompletely because the air supply is insufficient, and unburnt carbon (soot) is deposited. Adjusting the air holes lets more air in, so the fuel burns completely with a blue flame, saving fuel and avoiding smoke and carbon monoxide.
In simple words: Catenation and tetravalency give many compounds; more air gives complete combustion and no soot.

Teacher's Note:
1. A yellow flame and black soot indicate too little air.
2. The blue flame is the sign of complete combustion.

Question 6. Give any three uses of ethanoic acid.
Answer:
1. A 5–8% solution of ethanoic acid in water is vinegar, which is widely used as a preservative in pickles and as a flavouring in food.
2. It is used to make esters, which are used in perfumes and as flavouring agents.
3. It is used as a solvent and as a reagent in the chemical industry, for example in making dyes, plastics and artificial fibres.
In simple words: Ethanoic acid is used as vinegar, for esters and as an industrial chemical.

Teacher's Note:
1. Vinegar keeps bacteria away because of its acidity.
2. Pure ethanoic acid is called glacial acetic acid and is corrosive.

Question 7. What are the harmful effects of drinking alcohol?
Answer:
Step 1: Alcohol (ethanol) slows down the metabolic processes of the body and depresses the central nervous system.
Step 2: This causes lack of coordination, mental confusion, drowsiness, lowering of inhibitions and finally stupor. The person's sense of judgement, timing and muscular coordination is impaired, which leads to accidents.
Step 3: Long-term consumption damages the liver and other organs, causes addiction (alcoholism) and many health problems, and harms the family and society.
Value: Avoiding alcohol protects health, safety and relationships.
In simple words: Alcohol harms the nervous system, judgement and liver, and can lead to addiction.

Teacher's Note:
1. Drinking and driving is a major cause of road accidents.
2. Say no to peer pressure and seek help early for addiction.

Question 8. Intake of small quantity of methanol can be lethal. Comment.
Answer:
Step 1: In the liver, methanol is oxidised to methanal (formaldehyde).
Step 2: Methanal reacts rapidly with the components of cells and causes the protoplasm to get coagulated, in the same way as an egg is coagulated by cooking. Methanol also affects the optic nerve, causing blindness.
Conclusion: Even a very small amount of methanol can cause blindness or death, so it must never be consumed.
Value: Never drink spurious or illicit liquor, because it may contain methanol.
In simple words: Methanol turns into poisonous methanal in the body, which damages cells and eyes.

Teacher's Note:
1. Ethanol is also harmful in large amounts, but methanol is lethal even in small doses.
2. Industrial alcohol is made unfit for drinking by adding methanol.

Question 9. What is meant by denatured alcohol? How is it prepared?
Answer:
Step 1: Ethanol meant for industrial use is made unfit for drinking by adding poisonous substances such as methanol, and a dye (usually blue) so that it is easily identified. This is called denatured alcohol (methylated spirit).
Step 2: It is made by mixing pure ethanol with a small amount of methanol, pyridine and a blue dye.
Value: Denaturing prevents the misuse of industrial alcohol as a drink, because it is not taxed as liquor.
In simple words: Industrial alcohol is spoiled on purpose with poison and a dye so that nobody drinks it.

Teacher's Note:
1. The added dye helps to identify denatured alcohol.
2. Denatured alcohol is used as a solvent and a fuel.

Short and Long Answer Type Value Based Questions

Question 10. Ethanol is a renewable biofuel because it is made from biomass. Ethanol is a clear, colourless alcohol made from a variety of biomass materials. Ethanol producers mostly use food grains and crops with high starch and sugar content such as corn, sorghum, barley, sugar cane and sugar beets. The most common ethanol production processes today use yeast to ferment the starch and sugars in corn, sugar cane, and sugar beets. (A) Ethanol is a good solvent. Where can you use this property of ethanol? (B) Draw the structure of any carboxylic acid and the alcohol containing same number of C-atoms in this carboxylic acid. (C) What would be the products formed when ethanol: (i) is heated with excess concentrated sulphuric acid? (ii) reacts with sodium? Support your answer with balanced chemical equations.
Answer:
(A) As a good solvent, ethanol is used in medicines such as tincture of iodine, cough syrups and tonics, and in paints, perfumes and varnishes.
(B) Carboxylic acid: ethanoic acid, CH₃–C(=O)–OH. Alcohol with the same number of carbon atoms (two): ethanol, CH₃–CH₂–OH.
(C)(i) Heating with excess conc. H₂SO₄ at 443 K removes water (dehydration) and gives ethene: CH₃CH₂OH → CH₂=CH₂ + H₂O
(ii) With sodium it gives sodium ethoxide and hydrogen gas: 2CH₃CH₂OH + 2Na → 2CH₃CH₂ONa + H₂
Value: Ethanol made from crops by fermentation is a renewable fuel that reduces our dependence on petroleum.
In simple words: Ethanol is a solvent and a renewable fuel; with conc. H₂SO₄ it gives ethene, with sodium it gives hydrogen.

