SUBJECT – SCIENCE
CLASS – X (10th)
QUESTION FROM – Chemical Reactions And Equations – Chapter 1
ALL EXTRA & IMPORTANT / EXERCISE QUESTION ANSWER
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INTEXT QUESTIONS – Chemical Reactions And Equations – Chapter 1 (NCERT Solutions & All Extra And Important Questions) – Science (Class 10)
Q1. Why is the amount of gas collected in one of the test tubes in activity 1.7 (electrolysis of water
double of the amount collected in the other? Why should a magnesium ribbon be cleaned before burning in air?
Answer – Magnesium ribbon reacts with oxygen present in air to form a protective and inert layer of magnesium oxide on its surface. This layer is unreactive and prevents the ribbon from burning. Hence, it needs to be cleaned with sand paper before burning in air.
Q2. Write the balanced equation for the following chemical reactions :-
(i) Hydrogen + Chlorine —> Hydrogen chloride
(i) 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
Q3. Write a 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 the solution of sodium chloride.
(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.
Answer – (I) BaCl2(aq) + Na2SO4(aq) —> BaSO4(s) + 2NaCl(aq)
(II) NaOH(aq) + HCl (aq) —> NaCl (aq) + H2O (l)
Q4. A solution of a substance ‘X’ is used for white washing.
(i) Name the substance ‘X’ and write its formula.
(i) Write the reaction of the substance ‘X’ named in
(I) above with water.
Answer – (i) Substance X is calcium oxide or quicklime. Its Formulae is CAO.
(II) Quicklime reacts with water as :- CaO (s) + H2O (l) —> Ca(OH)2 (aq)
Q5. Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas.
Answer – The composition of water, i.e. the chemical formula H.O, suggests that the molar ratio of hydrogen and oxygen is 2 : 1. Therefore, when water is electrically decomposed, the constituent gases hydrogen and oxygen are produced in the same molar ratio, 2 : 1. Thus, the amount (volume) of hydrogen gas is double than that of oxygen gas. So, this gas is hydrogen.
Q6. Why does the colour of copper sulphate solution change when an iron nail is dipped in it ?
Answer – The colour of copper sulphate solution changes when an iron nail is dipped in it because iron being more reactive than copper, displaces copper metal from aqueous copper sulphate solution. Thus, blue colour of copper sulphate fades away to give green colour solution of ferrous sulphate.
Fe (s) + CuSO4 (aq) —> FeSO4 (aq) + Cu (s)
Q7. Give an example of a double displacement reaction other than the reaction of barium chloride with sodium sulphate ?
Answer – The following reaction is an example of a double displacement reaction :
2Naoh (aq) + H2SO4 (aq) —> Na2So4 + 2H2O (l)
Q8. Identify the substances that are oxidised and the substances that are reduced in the following reactions.
(i) 4Na(s) + O2(g) → 2Na2O(s)
Answer – Na has gained oxygen and forms Na2O. So, Na is oxidised and O2 is reduced.
(ii) CuO(s) + H2(g) → Cu(s) + H2O(l)
Answer – CuO has lost oxygen and forms Cu. So, Cu is reduced while H2 has gained oxygen, hence, its oxidised.
EXERCISE QUESTIONS – Chemical Reactions And Equations – Chapter 1 (NCERT Solutions & All Extra And Important Questions) – Science (Class 10)
Q1. Which of the statements about the reaction below are incorrect? 2PbO(s) + C(s) → 2Pb(s) + CO2 (g)
(a) Lead is getting reduced.
(b) Carbon dioxide is getting oxidised.
(c) Carbon is getting oxidised.
(d) Lead oxide is getting reduced.
Answer – The given reaction can be written in the form of two separate reaction :-
2PbO (s) —–> 2Pb (s)
C (s) —-> CO2(g)
Therefore, (I) and (ii) are incorrect, And (iii) and (iv) are correct statements.
Q2. Fe2O3 + 2Al → Al2O3 + 2Fe The above reaction is an example of a :-
(a) combination reaction.
(b) double displacement reaction.
(c) decomposition reaction.
(d) displacement reaction.
Answer – (iv) In the above reaction, Al is more reactive than Fe. So, it displaces Fe frpm Fe2O3 to form Al2O3. It is a displacement reactions.
Q3. What happens when dilute hydrochloric acid is added to iron fillings? 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.
Answer – Iron being more reactive than hydrogen, displaces hydrogen from the dilute hydrochloric acid. Thus, hydrogen gas and iron chloride a salt of iron are formed.
