CBSE Class 10 Science Ncert Solutions Chapter 1 – Chemical Reactions and Equations
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1. Why should a magnesium ribbon be cleaned before burning in air?
Ans. Magnesium ribbon is cleaned before burning to remove the protective layer of basic magnesium carbonate from the surface of magnesium ribbon.
2. Write the balance equation for the following reactions Give reasons for the following reactions?
i. Hydrogen + Chlorine → Hydrogen chloride
ii. Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
iii. Sodium + water → Sodium hydroxide + water
Ans. The chemical equations are as follows-
i. H2 + Cl2 2HCl.
ii. 3BaCl2 + Al2(SO4)3 3BaSO4 + 2AlCl3.
iii. 2Na + 2H2O 2NaOH + H2.
3. Write the balanced chemical equation with state symbols for the following reactions?
i. Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and solution of sodium chloride.
ii. Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.
Ans. Balance chemical reaction with state symbols are as follows-
(Page No. 10)
1. A solution of a substance ‘X’ is used for white washing
i. Name the substance ‘X’ and writes its formula.
ii. Write the reaction of the substance ‘X’ named in (i) above with water
Ans. i. The substance whose solution is water is used for white washing is calcium oxide. Its formula is CaO.
ii. CaO(s) + H2O(l) Ca(OH)2(aq)
2. Why the amount of gas collected in one of the tubes in Activity 1.7 double of the amount collected in the other? Name this gas.
Ans. The gas which is collected in double the amount in the electrolysis of water experiment is hydrogen. This is because water contains 2 parts of hydrogen element as compared to only 1 part of oxygen element.
(Page No. 13)
1. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
Ans. When iron nail is dipped in copper sulphate solution, than iron sulphate solution and copper solution and copper metal are formed:
In this reaction, iron displaces copper from copper sulphate solution. The deep blue colour of copper sulphate fades due to the formation of light green solution of iron sulphate.
2. Give an example of a double displacement reaction other than the one given in Activity 1.10.
Ans. An example of double displacement reaction is
3. Identify the substances that are oxidized and the substances that are reduced in the following reactions.
In this reaction, Na is oxidized because it combines with O2 to form Na2O. O2 is reduced because it is converted into Na2O.
In this reaction, CuO is reduced because it loses oxygen. H2 is oxidized because it combines with oxygen of CuO to form water
1. Which of the following statement about the reaction below are incorrect?
(a) Lead is getting reduced.
(b) Carbon dioxide is getting oxidized
(c) Lead oxide is getting oxidized
(d) Lead is getting reduced
i. (a) and (b)
ii. (a) and (c)
iii. (a), (b) and (c)
Ans. As statement (a) and (b) are incorrect, answer (i) is correct.
The above reaction is an example of a
(a) combination reaction
(b) double displacement reaction
(c) decomposition reaction
(d) displacement reaction
Ans. This is an example of displacement reaction because Fe in FeO3 has been displaced by Al. Hence correct answer is (d).
3. What happens when dilute hydrochloric acid is added to iron filling? Tick the correct answer
(a) Hydrogen gas and iron chloride are produced.
(b) Chlorine gas and iron hydroxide are produced
(c) No reaction takes place
(d) Iron salt and water are produced
Ans. Answer (a) is correct.
4. What is balanced chemical equation? Why should chemical equation be balanced?
Ans. The reaction in which the number of atoms of each element is equal on the reactant side and product side is called balanced equation.
Chemical reaction should be balanced because only a balanced equation tells us the relative quantities of different reactants and products involved in the reaction.
5. Translate the following statements into chemical equations and then balance them.
(a) Hydrogen gas combines with nitrogen to form ammonia.
(b) Hydrogen sulphide gas burns in air to give water and Sulpher dioxide.
