Class 10th science last 10 year papers chapter wise chapter 1 chemical reaction and equation part 4

 45.(a) Classify the following reactions into different types.

(b) Which of the above reaction(s) is/are precipitation reaction(s)? Why is a reaction called precipitation reaction?
Answer.
(a) (i) Precipitation reaction (Double displacement reaction)
(ii) Combination reaction (in) Decomposition reaction
(b) Reaction (i) is a precipitation reaction because one of the product formed is insoluble in water.

46. Write balanced equations for the following mentioning the type of reaction involved.
(i) Aluminium + Bromine —–> Aluminium bromide
(ii) Calcium carbonate—–> Calcium oxide + Carbon dioxide
(iii) Silver chloride—–>Silver + Chlorine
Answer.

47.(a) Why is respiration considered as an exothermic reaction?
(b) Define the terms oxidation and reduction.
(c) Identify the substance that is oxidised and reduced in the following reaction.

Answer. (a) It is because heat is evolved during respiration.
(b) Oxidation is a process in which O2 is added or H2 is removed or loss of electrons take place. Reduction is a process in which H2 is added or O2. is removed or gain of electrons take place.
(c) Zn is getting oxidised, CuO is getting reduced.

48. What is meant by
(i) precipitation reaction,
(ii) exothermic reaction,
(iii) oxidation reaction?
Write balanced chemical equations for an example of each.
Answer.(i) Precipitation reaction: The reaction in which two compounds exchange their
ions and the product formed is insoluble in water is called precipitation reaction.

(ii) Exothermic reaction: The reaction in Which heat is evolved is known as exothermic reaction.

(iii) Oxidation reaction: The reaction in which Og is added or H2 is removed or loss of electrons takes place is called oxidation reaction.

49.You might have noted that when copper powder is heated in a china dish, the surface of copper powder becomes coated with a black colour substance.
(i) How has this black coloured substance formed?
(ii) What is that black substance?
(iii) Write the chemical equation of the reaction that takes place.
Answer.
(i) Copper reacts with oxygen to form copper oxide which is black, i.e. oxidation of copper takes place.
(ii)Copper oxide

Very Short Answer Type Questions [1 Mark] -Year 2010

50. What happens chemically when quicklime is added to water filled in a bucket?
Answer. Quicklime reacts with water to form slaked lime and produces lot of heat and hissing sound.

51. On what basis is a chemical equation balanced?
Answer. A chemical reaction is balanced on the basis of law of conservation of mass.

52. What change in colour is observed when white silver chloride is left exposed to sunlight? State the type of chemical reaction in this change.
Answer. Silver chloride becomes grey. It is a photochemical decomposition reaction.

53. Write a balanced chemical equation for the reaction between sodium chloride
and silver nitrate indicating the physical state of the reactants and the products.
Answer.

Short Answer Type Questions[l] [2 Marks]

54. What happens when an aqueous solution of sodium sulphate reacts with an aqueous solution of barium chloride? State the physical conditions of reactants in which the reaction between them will not take place. Write the balanced chemical equation for the reaction and name the type of reaction.
Answer. White precipitate of barium sulphate is formed.
If both reactants are in solid state, then the reaction will not take place between them.

It is a double displacement as well as a precipitation reaction.

55. What is a redox reaction? When a magnesium ribbon burns in air with a dazzling flame and forms a white ash, is magnesium oxidised or reduced? Why?
Answer. The reactions in which oxidation (loss of electrons) and reduction (gain of electrons) take place simultaneously are called redox reactions.

Magnesium is getting oxidised because it is losing electrons to form Mg2+ and oxygen is gaining electrons to form O2-, therefore it is getting reduced.

56. Write any two observations in an activity which may suggest that a chemical reaction has taken place. Give an example in support of your answer.
Answer. Any two of these observations will suggest chemical reaction has taken place.
(i) Change in state.
(ii)Change in colour.
(iii) Evolution of gas.
(iv)Change in temperature.
For example, lead nitrate is white crystalline solid which on heating gives yellowish brown solid (lead monoxide). A brown gas and a colourless gas is also evolved. It shows chemical reaction has taken place.