Fe(s) + 2HCl (aq) —> FeCl2 (aq) + H2(g)
4. What is a balanced chemical equation? Why should chemical equations be balanced ?
Answer – A chemical change is represented by a chemical equation. When the number of atoms of different elements on reactant and product side are equal, then the chemical equation is called a balanced chemical equation.
It is important to balance a chemical equation because
(1) to validate the law of conservation of mass which states that the mass of reactants should be equal to the mass of the products. The total mass of a system is thus conserved. This law holds true only if number of atoms of reactants reacting together is equal to number of product atoms formed.
(2) a balanced chemical equation tells us about the physical state of the reactants and products whether they are solid (s), liquid (l) or gas (g) or aqueous (ag)
(3) it tells us about heat changes that can take place in a chemical reaction. A is the symbol of heat.
Hence, it is endothermic or exothermic can be deduced from a balanced chemical equations.
Q5. 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 sulpur dioxide.
(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
Q6. Balance the following chemical equations.
(a) HNO3 + Ca(OH)2 → Ca(NO3)2 + H2O
(b) NaOH + H2SO4 → Na2SO4 + H2O
(c) NaCl + AgNO3 → AgCl + NaNO3
(d) BaCl2 + H2SO4 → BaSO4 + HCl
Q7. 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) Aluminium + Copper chloride → Aluminium chloride + Copper
(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride
Q8. Write the balanced chemical equation for the following and identify the type of reaction in each case.
(a) Potassium bromide(aq) + 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)
Q9. What does one mean by exothermic and endothermic reactions? Give examples.
Q10. Why is respiration considered an exothermic reaction? Explain.
Q11. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
Q12. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.
Q13. What is the difference between displacement and double displacement reactions? Write equations for these reactions.
Answer – In a displacement reaction, a more reactive element displaces a less reactive element from its salt solution. But in a double displacement reaction, two atoms or groups from different compounds displace each other.
Chemical equation for single displacement,
Zn (s)+ CuSO4(aq) —> ZnSO4(aq) + Cu(s)
Here, Zn displaces Cu from its salt solution (CuSO4).
Chemical equation for double displacement,
BaCI2(aq) + K2SO(a) —> BaSO4(s)+ 2KCI(aq)
Here, Ba and K displace each other.
Q14. In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.
Answer – The reaction involved is :
2AgNO3(aq) + Cu(s) —-> Cu(NO3)2(aq) + 2Ag(s)
Q15. What do you mean by a precipitation reaction? Explain by giving examples ?
Answer –The reaction which is accompanied by the formation of an insoluble solid mass (called precipitate) is known as precipitation reaction, e.g.
1) When barium chloride solution is added to an aqueous solution of sodium sulphate, a white precipitate of barium sulphate is obtained
BaCl2(aą) + Na2SO4 (aq) —> BaSO4(s) 2NaCl (aq)
2) When silver nitrate is added to an aqueous solution of sodium chloride, a white precipitate
of silver chloride (AgCI), which is soluble in
NH4OH is obtained.
AgNO3 (ag) + NaCl(aą) → AgCl(s) ↓ + NaNO3 (aq)
Q16. Explain the following in terms of gain or loss of oxygen with two examples each.
(a) Oxidation (b) Reduction
Q17. 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 – Element X is copper and the black coloured compound is copper (II) oxide.
2Cu(s) + O2(g) —–> 2CuO(s)
Q18. Why do we apply paint on iron articles?
Answer – By applying paint on iron articles, they can be prevented from corrosion (rusting). Paint does not allow oxygen (from air) and water to come in contact with the surface of iron.
Q19. Oil and fat containing food items are flushed with nitrogen. Why?
Answer – Nitrogen is unreactive gas as compared to oxygen. Oil and fat present in the food items get oxidised and become rancid in the presence of air or oxygen. But such reaction is prevented in the
presence of nitrogen. Therefore, food items like potato chips etc., are packed with nitrogen gas to prevent them from rancidity for a long time.
Q20. Explain the following terms with one example each. (a) Corrosion (b) Rancidity.
Answer – (A) Corrosion – This process is commonly known as rusting of iron. Some other metals also get tarnished in this manner. When a metal is attacked by substances around it such as moisture, acids, etc., it is said to corrode and this process is called corrosion. The black coating on silver and the green coating on copper are other examples of corrosion.
(B) Rancidity – When fats and oils are oxidised, they become rancid and their smell and taste change.
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