CBSE Class 10 Science Chemical reactions and equations Sure Shot Questions Set 05

This Set 05 of sure shot questions for CBSE Class 10 Science Chapter 1, Chemical Reactions and Equations, gathers 17 board-style questions with stepwise answers. They test the law of conservation of mass, balancing, exothermic and endothermic changes, displacement and redox examples, and the activities on lead nitrate and iron nails.

Very Short Answer Questions

Question 1. Name and state the law which is kept in mind when we balance chemical equations.
Answer:
The law of conservation of mass: mass can neither be created nor destroyed in a chemical reaction, so atoms of each element must be equal on both sides.
In simple words: Total mass of reactants equals total mass of products.

Teacher's Note:
1. State the law in words.
2. This is why equations are balanced.

Question 2. Define redox reaction.
Answer:
A reaction in which oxidation and reduction occur simultaneously is called a redox reaction. Example: CuO + H₂ → Cu + H₂O
In simple words: One substance is oxidised while another is reduced.

Teacher's Note:
1. Give an example if possible.
2. Oxidation and reduction never occur alone.

Question 3. (a) Define corrosion. (b) What is the name given to the corrosion of iron? (c) What is the colour of coating formed on silver and copper? (d) What damage is caused by corrosion? (e) How can we prevent corrosion?
Answer:
(a) Corrosion is the slow eating away of the surface of a metal by air, moisture or chemicals.
(b) Corrosion of iron is called rusting.
(c) Silver forms a black coating and copper forms a green coating.
(d) Corrosion weakens metals; car bodies, bridges, railings and ships are damaged.
(e) By painting, oiling, greasing or galvanising the surface so that air and moisture cannot reach the metal.
In simple words: Air and moisture slowly eat metals; coatings protect them.

Teacher's Note:
1. Name the common example: rusting.
2. Air and moisture are both needed for iron.

Question 4. (a) Why is combustion reaction an oxidation reaction? (b) How will you test whether the gas evolved in a reaction is hydrogen?
Answer:
(a) Combustion is the burning of a substance in oxygen (air), in which the substance gains oxygen, so it is an oxidation reaction. Example: CH₄ + 2O₂ → CO₂ + 2H₂O + heat
(b) Bring a burning matchstick near the mouth of the tube. Hydrogen burns with a pop sound.
In simple words: Burning means combining with oxygen; hydrogen pops.

Teacher's Note:
1. The pop is the burning of hydrogen.
2. Do not inhale the gas.

Short Answer Questions

Question 5. Why should a magnesium ribbon be cleaned before burning in air?
Answer:
Magnesium kept in air gets covered with a layer of magnesium oxide. This layer prevents burning, so the ribbon is cleaned with sandpaper first.
In simple words: The oxide layer would stop the metal from burning.

Teacher's Note:
1. Name the layer: MgO.
2. Cleaning exposes fresh metal.

Question 6. (A) Why cannot a chemical change be normally reversed? (B) Why is it always essential to balance a chemical equation? (C) What happens when CO₂ gas is passed through lime water and why does it disappear on passing excess CO₂? (D) Can rusting of iron take place in distilled water?
Answer:
(A) A chemical change forms new substances with new bonds and properties, so it cannot easily be reversed to get the original ones back.
(B) The equation must obey the law of conservation of mass: atoms of each element must be equal on both sides.
(C) Lime water turns milky due to CaCO₃: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. On passing excess CO₂ the milkiness disappears because soluble calcium hydrogencarbonate forms: CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂
(D) Rusting needs both oxygen (air) and water. Iron kept in air-free (boiled) distilled water does not rust, but distilled water exposed to air contains dissolved oxygen, so rusting is very slow there and is faster in ordinary moist air.
In simple words: Reversing, balancing, milky lime water and rusting all depend on what substances are formed or present.

Teacher's Note:
1. Answer each part in one or two lines.
2. Rusting needs both air and water.

Question 7. Write the balanced equation for the following chemical reactions. (i) Hydrogen + Chlorine → Hydrogen chloride (ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride (iii) Sodium + Water → Sodium hydroxide + Hydrogen
Answer:
(i) H₂ + Cl₂ → 2HCl
(ii) 3BaCl₂ + Al₂(SO₄)₃ → 3BaSO₄ + 2AlCl₃
(iii) 2Na + 2H₂O → 2NaOH + H₂
In simple words: Write formulae, then balance.

Teacher's Note:
1. Check atoms on both sides.
2. Sodium reacts vigorously with water.

Question 8. Classify the following as exothermic and endothermic reactions: (a) Photosynthesis (b) Respiration (c) Burning of natural gas (d) Electrolysis of water
Answer:
(a) Photosynthesis: endothermic (absorbs light energy)
(b) Respiration: exothermic
(c) Burning of natural gas: exothermic
(d) Electrolysis of water: endothermic (needs electricity)
In simple words: Burning and respiration give out heat; photosynthesis and electrolysis take in energy.

Teacher's Note:
1. Heat released means exothermic.
2. Energy absorbed means endothermic.

Question 9. A white solid when dropped in water produces a hissing sound. What the solid may be? Give the chemical reaction for above. Name the product formed.
Answer:
The solid is quicklime (CaO).
CaO(s) + H₂O(l) → Ca(OH)₂(aq) + heat
The product is calcium hydroxide (slaked lime).
In simple words: Quicklime reacts with water, releasing heat.