Teacher's Note:
1. Concentrated H₂SO₄ acts as a dehydrating agent.
2. Ethanol blended with petrol reduces pollution.

Question 11. White vinegar, sometimes called distilled or spirit vinegar, has been a mainstay in households worldwide for thousands of years. It's easy to understand why. This versatile liquid presents a treasure trove of uses for cleaning, gardening and cooking. It even has medicinal applications too. (A) A child wants to know whether vinegar used at home is organic or not. What would be the chemical name and formula of vinegar? (B) Why is the pure form of ethanoic acid known as glacial acetic acid? (C) A teacher demonstrated the process of ester formation and then an ester was treated with alkali NaOH. What is the name of this process and why has this name been given to it?
Answer:
(A) Vinegar is a 5–8% solution of ethanoic acid (acetic acid), CH₃COOH, in water. Ethanoic acid is a carbon compound, so the vinegar is organic in nature.
(B) The melting point of pure ethanoic acid is 290 K (about 17 °C), so in cold weather it freezes into ice-like flakes that look like a glacier. Because of this the pure acid is called glacial acetic acid.
(C) The process is saponification: CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH. It is called saponification because the same reaction of an ester (fat or oil) with alkali is used to make soap.
Value: Vinegar is a safe, natural preservative for pickles and a household cleaner.
In simple words: Vinegar is dilute ethanoic acid; pure ethanoic acid freezes like ice in winter; alkali breaks an ester back into alcohol and a salt, which is how soap is made.

Teacher's Note:
1. Do not confuse vinegar (5–8% in water) with glacial acetic acid (pure).
2. Alkaline hydrolysis of an ester is called saponification.

Question 12. Detergents have many advantages over soaps but are still quite harmful. Explain.
Answer:
Advantages: Detergents work even in hard water because they do not form insoluble scum with Ca²⁺ and Mg²⁺ ions, and they have a strong cleansing action.
Harmful effect: Many synthetic detergents have branched hydrocarbon chains which micro-organisms cannot break down. They are non-biodegradable, accumulate in rivers and lakes, form foam, cut off oxygen supply to aquatic life and cause water pollution. Detergents containing phosphates also cause excess growth of algae.
Value: Use detergents in limited amounts and prefer biodegradable ones.
In simple words: Detergents clean well in hard water but pollute water because they do not decompose.

Teacher's Note:
1. Biodegradable means it can be broken down by micro-organisms.
2. Straight-chain detergents are biodegradable.

Question 13. Answer the following questions: (i) Explain the formation of scum when hard water is treated with soap. (ii) People use a variety of methods to wash clothes. Usually after adding the soap, they 'beat' the clothes on a stone, or beat it with a paddle, scrub with a brush or the mixture is agitated in a washing machine. Why is agitation necessary to get clean clothes?
Answer:
(i) Hard water contains calcium and magnesium salts. The soap (sodium salt of a fatty acid) reacts with the Ca²⁺ and Mg²⁺ ions to form insoluble calcium and magnesium salts of the fatty acid, a curdy white precipitate called scum. A lot of soap is wasted and no lather is produced.
2C₁₇H₃₅COONa + CaCl₂ → (C₁₇H₃₅COO)₂Ca↓ + 2NaCl
(ii) Soap molecules form micelles that trap the oily dirt in their centre. Agitation (beating, scrubbing or the washing machine) dislodges the dirt from the fibres of the cloth and spreads the micelles in the water, so that the dirt is washed away with the rinse water.
In simple words: Hard water spoils soap into scum; agitation lifts dirt trapped in micelles off the cloth.

