# Chemical Reactions and Equations

## Class 10 NCERT Science

### NCERT

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

##### Solution :

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 the balanced equation for the following chemical reactions.$\\$ (i) Hydrogen + Chlorine $\rightarrow$ Hydrogen chloride$\\$ (ii) Barium chloride + Aluminium sulphate $\rightarrow$ Barium sulphate +Aluminium chloride$\\$ (iii) Sodium + Water $\rightarrow$ Sodium hydroxide + Hydrogen

(i)$H_{2(g)}+Cl_{2(g)} \rightarrow 2HCl_{(g)}$$\\ (ii)3BaCl_{2(s)}+Al_2(SO_4)_{3(s)} \rightarrow 3BaSO_{4(s)}+2AlCl_{3(s)}$$\\$ (iii)$2Na_{(s)}+2H_2O_{(l)}\rightarrow 2NaOH_{(aq)}+H_{2(g)}$$\\ 3 Write a balanced chemical equation with state symbols for the following reactions.\\ (i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.\\ (ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water)to produce sodium chloride solution and water. ##### Solution : (i)BaCl_{2(aq)}+Na_2SO_{4(s)} \rightarrow BaSO_{4(s)}+2NaCl_{(aq)}$$\\$ (ii)$NaOH_{(aq)}+HCl_{(aq)}\rightarrow NaCl_{(aq)}+H_2O_{(l)}$$\\ 4 A solution of a substance 'X’ is used for white washing.\\ (i) Name the substance 'X’ and write its formula.\\ (ii) Write the reaction of the substance 'X’ named in (i) above with water. ##### Solution : (i) The substance ‘X’ is calcium oxide. Its chemical formula is Ca O.$$\\$ (ii) Calcium oxide reacts vigorously with water to form calcium hydroxide (slaked lime).$\\$ $\ \ \ \ \ CaO_{(s)} \ \ \ \ \ \ \ \ \ \ \ \ + H_2O_{(l)} \ \ \ \ \ \rightarrow \ \ \ Ca(OH)_{2(aq)}$$\\ Calcium oxide(Quik line) + Water \rightarrow Calcium hydroxide (Slaked lime) 5 Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas. ##### Solution : Water (H_2 O) contains two parts hydrogen and one part oxygen. Therefore, the amount of hydrogen and oxygen produced during electrolysis of water is in a 2:1 ratio. During electrolysis, since hydrogen goes to one test tube and oxygen goes to another, the amount of gas collected in one of the test tubes is double of the amount collected in the other. 6 Why does the colour of copper sulphate solution change when an iron nail is dipped in it? ##### Solution : When an iron nail is placed in a copper sulphate solution, iron displaces copper from copper sulphate solution forming iron sulphate, which is green in colour.\\ Fe_s+ CuSO_{4(aq)} \rightarrow FeSO_{4(aq)} +Cu_{(s)}$$\\$ Therefore, the blue colour of copper sulphate solution fades and green colour appears.

7   Give an example of a double displacement reaction other than the one given in Activity1.10.

##### Solution :

Sodium carbonate reacts with calcium chloride to form calcium carbonate and sodium chloride. $\underset{\text{Sodium carbonate}}{Na_2CO_{3(aq)}} + \underset{\text{Calcium chloride}}{CaCl_{2(aq)}} \rightarrow \underset{\text{Calcium carbonate}}{CaCO_{3(s)}} + \underset{\text{Sodium chloride}}{2NaCl_{aq}} $$\\ In this reaction, sodium carbonate and calcium chloride exchange ions to form two new compounds. Hence, it is a double displacement reaction. 8 Identify the substances that are oxidised and the substances that are reduced in the following reactions.\\ (i)4Na_{(s)}+O_{2(g)} \rightarrow 2Na_2O_{(s)}$$\\$ (ii)$CuO_{(s)}+H_{2(g)} \rightarrow Cu(s)+H_2O_{(l)}$

##### Solution :

(i) Sodium $(Na)$ is oxidised as it gains oxygen and oxygen gets reduced. (ii) Copper oxide $(CuO)$ is reduced to copper $(Cu)$ while hydrogen $(H_2 )$ gets oxidised to water $(H_2 O).$

##### Solution :

(d) The given reaction is an example of a $\underline{\text{displacement reaction.}}$

11   What happens when dilute hydrochloric acid is added to iron filings? Tick the correct answer.

##### Solution :

(a) Hydrogen gas and iron chloride are produced. The reaction is as follows: $\\$ $Fe_{(s)}+2HCl_{(aq)} \rightarrow FeCl_{2(aq)} +H_2 \uparrow$

12   What is a balanced chemical equation? Why should chemical equations be balanced?

##### Solution :

A reaction which has an equal number of atoms of all the elements on both sides of the chemical equation is called a balanced chemical equation.The law of conservation of mass states that mass can neither be created nor destroyed.Hence, in a chemical reaction, the total mass of reactants should be equal to the total mass of the products. It means that the total number of atoms of each element should be equal on both sides of a chemical equation. Hence, it is for this reason that chemical equations should be balanced.