Teacher's Note:
1. The reaction is exothermic.
2. It is also a combination reaction.

Question 10. (A) What are the limitations seen in the given chemical reaction? CO + 2H₂ → CH₃OH (B) What type of chemical reaction is it?
Answer:
The equation does not tell the conditions needed (high pressure, catalyst), the physical states, the rate of the reaction or the heat change.
It is a combination reaction.
In simple words: It tells which substances react but not the conditions.

Teacher's Note:
1. List what the equation does not show.
2. Say the type of reaction.

Short Answer Questions (3 Marks)

Question 11. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
Answer:
Iron is more reactive than copper, so it displaces copper from the solution.
Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
Blue CuSO₄ changes to light green FeSO₄ and a brownish layer of copper forms on the nail.
In simple words: Iron pushes copper out, so blue turns to green.

Teacher's Note:
1. It is a displacement reaction.
2. Mention both colour changes.

Question 12. List four observations that help us to determine whether a chemical reaction has taken place.
Answer:
(i) Evolution of a gas
(ii) Change in temperature
(iii) Change in state
(iv) Change in colour
(Formation of a precipitate is also an observation.)
In simple words: Gas, heat, state, colour.

Teacher's Note:
1. Give an example for each.
2. Precipitate formation can also be added.

Question 13. “Oxidation and reduction processes occur simultaneously.” Justify this statement with the help of an example.
Answer:
CuO + H₂ → Cu + H₂O
CuO loses oxygen: it is reduced. H₂ gains oxygen: it is oxidised.
Both changes happen in the same reaction at the same time.
In simple words: One substance is oxidised while the other is reduced.

Teacher's Note:
1. Name the substance oxidised and reduced.
2. Use the oxygen gain or loss idea.

Question 14. A child adds a few zinc granules and dilute hydrochloric acid in the conical flask. List the observations that the child makes to illustrate that a chemical reaction has taken place. Write two more observations along with the examples which help us determine that a chemical reaction has taken place.
Answer:
Observations: bubbles of a colourless gas (hydrogen) are evolved and the flask becomes warm. Zn + 2HCl → ZnCl₂ + H₂
Two more observations: change in colour, for example Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃ (yellow precipitate forms); change in state or temperature, for example CaO + H₂O → Ca(OH)₂ + heat (the mixture becomes hot).
In simple words: Bubbles and warmth prove a chemical change; colour or state changes also do.

Teacher's Note:
1. Write the equation for the zinc reaction.
2. Give one example for each extra observation.

Long Answer Questions

Question 15. Identify the type of chemical reaction and write balanced chemical equation for each of the following: (a) Barium chloride solution is mixed with copper sulphate solution and a white precipitate is observed. (b) On heating copper powder in a china dish, the surface of copper powder becomes black. (c) On heating green ferrous sulphate crystals, a reddish brown solid is left and gases having smell of burning sulphur are noticed. (d) Iron nails when left dipped in blue copper sulphate solution become brownish in colour and blue colour of copper sulphate solution turns to light green. (e) Quicklime reacts vigorously with water releasing a large amount of heat.
Answer:
(a) BaCl₂(aq) + CuSO₄(aq) → BaSO₄(s) + CuCl₂(aq): double displacement (precipitation) reaction
(b) 2Cu(s) + O₂(g) →^heat 2CuO(s): combination (oxidation) reaction
(c) 2FeSO₄(s) →^heat Fe₂O₃(s) + SO₂(g) + SO₃(g): decomposition reaction
(d) Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s): displacement reaction
(e) CaO(s) + H₂O(l) → Ca(OH)₂(aq) + heat: combination reaction
In simple words: Name the type by looking at what happens to the reactants.

Teacher's Note:
1. Balance before naming.
2. Give state symbols where possible.

Question 16. (a) Design an activity to demonstrate the decomposition reaction of lead nitrate. (b) List two main observations. (c) Write balanced chemical equation for the reaction stating the physical state of the reactant and the products.
Answer:
(a) Take a small amount of lead nitrate powder in a boiling tube. Hold it with a pair of tongs and heat it first gently and then strongly over a flame.
(b) Brown fumes of nitrogen dioxide are given off, and the white solid changes to a yellow residue of lead oxide.
(c) 2Pb(NO₃)₂(s) →^heat 2PbO(s) + 4NO₂(g) + O₂(g)
In simple words: Heating breaks lead nitrate into PbO, NO₂ and O₂.

Teacher's Note:
1. Hold the tube at an angle.
2. Do the activity in a fume cupboard.

Question 17. You are provided with three nails and copper sulphate solution in a test tube. (A) Design an activity to demonstrate displacement reaction between iron nails and copper sulphate solution. (C) Write your observations. (D) Write balanced chemical equation for the reaction stating the physical state of the reactants and products. (E) What inference do you draw from this activity?
Answer:
(A) Clean the nails with sandpaper. Place two nails in the test tube containing copper sulphate solution and keep the third nail outside for comparison. Leave for about 20 minutes.
(C) The nails in the solution become brownish and the blue colour of the solution changes to light green.
(D) Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
(E) Iron is more reactive than copper and displaces it from its solution.
In simple words: Iron pushes copper out of its salt solution.

Teacher's Note:
1. Use the third nail for comparison.
2. Always mention both observations.