NCERT Solutions Class 10 Science
The NCERT Solutions in English Language for Class 10 Science Chapter – 1 (Chemical Reactions and Equations) has been provided here to help the students in solving the questions from this exercise.
Chapter – 1 (Chemical Reactions and Equations)
Questions
1. Why should a magnesium ribbon be cleaned before burning in the air?
Answer – Magnesium is very reactive metal. When stored it reacts with oxygen to form a layer magnesium oxide on its surface. This layer of magnesium oxide is quite stable and prevents further reaction of magnesium with oxygen. The magnesium ribbon is cleaned by sand paper to remove this layer so that the underlying metal can be exposed into air.
2. Write a balanced equation for the following chemical reactions.
(i) Hydrogen + Chloride → Hydrogen chloride
Answer – H2 + Cl2 → 2HCl
(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
Answer – 3BaCl2 + Al2(SO4)3 →3BaSO4 + 2AlCl3
(iii) Sodium + Water → Sodium hydroxide + Hydrogen
Answer – 2Na + 2H2O → 2NaOH + H2
3. 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 solution of Sodium chloride.
Answer – BaCl2 + Na2SO4 → BaSO4 + 2NaCl
(ii) Sodium hydroxide solution in water reacts with the hydrochloric acid solution to produce Sodium chloride solution and water.
Answer – NaOH + HCl → NaCl + H2O
Questions
1. A solution of a substance, ‘X,’ is used for whitewashing.
(i) Name the substance ‘X’ and write its formula.
Answer – The substance ‘X’ which is used in whitewashing is quick lime or Calcium Oxide and its formula is CaO.
(ii) Write the reaction of the substance ‘X’ named in (i) above with water.
Answer – CaO + H2O → Ca(OH)2
2. Why is the amount of gas collected in one of the test tubes in Activity 1.7 double the amount collected in the other? Name this gas.
Answer – In Activity 1.7, water is electrolysed to give H2 gas at one electrode and O2 gas at the other electrode.
2H2O(l) → 2H2(g) + O2(g)
Thus two molecules of water on electrolysis give two molecules of hydrogen gas and one molecule of oxygen gas or in other words the amount of hydrogen gas collected would be double than that of oxygen gas.
Questions
1. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
Answer – When an iron nail is placed in a copper sulphate solution, iron displaces copper from copper sulphate solution forming iron sulphate, which is green in colour.
Fe + CuSO4 → FeSO4 + Cu
2. Give an example of a double displacement reaction other than the one given in Activity 1.10.
Answer – Sodium carbonate reacts with calcium chloride to form calcium carbonate and sodium chloride. The chemical reaction is given below.
Ag+ + NO3– + Na+ + Cl– → AgCl + Na+ + NO3–
3. Identify the substances that are oxidised and that are reduced in the following equation.
(i) 4Na(s) + O2(g) → 2Na2O(s)
Answer – Sodium (Na) is oxidised as it gains oxygen and oxygen gets reduced.
(ii) CuO(s) + H2(g) → Cu(s) + H2O(l)
Answer – Copper oxide (CuO) is reduced to copper (Cu) while hydrogen (H2) gets oxidised to water (H2O).
Exercise
1. 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
(i) (a) and (b)
(ii) (a) and (c)
(iii) (a), (b) and (c)
(iv) all the above
Answer – (i) (a) and (b).
(a) Because Oxygen is being removed and (b) Because the removed oxygen from Lead is added to the elemental Carbon.
2. 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 – (d) Displacement reaction.
This is an example of displacement reaction because Fe in FeO3 has been displaced by Al.
3. 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 – (a) Hydrogen gas and Iron chloride are produced.
The Chlorine from Hydrogen chloride is displaced by the Iron fillings to undergo the following reaction. 2HCl + Fe → FeCl2 + H2
4. What is a balanced chemical equation? Why should a chemical equation be balanced?
Answer – A balanced equation is the one in which number of different atoms on both the reactant and product sides are equal. Balancing chemical equations is necessary for the reaction should obey The Law of Conservation of mass. Balancing the chemical equation has no defined method and is purely a trial-and-error attempt.
5. Translate the following statements into chemical equations and balance them.
(a) Hydrogen gas combines with nitrogen to form ammonia.
Answer –
Unbalanced: H2 + N2 → NH3
Balanced: 3H2 + N2 → 2NH3
(b) Hydrogen sulphide gas burns in the air to give water and sulphur dioxide.
