CBSE Class 10 Science Advanced Chemistry Chemical Reactions and Equations Very Short Answer Questions Set 01

Very Short Answer Type

Question 1. A shiny brown-coloured element 'X' on heating in air becomes black in colour. Name the element 'X' and the black coloured compound formed.
Answer:
'X' is copper (Cu) and the black compound is copper(II) oxide (CuO).
\( \ce{2Cu + O2 ->[\Delta] 2CuO} \)
In simple words: Shiny brown copper combines with oxygen of air on heating and turns black.

Teacher's Note:
1. Copper is the only common brown shiny metal, which gives the clue.
2. Passing hydrogen over hot CuO brings the brown copper back.

Question 2. In the reaction \( \ce{2KClO3 ->[\Delta, MnO2] 2KCl + 3O2} \), what do the symbol Δ and \( \ce{MnO2} \) written on the arrow signify ?
Answer:
The symbol Δ shows that the reaction needs heat.
\( \ce{MnO2} \) is a catalyst; it speeds up the reaction and is not used up.
In simple words: Delta means the mixture is heated and MnO2 is the helper that makes the reaction faster.

Teacher's Note:
1. Conditions of a reaction are always written above or below the arrow.
2. The catalyst is not part of the balanced equation, as it appears unchanged at the end.

Question 3. Which among the following changes are exothermic or endothermic in nature ? (a) Decomposition of ferrous sulphate (b) Dilution of sulphuric acid (c) Dissolution of sodium hydroxide in water (d) Dissolution of ammonium chloride in water
Answer:
(a) Decomposition of ferrous sulphate: endothermic (heat is absorbed).
(b) Dilution of sulphuric acid: exothermic (the solution becomes hot).
(c) Dissolution of sodium hydroxide in water: exothermic.
(d) Dissolution of ammonium chloride in water: endothermic (the solution becomes cold).
In simple words: If the vessel turns hot, heat was released; if it turns cold, heat was absorbed.

Teacher's Note:
1. Decomposition reactions that need heating are endothermic.
2. Always add acid to water slowly, never water to acid, as dilution releases much heat.

Question 4. Complete the missing components/variables given as x and y in the following reactions. (a) \( \ce{Pb(NO3)2(aq) + 2KI(aq) -> PbI2(x) + 2KNO3(y)} \) (b) \( \ce{Cu(s) + 2AgNO3(aq) -> Cu(NO3)2(aq) + x(s)} \) (c) \( \ce{Zn(s) + H2SO4(aq) -> ZnSO4(x) + H2(y)} \) (d) \( \ce{CaCO3(s) -> CaO(x) + CO2(y)} \)
Answer:
(a) x = s (lead iodide is a yellow precipitate), y = aq.
(b) x = Ag (copper displaces silver).
(c) x = aq, y = g.
(d) x = s, y = g.
In simple words: Use the state symbols (s), (l), (g), (aq) according to whether the substance is a solid, a liquid, a gas or dissolved in water.

Teacher's Note:
1. A precipitate is (s), while a salt that stays dissolved is (aq).
2. Gases like H2 and CO2 are written (g).

Question 5. Define the term rancidity. [CBSE 10th Board 2014, 1 Mark]
Answer:
Rancidity is the unpleasant smell and taste that develops in food containing fats and oils when it is oxidised by the oxygen of air on long storage.
In simple words: Oily food that turns stale and smelly because of oxidation is called rancid.

Teacher's Note:
1. Rancidity is prevented by antioxidants, nitrogen packing, cold storage and airtight containers.
2. It is an oxidation (redox) change that is harmful in daily life.

Question 6. Why are decomposition reactions called the opposite of combination reactions ? [CBSE 10th Board 2015, 1 Mark]
Answer:
In a combination reaction two or more substances join to form one product, whereas in a decomposition reaction a single compound breaks down into two or more simpler substances.
For example, \( \ce{CaO + CO2 -> CaCO3} \) is combination and \( \ce{CaCO3 -> CaO + CO2} \) is its decomposition.
In simple words: One reaction joins substances and the other splits them.

Teacher's Note:
1. Writing a pair of reverse equations is a neat way to show this.
2. Combination is usually exothermic, and decomposition usually needs energy.

Question 7. What happens when silver nitrate is mixed with a solution of sodium chloride ?
Answer:
A white precipitate of silver chloride is formed, and colourless sodium nitrate stays in solution.
\( \ce{AgNO3(aq) + NaCl(aq) -> AgCl(s) + NaNO3(aq)} \)
In simple words: The two clear solutions give a white solid, silver chloride.

Teacher's Note:
1. This is a precipitation reaction and a double displacement reaction as well.
2. Silver chloride turns grey in sunlight.

Question 8. What do you mean by exothermic reactions ?
Answer:
A chemical reaction in which heat (energy) is released is called an exothermic reaction.
Example: \( \ce{CaO + H2O -> Ca(OH)2 + "heat"} \)
In simple words: An exothermic reaction gives out heat to the surroundings.

Teacher's Note:
1. Burning, respiration and neutralisation are all exothermic.
2. In an equation, heat released is written on the product side.