(c) Barium chloride reacts with aluminum sulphate to give aluminum chloride and precipitate of barium sulphate
6. Balance the following chemical equations:
Ans. Balanced chemical equation are:
7. Write the balanced chemical equations for the following reactions.
(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
(b) Zinc + Silver nitrate → Zinc nitrate + Silver
(c) Aluminum + Copper chloride → Aluminum chloride +Copper
(d) Barium chloride + Potassium sulphate → Barium sulphate + potassium chloride
Ans. Balanced chemical equation for reactions are:
8. Write the balanced chemical equation for the following and identify the type of reaction in each case.
(a) Potassium bromide (s) + Barium iodide (aq) → Potassium iodide (aq) + Barium bromide(s)
(b) Zinc carbonate (s) → Zinc oxide (s) + Carbon dioxide (g)
(c) Hydrogen (g) + Chlorine (g) → Hydrogen chloride (g)
(d) Magnesium (s) + Hydrochloric acid (aq) → Magnesium chloride (aq) + Hydrogen (g)
Ans. Balanced equations are:
(a) ; Double Displacement Reaction
(b) ; Decomposition Reaction
(c) ; Combination Reaction
(d) ; Displacement Reaction
9. What does one mean by exothermic and endothermic reactions? Give examples.
Ans. A reaction in which energy is released in the form of heat or light is called exothermic reaction. Example of exothermic reaction are:
A reaction in which energy is absorbed from the surrounding and cooling is produced is called endothermic reaction. Example of endothermic reaction are:
10. Why respiration is considered an exothermic reaction? Explain.
Ans. During respiration, we inhale oxygen from the atmosphere which reacts with glucose in your body cells to produce carbon dioxide and water.
Heat is liberated in this process; hence respiration is considered an exothermic reaction.
11. Why decomposition reactions are called the opposite of combination reactions? Write equations for these reactions.
In a decomposition reaction, a single substance breaks down into two or more substances while in a combination reaction, two or more substances react to produce one substance. Therefore, decomposition reactions are called opposite of combination reactions.
Example of decomposition reaction: Example of combination reaction:
12. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.
Ans. Decomposition by heat:
Decomposition by electricity:
Decomposition by light:
13. What is difference between displacement and double displacement reactions? Write equations for these reactions.
Ans. In displacement reaction, more reactive element displaces the less reactive element from its compound. For example
But in double displacement reaction, exchange of ions takes place. For example
14. In refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.
Ans. The reaction involved is:
AgNO3(aq) + Cu(s) Cu(NO3)2(aq) + 2 Ag(s)
Copper + SilverNitrate CopperNitrate + Silver
15. What do you mean by a precipitation reaction? Explain by giving examples.
Ans. A chemical reaction in which an insoluble substance (precipitate) is formed is called precipitation reaction. For example
16. Explain the following in terms of gain and loss of oxygen with two examples each?
Ans. Oxidation- addition of oxygen or removal of hydrogen in a chemical reaction is called oxidation reaction. For example
Reduction- addition of hydrogen or removal of oxygen in a chemical reaction is called oxidation reaction. For example
17. A shiny brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.
Ans. The brown coloured element ‘X ‘ is copper. On heating in air it forms copper oxide, which is black in colour.
18. Why do we apply paint on iron articles?
Ans. We apply paint on iron articles to prevent rusting. Iron articles do not come in contact of atmospheric oxygen and moisture and thus the rusting is prevented.
19. Oil and fat containing food items are flushed with nitrogen. Why?
Ans. Oil and fat containing items get rancid due to oxidation with atmospheric oxygen. To prevent rancidity food items are flushed with nitrogen. Nitrogen do not reacts with oil and fat containing items.
20. Explain the following terms with one example each.
Ans. Corrosion– action of air, water, acid or other substance on metal surface to form oxides and carbonates is called corrosion. Corrosion of iron is called rusting. Green coating on copper and black coating on silver is examples of corrosion.
Rancidity-change in smell of food item containing fat and oil when kept open for longer time due to oxidation is called rancidity. To prevent rancidity food items are flushed with nitrogen or kept in airtight containers.