Teacher's Note:
1. Scum is the insoluble Ca and Mg salt of the soap.
2. Rinsing is needed to wash the micelles away.

Question 14. Answer the following questions: (i) Explain the process of preparation of soap in laboratory. (ii) Why is common salt (sodium chloride) added during the preparation of soap? (iii) Why is soap not suitable for washing clothes when the water is hard?
Answer:
(i) Take about 20 mL of castor oil (or cottonseed, linseed or soyabean oil) in a beaker and add 30 mL of 20% sodium hydroxide solution. Heat the mixture with constant stirring till a paste of soap is formed (saponification). Add 5–10 g of common salt, stir well and allow to cool. Solid soap separates out and can be cut into bars.
(ii) Soap is somewhat soluble in water. Common salt is added to precipitate (salt out) all the soap from the solution, so that it floats on top and can be collected.
(iii) In hard water, soap reacts with Ca²⁺ and Mg²⁺ ions to form insoluble scum, so a large amount of soap is wasted before it can clean.
In simple words: Oil plus sodium hydroxide gives soap; salt separates it; hard water turns it into scum.

Teacher's Note:
1. Soap making is the saponification of oils and fats with alkali.
2. Glycerol is formed as a by-product.

Question 15. Answer the following questions: (a) Describe a chemical test to distinguish between ethanol and ethanoic acid. (b) Give reason for the following: (i) Ethanol is used in the preparation of tincture iodine. (ii) Ethanoic acid is used in the preservation of pickles.
Answer:
(a) Add a pinch of sodium hydrogencarbonate (or sodium carbonate) to each liquid. Ethanoic acid gives a brisk effervescence of CO₂ gas, which turns lime water milky. Ethanol gives no effervescence.
CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂
(b)(i) Ethanol is a good solvent, so it dissolves iodine to make the antiseptic tincture of iodine.
(ii) Ethanoic acid (as vinegar) creates an acidic condition in which bacteria and fungi cannot grow, so it preserves pickles for a long time.
In simple words: Only the acid fizzes with baking soda; ethanol dissolves iodine and vinegar keeps pickles from spoiling.

Teacher's Note:
1. Litmus is another test: ethanoic acid turns blue litmus red.
2. Tincture of iodine is a solution of iodine in alcohol.

Question 16. The dirty clothes has grease or oil particles which hold dirt on clothes. For washing these clothes soap is required because grease particles do not dissolve in water. Only water is not sufficient to remove dirt even at hot condition. Why?
Answer:
Step 1: Grease and oil are non-polar substances, so they do not dissolve in water, which is polar. Hot water only makes the grease more fluid; it does not dissolve it.
Step 2: A soap molecule has an ionic (hydrophilic) head and a long hydrocarbon (hydrophobic) tail. In water the soap molecules form micelles with the tails pointing inwards.
Step 3: The tails trap the oily particles in the centre while the ionic heads stay in water. The micelles get dispersed in water, so the grease and the dirt with it are washed away.
In simple words: Soap micelles trap grease inside them and carry it into water; plain water cannot do this.

Teacher's Note:
1. Like dissolves like: oil dissolves in the hydrocarbon tails.
2. Micelles are large enough to scatter light, so soap solution looks cloudy.

Question 17. The cleaning action of soap is very effective in soft water but it is not effective in hard water. If detergents are used for cleaning purpose, it will be effective in both, soft water as well as hard water. why?
Answer:
Step 1: Hard water contains calcium and magnesium salts. Soap, the sodium or potassium salt of a long chain fatty acid, reacts with the Ca²⁺ and Mg²⁺ ions and forms insoluble calcium and magnesium salts called scum. The scum sticks to clothes and soap loses its cleansing power.
Step 2: A detergent is the sodium salt of a long chain sulphonic acid (for example, benzene sulphonic acid). Its calcium and magnesium salts are soluble in water, so no scum is formed.
Step 3: Therefore a detergent lathers easily and cleans in hard water as well as in soft water.
In simple words: Soap makes insoluble scum in hard water, but a detergent's Ca and Mg salts dissolve, so it keeps working.

