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Page No 72:

Question 1:

Which of the following reactions is a decomposition reaction?
(a) CaCO3 ⟶CaO + CO2
(b) AgNO3 + NaCl ⟶ AgCl + NaNO3
(c) CuO + 2HCl ⟶ CuCl2 + H2O
(d) C + O2 ⟶ CO2

Answer:

(a) CaCO3 ⟶ CaO + CO2

This is a decomposition reaction because in this reaction calcium carbonate decomposes to produce calcium oxide and carbon dioxide.

Page No 72:

Question 2:

Which of the following is not a combination reaction?
(a) CaO + CO2 ⟶ CaCO3
(b) 2HgO ⟶ 2Hg + O2
(c) 2H2 + O2 ⟶ 2H2O
(d) C + O2 ⟶ CO2

Answer:

(b) 2HgO ⟶ 2Hg + O2
It is a decomposition reaction but not a combination reaction.

Page No 72:

Question 3:

Which one of the sets below represents the correct values for the coefficients x, y and z in the following equation?
xSO3 + yH2O ⟶zH2SO4

(a) x-1; y-2; z-2
(b) x-2; y-2; z-2
(c) x-1; y-1; z-2
(d) x-1; y-1; z-1

Answer:

(d) x-1; y-1; z-1

 These are the correct values for the coefficients x, y and z in the equation. The equation is written as follows:

        1SO3 (g)           +       1H2O (l)        ⟶      1H2SO4 (aq)
  Sulphur dioxide                Water                       Sulphuric acid



Page No 73:

Question 4:

The equation Mg + O2 ⟶ MgO is
(a) correct and balanced
(b) incorrect since magnesium does not react with oxygen.
(c) correct and not balanced.
(d) incorrect since oxygen should be written as O and not O2.

Answer:

(c) correct and not balanced

Equation is correct. Balanced equation is as follow:
 
2Mg (s)       +         O2 (g)         ⟶          2MgO (s)
Magnesium          Oxygen                    Magnesium oxide

Page No 73:

Question 5:

phosphorus + chlorine ⟶ phosphorus  chloride
(P valency = 3)
The correct balanced equation for this reaction is:

(a) 2P + 3Cl2 ⟶ 2PCl3
(b) P + 3Cl ⟶PCl3
(c) 6P + Cl2 ⟶ 2P3Cl
(d) P + 3Cl2 ⟶ 2PCl3

Answer:

(a) 2P + 3Cl2 ⟶ 2PCl3

This equation is balanced because number of atoms of reactants and products are equal.

Page No 73:

Question 6:

Match the compounds in column I to their formulae in column II.
 

Column I                        Column II
A. sodium sulphide i. Na2SO4
B. magnesium sulphide ii. MgSO4
C. sodium sulphate iii. Na2S
D. magnesium sulphate iv. MgS

(a) A-i; B-ii; C-iii; D-iv
(b) A-iii; B-iv; C-i; D-ii
(c) A-iv; B-iii; C-ii; D-i
(d) A-ii; B-i; C-iv; D-iii

Answer:

(b) A-iii; B-iv; C-i; D-ii

Because, compounds with S2- and SO42- as anions are represented as sulphides and sulphates, respectively.

Page No 73:

Question 7:

Which of these molecules is composed of exactly two atoms?
(a) water
(b) sodium chloride
(c) sodium hydroxide
(d) calcium carbonate

Answer:

(b) sodium chloride

Sodium chloride consists of exactly two atoms. These are sodium and chlorine atoms.

Page No 73:

Question 8:

Which of the following will result in a chemical change?
(a) mixing salt and water
(b) heating candle wax
(c) adding curd to milk
(d) adding water to milk

Answer:

(b) heating candle wax

Because heating of candle wax converts solid wax into liquid water and gaseous carbon dioxide. So, this chemical reaction is characterised by change in physical state.

Page No 73:

Question 9:

MgCO3: magnesium carbonate:: ______________________: zinc carbonate
(a) ZnCO3
(b) Zn(CO3)2
(c) Zn2CO3
(d) ZnCO3

Answer:

(a) ZnCO3
In this compound, Zinc is present as Zn2+ cation and carbonate as CO32- anion. Both these ions satisfy each other's valency to form compound zinc carbonate (ZnCO3).

Page No 73:

Question 10:

Which of the following represents a correct chemical reaction?
(a) nitrogen + oxygen ⟶ hydrogen oxide
(b) carbon + chlorine ⟶ potassium chloride
(c) sulphur + oxide ⟶ sulphur dioxide
(d) sodium + chlorine ⟶ magnesium chloride

Answer:

(c) sulphur + oxide ⟶ sulphur dioxide

    S (g)   +     O2 (g)     ⟶          SO2 (g)
Sulphur        Oxygen               Sulphur dioxide

Page No 73:

Question 11:

Name the following compounds:
CaO, KNO3, HgS, H2O, PbSO4, NaOH, CaCl2, MgCO3, CO2, Zn(OH)2

Answer:

CaO  -  Calcium (II) oxide
KNO3  -  Potassium (I) nitrate
HgS  -   Mercury (II) sulphide
H2O  -  Water (Hydrogen oxide)
PbSO4   -   Lead (II) sulphate
NaOH  -   Sodium hydroxide
CaCl2  -  Calcium (II) chloride
MgCO3  -  Magnesium (II) carbonate
CO2  -  Carbon dioxide
Zn(OH)2  -  Zinc (II) hydroxide

Page No 73:

Question 12:

Define a combination reaction.