Question 9. What do you mean by endothermic reactions ?
Answer:
A chemical reaction in which heat (energy) is absorbed is called an endothermic reaction.
Example: \( \ce{C(s) + 2S(s) ->[heat] CS2(l)} \); photosynthesis also absorbs light energy.
In simple words: An endothermic reaction takes in energy from the surroundings.

Teacher's Note:
1. Thermal decomposition and photosynthesis are endothermic.
2. In an equation, heat absorbed is written with a minus sign on the product side.

Question 10. What do you mean by corrosion ?
Answer:
Corrosion is the slow eating up of a metal by the action of air, moisture and gases such as oxygen, carbon dioxide and hydrogen sulphide, which changes the metal into its oxide, carbonate or sulphide.
Examples: rusting of iron, green coating on copper and blackening of silver.
In simple words: Corrosion is the gradual damage to metals when they react with substances in the air.

Teacher's Note:
1. Rusting is the corrosion of iron only.
2. Painting, oiling, galvanisation and alloying prevent corrosion.

Question 11. What reaction takes place during the digestion of food in our body ?
Answer:
During digestion, large food molecules are broken into smaller ones, such as starch into glucose, which is a decomposition reaction (assisted by enzymes).
The glucose is later oxidised in the cells, which is an exothermic oxidation reaction (respiration): \( \ce{C6H12O6 + 6O2 -> 6CO2 + 6H2O + "energy"} \)
In simple words: Food is first broken into small molecules and then burnt slowly in the cells to give energy.

Teacher's Note:
1. Digestion is decomposition, respiration is oxidation.
2. Both together provide the energy for the body.

Question 12. Give an example of a double displacement reaction accompanied by evolution of a gas.
Answer:
\( \ce{Na2CO3 + 2HCl -> 2NaCl + H2O + CO2 ^} \)
The two compounds exchange ions, and the carbonic acid formed breaks down to water and carbon dioxide gas, which escapes as bubbles.
In simple words: Sodium carbonate and hydrochloric acid exchange ions and bubble out carbon dioxide.

Teacher's Note:
1. Carbonates and hydrogencarbonates with acids always give CO2.
2. Another example is \( \ce{FeS + H2SO4 -> FeSO4 + H2S ^} \).

Question 13. Why does iron corrode but aluminium does not ?
Answer:
The rust formed on iron is flaky and falls off, exposing fresh iron to air and moisture, so the corrosion continues.
Aluminium forms a thin, tough layer of aluminium oxide (\( \ce{Al2O3} \)) that sticks firmly to the surface and protects the metal underneath, so further corrosion stops.
In simple words: Aluminium makes its own protective coating, but rust on iron peels off.

Teacher's Note:
1. This oxide layer is why aluminium utensils stay bright.
2. Anodising makes this layer even thicker.

Question 14. What will happen if an iron rod is dipped in \( \ce{CuSO4} \) solution ?
Answer:
Iron is more reactive than copper, so it displaces copper from the solution.
The blue colour of the solution fades to pale green and a reddish brown layer of copper deposits on the iron rod.
\( \ce{Fe(s) + CuSO4(aq) -> FeSO4(aq) + Cu(s)} \)
In simple words: Iron pushes copper out of copper sulphate, so the rod gets a brown coating.

Teacher's Note:
1. It is a single displacement reaction.
2. The pale green colour is that of iron(II) sulphate.

Question 15. Give the characteristic tests for the following gases : (a) \( \ce{CO2} \) (b) \( \ce{SO2} \) (c) \( \ce{O2} \) (d) \( \ce{H2} \)
Answer:
(a) CO₂ turns lime water milky: \( \ce{Ca(OH)2 + CO2 -> CaCO3 v + H2O} \).
(b) SO₂ has a pungent smell of burning sulphur and turns acidified potassium dichromate paper from orange to green.
(c) O₂ makes a glowing splinter burst into flame.
(d) H₂ burns with a pop sound when a burning splinter is brought near.
In simple words: Each gas has its own simple test: milky lime water, burning-sulphur smell, relighting a splinter and a pop sound.

Teacher's Note:
1. Learn these four tests as a set, since they are often asked in practical questions.
2. CO2 in excess turns the milkiness clear again.

Question 16. Why do we apply paint on iron articles ?
Answer:
Paint cuts off the contact of iron with moisture and air, both of which are needed for rusting.
So the paint prevents rusting.
In simple words: Paint acts as a shield between iron and air and water.

Teacher's Note:
1. Greasing, oiling and galvanisation work on the same idea.
2. Paint must be renewed when it peels off.

Question 17. Oil and fat containing food items are flushed with nitrogen. Why ?
Answer:
Nitrogen is an inert gas and does not react with the food.
Flushing the pack with nitrogen removes oxygen, so the fats and oils cannot be oxidised, and rancidity is prevented.
In simple words: No oxygen in the packet means the food cannot turn rancid.

Teacher's Note:
1. Potato chips packets are filled with nitrogen for this reason.
2. The puffy pack also protects the chips from being crushed.