Answer –
Unbalanced: H2S + O2 → H2O + SO2
Balanced: 2H2S + 3O2 → 2H2O + 2SO2
(c) Barium chloride reacts with aluminium sulphate to give Aluminium chloride and a precipitate of barium sulphate.
Answer –
Unbalanced: BaCl2 + Al2(SO4)3 → AlCl3 + BaSO4
Balanced: 3BaCl2 + Al2(SO4)3 → 2AlCl3 + 3BaSO4
(d) Potassium metal reacts with water to give potassium hydroxide and Hydrogen gas.
Answer –
Unbalanced: K + H2O → KOH + H2
Balanced: 2K + 2H2O → 2KOH + H2
6. Balance the following chemical equations.
(a) HNO3 + Ca(OH)2 → Ca(NO3)2 + H2O
Answer – 2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H2O
(b) NaOH + H2SO4 → Na2SO4 + H2O
Answer – 2NaOH + H2SO4 → Na2SO4 + 2H2O
(c) NaCl + AgNO3 → AgCl + NaNO3
Answer – NaCl + AgNO3 → AgCl + NaNO3
(d) BaCl2 + H2SO4 → BaSO4 + HCl
Answer – BaCl2 + H2SO4 → BaSO4 + 2HCl
7. Write the balanced chemical equation for the following reactions.
(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
Answer – 2Ca(OH)2 + 2CO2 → 2CaCO3 + 2H2O
(b) Zinc + Silver nitrate → Zinc nitrate + Silver
Answer – Zn + 2AgNO3 → Zn(NO3)2 + 2Ag
(c) Aluminium + Copper chloride → Aluminium chloride + Copper
Answer – 2Al + 3CuCl2 → 2AlCl3 + 3Cu
(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride
Answer – BaCl2 + K2SO4 → BaSO4 + 2KCl
8. Write a balanced chemical equation for the following and identify the type of reaction of each case.
(a) Potassium bromide (aq) + Barium iodide (aq) → Potassium iodide (aq) + Barium bromide(s)
Answer – 2KBr + BaI2 → 2KI + BaBr2 (Double Displacement Reaction)
(b) Zinc carbonate (s) → Zinc oxide (s) + Carbon dioxide (g)
Answer – ZnCO3 → ZnO + CO2 (Decomposition Reaction)
(c) Hydrogen (g) + Chlorine (g) → Hydrogen chloride (g)
Answer – H2 + Cl → 2HCl (Combination Reaction)
(d) Magnesium (s) + Hydrochloric acid (aq) → Magnesium chloride (aq) + Hydrogen (g)
Answer – Mg + 2HCl → MgCl2 + H2 (Displacement Reaction)
9. What is meant by exothermic and endothermic reactions? Give examples.
Answer –
Exothermic reactions : Those reactions in which heat is evolved are known as exothermic reactions. An exothermic reaction is indicated by writing “+ Heat”on the products side of an equation.
Example :
(i) C (s) + O2 (g) → CO2 (g) + Heat
(ii) N2 (g) + 3H2 (g) → 2NH3 (g) + Heat
Endothermic reactions : Those reactions in which heat is absorbed are known as endothermic reactions. An endothermic reaction is usually indicated by writing “Heat” on the product side of a chemical equation.
Examples :
(i) C (s) + 2S (s) → CS2 (l) – Heat
(ii) N2 (g) + O2 (g) → 2NO(g) – Heat
10. Why is respiration considered to be an exothermic reaction?
Answer – Energy is required to support life. Energy in our body is obtained from the food we eat. During digestion, large molecules of food are broken down into simpler substances such as glucose. Glucose combines with oxygen in the cells and provides energy. The special name of this combustion reaction is respiration.
The reaction taking place is: C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy
11. Why are decomposition reactions called the opposite of Combination reactions? Write equations for decomposition reactions.
Answer –
Combination Reactions – Two or more substances react to form a single product in a Combination Reaction. Usually, heat or energy is produced in such reactions.
Example : CaO(s) + H 2 O(l) → Ca(OH) 2 (aq)
Decomposition Reactions – A single reactant breaks down into more than one product in a Decomposition Reaction. Usually, heat or energy is used in such reactions.