Answer:

A reaction in which two or more atoms (or groups of atoms) combine to form a single compound is called combination reaction. For example, magnesium combines with oxygen to form magnesium oxide.
 
   Mg (s)      +      O(g)                  MgO (s)
Magnesium      Oxygen                  Magnesium oxide

Page No 73:

Question 13:

Explain a decomposition reaction with an example. Can an element undergo a decompostion  reaction? Why or why not?

Answer:

Decomposition reaction is a reaction in which a compound breaks into two or more simpler atoms (or group of atoms). For example, calcium carbonate on heating decomposes into calcium oxide and carbon dioxide.

    CaCO3 (s)        Heat        CaO (s)        +          CO2 (g)
Calcium carbonate        Calcium oxide          Carbon dioxide

An element cannot undergo a decomposition reaction because an element exists in its simplest form and thus cannot be further split up chemically.

Page No 73:

Question 14:

Write the following as balanced chemical equations. Also identify the type of reaction in each case.
(i) Zn + O2 ⟶ ZnO
(ii) S + O2 ⟶ SO2
(iii) Na + Cl2 ⟶ NaCl
(iv) Na2O + H2O ⟶ NaOH
(v) HgO ⟶ Hg + O2
(vi) CuCO3 ⟶ CuO + CO2

Answer:

(i) 2Zn + O2 ⟶ 2ZnO
It is a combination reaction.
(ii) S + O2 ⟶ SO2
It is a combination reaction.
(iii) 2Na + Cl2 ⟶ 2NaCl
It is a combination reaction.
(iv) Na2O + H2O ⟶ 2NaOH
It is a combination reaction.
(v) 2HgO ⟶ 2Hg + O2
It is a decomposition reaction.
(vi) CuCO3 ⟶ CuO + CO2
It is a decomposition reaction.



Page No 74:

Question 15:

Write the formula of the following compounds.
calcium hydroxide, copper chloride, iron sulphate, sodium carbonate, copper nitrate, copper oxide, lead sulphide, magnesium chloride, zinc sulphate, potassium hydroxide.

Answer:

Calcium hydroxide - Ca(OH)2
Copper chloride - CuCl2
Iron sulphate - FeSO4
Sodium carbonate - Na2CO3
Copper nitrate - Cu(NO3)2
Copper oxide - CuO
Lead sulphide - PbS
Magnesium chloride - MgCl2
Zinc sulphate - ZnSO4
Potassium hydroxide - KOH

Page No 74:

Question 16:

Find out the mistakes (if any) in the formulae of compounds given here.
Fe2S2, NaSO4, K2CO3, Zn(CO3), Cu1O1, HgO, CA(NO3)2, PbCO3, K2S1, MgSO4, Na(CO)3

Answer:

CA(NO3)2, Fe2S2, NaSO4 and Na(CO)3 are the incorrect formulae, while K2CO3, Zn(CO3), Cu1O1, HgO,  PbCO3, K2S1 and MgSO4 are correct formulae. Corrected formulae for the incorrect ones are as follows:

FeS is the correct formula for iron sulphide.
Na2SO4 is the correct formula for sodium sulphate.
Na2(CO3) is the correct formula for sodium carbonate.
Ca(NO3)2 is the corrected formula for calcium nitrate.

Page No 74:

Question 17:

Convert the following word equations into balanced chemical equations.
(a) Carbon monoxide (CO) burns in oxygen to form carbon dioxide.
(b) Carbon combines with oxygen to form carbon dioxide.
(c) Sodium combines with chlorine to form sodium chloride.

Answer:

(a)   2CO (g)         +          O2 (g)                2CO2 (g)
  Carbon monoxide        Oxygen                  Carbon dioxide

(b)     C (s)            +            O2 (g)                  CO2 (g)
      Carbon                       Oxygen                  Carbon dioxide

(c)    2Na (s)            +        Cl2 (g)                       2NaCl (s)
      Sodium                      Chlorine                      Sodium chloride

Page No 74:

Question 18:

Which of the changes given below are chemical changes? Give reasons for your answer.
(a) ice melting
(b) iron rusting
(c) wet hair drying
(d) magnesium burning
(e) candle wax melting
(f) a lump of sodium carbonate being crushed
(g) clothes being ironed
(h) carbon burning in air to give carbon dioxide
(i) custard being made
(j) burning of petrol

Answer:

(a) Melting of ice is not a chemical change but a physical change because molecules of water remain same but their physical state change.
(b) Rusting of iron is a chemical change because iron reacts with water and oxygen to give iron oxide (rust), which is reddish brown in colour. This reaction is characterised by change in colour.
(c) There is no chemical change in the hair while drying.Thus, it is not a chemical change.
(d) Magnesium burning is a chemical change because it combines with oxygen to form magnesium oxide. Here, chemical properties of magnesium and oxygen is different from the product magnesium oxide.
(e) Candle wax melting is a chemical change because solid wax changes into liquid water and gaseous carbon dioxide.
(f) When a lump of sodium carbonate is crushed, there is no change in its chemical properties.
(g) Ironing is a physical change because there is no chemical change in the fabric of the cloth being ironed.
(h) Carbon burns in air to form carbon dioxide, which is a chemical change. This reaction is characterised by the evolution of gas.