13   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.

##### Solution :

(a)$3H_{2(g)}+N_{2(g)}\rightarrow 2NH_{3(g)}$$\\ (b)2H_2S_{(g)}+3O_{2(g)} \rightarrow 2H_2O_{(l)}+2SO_{2(g)}$$\\$ (c)$3BaCl_{2(aq)}+Al_2(SO_4)_{3(aq)} \rightarrow 2AlCl_{3(aq)}+3BaSO_{4(s)}$$\\ (d)2K_{(s)}+2H_2O_{(l)}\rightarrow 2KOH_{(aq)}+h_{2(s)}$$\\$

14   Balance the following chemical equations.$\\$ (a)$HNO_3 +Ca(OH)_2 \rightarrow Ca(NO_3)_2+H_2O$$\\ (b)NaOH+H_2SO_4 \rightarrow Na_2SO_4+H_2O$$\\$ (c)$NaCl+AgNO_3 \rightarrow AgCl+NaNO_3$$\\ (d) BaCl_2+H_2SO_4 \rightarrow BaSO_4+HCl ##### Solution : (a)2HNO_3 +Ca(OH)_2 \rightarrow Ca(NO_3)_2+2H_2O$$\\$ (b)$2NaOH+H_2SO_4 \rightarrow Na_2SO_4+2H_2O$$\\ (c)NaCl+AgNO_3 \rightarrow AgCl+NaNO_3$$\\$ (d) $BaCl_2+H_2SO_4 \rightarrow BaSO_4+2HCl$

15   Write the balanced chemical equations for the following reactions.$\\$ (a) Calcium hydroxide + Carbon dioxide $\rightarrow$Calcium carbonate + Water$\\$ (b) Zinc + Silver nitrate $\rightarrow$ Zinc nitrate + Silver $\\$ (c) Aluminium + Copper chloride $\rightarrow$ Aluminium chloride + Copper$\\$ (d) Barium chloride + Potassium sulphate $\rightarrow$ Barium sulphate +Potassium chloride

##### Solution :

(a)$Ca(OH)_2+CO_2\rightarrow CaCO_3+H_2O$$\\ (b)Zn+ 2AgNO_3 \rightarrow Zn(NO_3)_2 +2Ag$$\\$ (c )$2Al+3CuCl_22\rightarrow AlCl_3+3Cu$$\\ (d)BaCl_2+K_2SO_4\rightarrow BaSO_4+2KCl$$\\$

16   Write the balanced chemical equation for the following and identify the type of reaction in each case.$\\$ (a)Potassium bromide (aq) + Barium iodide (aq) $\rightarrow$ Potassium iodide (aq) + Barium bromide(s)$\\$ (b) Zinc carbonate (s) $\rightarrow$ Zinc oxide (s) + Carbon dioxide (g)$\\$ (c) Hydrogen (g) + Chlorine (g) $\rightarrow$ Hydrogen chloride (g)$\\$ (d) Magnesium (s) + Hydrochloric acid (aq) $\rightarrow$ Magnesium chloride (aq) + Hydrogen (g)

##### Solution :

(a)$2KBr_{(aq)}+Bal_{2(aq)}\rightarrow 2Kl_{(aq)}+BaBr_{2(s)}; \text{Double displacement reaction}$$\\ (b)ZnCO_{3(s)} \rightarrow ZnO_{(s)}+CO_{2(g)}; \text{Decomposition reaction}$$\\$ (c)$H_{2(g)}+Cl_{2(g)}\rightarrow 2HCl_{(g)};\text{Combination reaction }$$\\$ (d)$Mg_{(s)}+2HCl_{(aq)}\rightarrow MgCl_{2(aq)}\rightarrow MgCl_{2(aq)}+H_{2(g)};\text{Displacement reaction}$