Example : CaCO 3 (s) → CaO(s) + CO 2 (g)
12. Write one equation each for decomposition reactions in which energy is supplied in the form of heat, light or electricity.
Answer –
(a) Thermal Decomposition Reaction (Thermolysis) Decomposition of potassium chlorate: When heated strongly, potassium chlorate decomposes into potassium chloride and oxygen. This reaction is used for the preparation of oxygen.
2KClO3 + Heat → 2KCl + 3O2
(b) Electrolytic Decomposition Reaction (Electrolysis) Decomposition of sodium chloride: On passing electricity through molten sodium chloride, it decomposes into sodium and chlorine.
2NaCl + electricity → 2Na + Cl2
(c) Photodecomposition Reaction (Photolysis) Decomposition of hydrogen peroxide: In the presence of light, hydrogen peroxide decomposes into water and oxygen.
2H2O2 + light → 2H2O
13. What is the difference between displacement and double displacement reactions? Write relevant equations for the above.
Answer – In a displacement reaction, one element takes the place of another in a compound dissolved in a solution. Example : Fe(s) + CuSO4 (aq) → FeSO4 (aq) + Cu(s)
In a double displacement reaction, one component each of both the reacting molecules get exchanged to form the products.
Example : HCl (aq) + NaOH(aq) → NaCl (aq) + H2O (aq)
14. In the refining of Silver, the recovery of silver from Silver nitrate solution involves displacement reaction by Copper metal. Write down the reaction involved.
Answer – The chemical equation for the displacement reaction is :
Cu(s) + 2AgNO3(aq) → Cu(NO3)2(aq) + 2Ag(s)
15. What do you mean by a precipitation reaction? Explain by giving examples.
Answer – A reaction in which an insoluble solid (called precipitate) is formed is called a precipitation reaction.
Examples :
Na2CO3 (aq) + CaCl2 (aq) → CaCO3 (s) + 2NaCl (aq)
In this reaction, calcium carbonate is obtained as a precipitate. Hence, it is a precipitation reaction.
Na2SO4 (aq) + BaCl2 (aq) → BaSO4 (s) + 2NaCl (aq)
In this reaction, barium sulphate is obtained as a precipitate.
16. Explain the following in terms of the gain of oxygen with two examples each.
(a) Oxidation
Answer – In a chemical reaction, when the oxygen is added to the element to form its respective oxide it is the element being oxidised.
Example:
4Na(s) + O2(g) → 2Na2O(s)
H2S + O2 → H2O + SO2
(b) Reduction
Answer – In a chemical reaction, when the oxygen is removed from the compound, then it is said to be reduced.
Example:
CuO(s) + H2(g) → Cu(s) + H2O(l)
2HgO → 2Hg + O2
17. A shiny brown coloured element ‘X’ on heating in the air becomes black in colour. Name the element ‘X’ and the black-coloured compound formed.
Answer – ‘X’ is copper (Cu) and the black-coloured compound formed is copper oxide (CuO). The equation of the reaction involved on heating copper is given below.
2Cu(s) + O2(g) → 2CuO(s)
18. Why do we apply paint on iron articles?
Answer – Iron articles are painted because it prevents them from rusting. When painted, the contact of iron articles from moisture and air is cut off. Hence, rusting is prevented. So presence of air and moisture is essential for rusting to take place.
19. Oil and Fat containing food items are flushed with Nitrogen. Why?
Answer – The main purpose of flushing Nitrogen into food packets that contain oil and fat items is to prevent Rancidity which occurs when the oil or fat reacts with the oxygen letting out an unpleasant smell and taste. Therefore, by flushing Nitrogen, an unreactive surrounding is created, thus preventing rancidity.
20. Explain the following terms with one example each.
(a) Corrosion
Answer – Corrosion is defined as a process where materials, usually metals, deteriorate as a result of a chemical reaction with air, moisture, chemicals, etc. For example, iron, in the presence of moisture, reacts with oxygen to form hydrated iron oxide.
4Fe + 3O2 + nH2O → 2Fe2O3.nH2O
(b) Rancidity
Answer – The process of oxidation of fats and oils that can be easily noticed by the change in taste and smell is known as rancidity. For example, the taste and smell of butter changes when kept for long.
Rancidity can be avoided by:
1. Storing food in air tight containers
2. Storing food in refrigerators
3. Adding antioxidants
4. Storing food in an environment of nitrogen