(i) Formation of custard is a chemical change because the chemical properties of milk is different from the custard.
(j) Burning of petrol is a chemical reaction. It is a combustion reaction in which petrol burns with oxygen in air to evolve various gases.

Page No 74:

Question 19:

A  compound has a formula KOH. What elements does it contain?

Answer:

Compound KOH contains K (potassium), O (oxygen) and H (hydrogen) elements.

Page No 74:

Question 20:

Fill in the blanks.
H2O is the _______________ of water. It also represents _______________ molecule of water. There are ________________ atoms of hydrogen and _______________ atom of oxygen in H2O. 2H2O represents _________________ molecules of water. There are _____________ atoms of hydrogen and _______________ atoms of oxygen in 2H2O.

Answer:

H2O is the formula of water. It also represents one molecule of water. There are two atoms of hydrogen and one atom of oxygen in H2O. 2H2O represents two molecules of water. There are four atoms of hydrogen and two atoms of oxygen in 2H2O.

Page No 74:

Question 21:

 The formula of sodium sulphate is Na2SO4.
(a) Name the elements present in it.
(b) Write the number of different atoms present in one molecule of this compound.

Answer:

(a) The elements present in sodium sulphate are Na (sodium), S (sulphur) and O (oxygen).
(b) One molecule of this compound contains two atoms of sodium, one atom of sulphur and four atoms of oxygen.

Page No 74:

Question 22:

Match the terms in column I to those in column II.
 

Column I Column II
a. balanced equation i. substances that start the reaction
b. reactants ii. the number of atoms of each kind is the same on each side
c. products iii. joining together of two elements or compounds
d. symbol iv. substances formed as a result of a reaction
e. combination v. breaking up of a substance
f. decomposition vi. one atom of an element

Answer:

(a)    -    (ii)
(b)    -    (i)
(c)    -    (iv)
(d)    -    (vi)
(e)    -    (iii)
(f)    -     (v)



Page No 75:

Question 23:

Check the formulae of these compounds. Correct the mistakes and then balance the equation.
(a) CaO + HNO3 ⟶ CaNO3 + H2O
(b) Ca(CO3)2 ⟶ CaO2 + CO2
(c) Zn2O2 + H1Cl1 ⟶ZnCl2 + H2O
(d) Hg2O2 + ⟶ Hg + O
(e) MG + H2SO4 ⟶ Mg2(SO4)2 + H2

Answer:

(a) Formula of calcium nitrate is Ca(NO3)2, not CaNO3. The balanced equation is as follows:
 
    CaO      +        2HNO3   ⟶      Ca(NO3)2     +       H2O
Calcium oxide    Nitric acid          Calcium nitrate      Water

(b) Formula of calcium carbonate is Ca(CO3), not Ca(CO3)2. Also, formula of calcium oxide is
CaO, not CaO2. The balanced equation is as follows:
   
Ca(CO3)            ⟶             CaO          +          CO2
Calcium carbonate            Calcium oxide      Carbon dioxide

(c) Formula of zinc oxide is ZnO, not Zn2O2. Also, formula of hydrochloric acid is written as HCl, not H1Cl1. The balanced equation is as follows:
    
ZnO           +           2HCl             ⟶          ZnCl2     +       H2O
 Zinc oxide       Hydrochloric acid            Zinc chloride        Water

(d) Oxygen gas exists as O2​, not O. The balanced equation is as follows:
    
    Hg2O2                  ⟶          2Hg       +       O2
Mercury (I) peroxide              Mercury          Oxygen

(e) Mg is the correct representation for magnesium, not MG. Also, formula of magnesium sulphate is Mg(SO4), not Mg2(SO4)2. The balanced equation is as follows:
   
  Mg        +        H2SO4         ⟶           Mg(SO4)           +         H2
Magnesium     Sulphuric acid          Magnesium sulphate    Hydrogen

Page No 75:

Question 24:

What does each of the following represent?
(a) 2H
(b) 4H2O
(c) Cl2
(d) NaCl
(e) KNO3
(f) 2CuSO4

Answer:

(a) 2H represents two atoms of hydrogen element.
(b) 4H2O represents four molecules of water.
(c) Cl2 represents a chlorine molecule.
(d) NaCl represents a molecule of compound sodium chloride.
(e) KNO3 represents a molecule of compound potassium nitrate.
(f) 2CuSO4 represents two molecules of copper sulphate.



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