# NCERT Solutions for class 10th Science Chapter 1 Chemical Reaction and Equations

##### NCERT Intex Questions

Question 1. Why should a magnesium ribbon be cleaned before burning in the air?

Sol. The magnesium ribbon that we use has a coating of magnesium oxide on its surface, which slows down the burning of the magnesium ribbon. Therefore, before burning it in the air, it is cleaned with sandpaper which removes the protective layer of magnesium oxide from the surface of the magnesium ribbon, so that it can easily burn.

Question 2. Write the balanced equation for the following chemical reactions

Question 3. Write a balanced chemical equation with state symbols for the following reactions.
i) Solutions of barium chloride and sodium sulfate in water react to give insoluble barium sulfate 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.

Question 4. A solution of a substance ‘X’ is used for whitewashing.
(i)
Name the substance ‘X’ and write its formula is CaO.
(ii) Write the reaction of the substance ‘X’ named in (i) above with water.

Sol. (i) Substance X is calcium oxide whose solution we use in water is used for whitewashing. The formula of substance X is CaO.
(ii) Calcium oxide reacts vigorously with water to produce slaked lime (calcium hydroxide) releasing a large amount of heat.

Question 5. Why is the amount of gas collected in one of the test tubes in Activity 1.7 (of ncert textbook) double the amount collected in the other? Name this gas.

Sol. The gases formed at two electrodes are produced due to the decomposition of water on passing electricity. Since the electrolysis of water produces 2 volumes of hydrogen gas and 1 volume of oxygen gas, thus we can say that the ratio of hydrogen and oxygen in water is 2:1 by volume and hence, the amount of gas collected in one test tube will be double the amount collected in other.

The gases are hydrogen and oxygen

Question 6. Why does the color of copper sulfate solution change when an iron nail is dipped in it?

Sol. As iron is more reactive than copper it displaces copper from copper sulphate solution.

In this way, the concentration of copper sulphate decreases while the concentration of ferrous sulphate increases. As a result, the color changes from blue to green.

Note: It is an example of a displacement reaction, in which blue colour of copper sulphate solution turns green into ferrous sulphate solution and the grey nails turn reddish brown due to the deposition of copper.

Note: Double displacement reaction occurs most often between ionic compounds, although the bonds formed between the chemical species may be either ionic or covalent in nature. Â

Question 8 : Identify the substances that are oxidized and the substances that are reduced in the following reactions.

Sol. (i) In this reaction, sodium is oxidised because it combines with oxygen to form sodium oxide. Oxygen is reduced because it is converted into sodium oxide.

(ii) In this reaction, CuO is reduced because it loses oxygen. H2 is oxidized because it combines with oxygen of CuO to form water.

#### Excersice

Question 1. Which of the statements about the reaction below are incorrect?

• Carbon dioxide is getting oxidised.
• Carbon is getting oxidised.
• Lead oxide is getting reduced.

(a) (i) and (ii) (b) (i) and (iii)
(c) (i), (ii) and (iii) (d) all

Sol. (a) The statements (i) and (ii) are incorrect because PbO is getting reduced, not Pb, similarly C is getting oxidised not CO2.

Question 2. The below reaction is an example of a

• combination reaction
• double displacement reaction
• decomposition reaction
• displacement reaction

Sol. (d) The given reaction is an example of a displacement reaction as in this reaction Al displaces Fe from Fe2O3.

Question 3. What happens when dilute hydrochloric acid is added to iron fillings? Tick the correct answer.

• Hydrogen gas and iron chloride are produced.
• Chlorine gas and iron hydroxide are produced.
• No reaction takes place.
• Iron salt and water are produced.

Sol. (a) When dilute hydrochloric acid is added to iron fillings, hydrogen gas and iron chloride are produced

Question 4. What is a balanced chemical equation? Why should chemical equations be balanced?

Sol. An equation in which the number of atoms of each element is equal on the reactant side and product side is called a balanced equation, e.g.,

The chemical equation should be balanced because only a balanced equation tells us the relative quantities of different reactants and products involved in the reaction.

Question 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 sulphur 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.

Question 6. Balance the following chemical equations.

Question 7. Write the balanced chemical equations for the following reactions.

Question 8. Write the balanced chemical equation for the following and identify the type of reaction in each case.

Question 9. What does one mean by exothermic and endothermic reaction? Give examples.

Question 10. Why is respiration considered an exothermic reaction? Explain.

Sol. Respiration is the most important biochemical reaction that releases energy in the cell. When we breathe air (oxygen) enters into our lungs and this oxygen binds itself to hemoglobin present in red blood cells (RBC). It is then carried to millions of cells present in our body. Respiration occurs in these cells and is accompanied by the combustion of glucose (C6H12O6) producing carbon dioxide (CO2) and water (H2O). A large amount of energy is released in this chemical reaction.

Since the process is accompanied by the release of energy it is considered as an exothermic reaction.

Question 11. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.

Sol. During decomposition reaction, a single reactant decomposes to form two or more products, whereas in combination two or more reactants react to form a single product, e.g.,

Question 12. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.

Question 13. What is the difference between displacement and double displacement reactions? Write equations for these reactions.

Sol. Displacement reaction: In this reaction, an element which is more reactive displaces the other element which is less reactive from its salt solution, e.g.,

Double displacement reaction: In this reaction exchange of ions between two reactants takes place, e.g.,

Question 14. In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.

Sol. When copper is mixed in silver nitrate solution, it displaces the silver because copper is more reactive than silver.

Question 15. What do you mean by a precipitation reaction? Explain by giving examples.

Sol. Those reactions which produce an insoluble product or precipitate are known as precipitation reactions, e.g.,

Note: Precipitation reaction occurs when two solutions are mixed together and a solid separates from the solution. It helps to determine the presence of various ions in the solution.

Question 16. Explain the following in terms of gain or loss of oxygen with two examples each.
(a) Oxidation (b) Reduction

Sol. (a) Oxidation: The reaction in which gain of oxygen takes place is called oxidation, e.g.,

Reduction: The reaction in which loss of oxygen takes place is called reduction, e.g.,

Question 17. A shiny brown colored element ‘X’ on heating in the air becomes black in color. Name the element ‘X’ and the black-coloured compound formed.

Sol. The shiny brown-colored element ‘X’ is copper. When it is heated in air, it becomes black due to a deposit of copper oxide.

Question 18. Why do we apply paint on iron articles?

Sol. To prevent iron articles from rusting, we apply paint on them, by which iron does not come in contact with air and moisture which cause rusting.

Question 19. Oil and fat-containing food items are flushed with nitrogen. Why?

Sol. When the oil and fat-containing food is surrounded by unreactive gas nitrogen, there is no oxygen to cause its oxidation and make it rancid. So, to keep food items fresh and save from the oxidizing effect of oxygen, food items are flushed with nitrogen gas.

Note: Nitrogen acts as an antioxidant to stop the oxidation of oils and fats

Question 20. Explain the following terms with one example each.
(a) CorrosionÂ Â Â Â Â Â Â Â Â Â Â Â Â Â  (b)Â  Rancidity

Sol. (a) Corrosion: It is a process in which metals are eaten up gradually by the action of air, moisture, or a chemical on their surface. E.g., iron forms a reddish brown-colored coating on its outer surface due to corrosion.
(b) Rancidity: When fats and oil-containing food oxidized with air or oxygen, their smells and tastes change. This process is called rancidity, e.g., cooking oil kept for a long time will become rancid.