Teacher's Note:
1. Soft water has hardly any Ca²⁺ or Mg²⁺ ions.
2. Compare the two by the formation of scum.

Question 18. Detergents have number of advantages over soaps but detergents are the main pollutants to cause water pollution. Why?
Answer:
Step 1: Some detergents have branched hydrocarbon chains and are non-biodegradable, that is, they cannot be decomposed by micro-organisms.
Step 2: They therefore accumulate in rivers, lakes and ponds, produce foam, and reduce the oxygen available to aquatic life.
Step 3: Phosphates present in many detergents also cause excessive growth of algae (eutrophication), which harms water bodies.
Value: We should use detergents sparingly and choose biodegradable ones to protect our water sources.
In simple words: Detergents that do not decompose pile up in water and harm aquatic life.

Teacher's Note:
1. Soaps are biodegradable, which makes them eco-friendly.
2. Detergent molecules can be changed to straight chains to make them biodegradable.

Question 19. Give the chemical equation involved in the formation of scum.
Answer:
Scum is the curdy white precipitate formed when soap reacts with the calcium or magnesium salts present in hard water.
2C₁₇H₃₅COONa + CaCl₂ → (C₁₇H₃₅COO)₂Ca↓ + 2NaCl
(sodium stearate, soap + calcium chloride → calcium stearate, scum + sodium chloride)
Similarly, with magnesium sulphate: 2C₁₇H₃₅COONa + MgSO₄ → (C₁₇H₃₅COO)₂Mg↓ + Na₂SO₄
In simple words: Soap plus the calcium or magnesium salt in hard water gives an insoluble calcium or magnesium soap.

Teacher's Note:
1. Sodium stearate stands for a typical soap.
2. Chlorides and sulphates of Ca and Mg cause this hardness.

Question 20. Answer the following questions: (i) Give a test that can be used to differentiate chemically between butter and cooking oil. (ii) How does graphite acts as a lubricant?
Answer:
(i) Cooking oil contains unsaturated fatty acids, whereas butter contains saturated fats. Add a few drops of bromine water (or alkaline potassium permanganate) to each sample and shake. The cooking oil decolourises it, but butter does not.
(ii) In graphite the carbon atoms are arranged in flat hexagonal layers. The layers are held together by weak forces, so they slide easily over each other. Hence graphite is used as a dry lubricant in machines, often mixed with oil or petroleum jelly.
In simple words: Oil (unsaturated) decolourises bromine water, butter does not; graphite is slippery because its layers slide.

Teacher's Note:
1. Unsaturated fats can be hydrogenated into solid fats.
2. Diamond cannot be used as a lubricant because it is a rigid network.

Question 21. Explain the following: (i) CH₃COOH is a weak acid. (ii) Propene undergoes addition reaction. (iii) The gas stoves have inlets for air.
Answer:
(i) Ethanoic acid ionises only partially in water, giving few H⁺ ions, so it is a weak acid, unlike HCl, which ionises completely.
(ii) Propene, CH₃–CH=CH₂, is an unsaturated hydrocarbon with a carbon–carbon double bond. The double bond opens up and adds atoms such as hydrogen or bromine.
(iii) The inlets supply enough air (oxygen) so that the gas burns completely with a clean blue flame, giving more heat and no soot or poisonous carbon monoxide.
Value: Proper air supply saves fuel and protects our health.
In simple words: Partial ionisation makes a weak acid; double bonds add atoms; air holes give complete burning.

Teacher's Note:
1. Weak acid means partly ionised, not necessarily dilute.
2. Smoky flames waste fuel.

Question 22. Give four uses of ethyl alcohol.
Answer:
1. It is used as a solvent for paints, varnishes, perfumes, and in medicines such as tincture of iodine, cough syrups and tonics.
2. It is used as an antiseptic to sterilise the skin and surgical instruments.
3. It is mixed with petrol as a fuel (power alcohol), which is renewable and reduces pollution.
4. It is used as a raw material to prepare other compounds such as ethanoic acid, ethene and esters, and as an antifreeze.
In simple words: Ethanol is a solvent, antiseptic, fuel and raw material.

Teacher's Note:
1. Ethanol used in industry is denatured so that it is not drunk.
2. Power alcohol is petrol blended with ethanol.