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Solutions for Chemistry, Class 7, ICSE
(a) Define a chemical reaction.
(b) What is a chemical equation?
(c) Why do we need to balance chemical equations?
(a) Any chemical change in matter which involves its transformation into one or more new substances is called a chemical reaction.
(b) A chemical reaction can be represented with the help of symbols or the formulae of the elements and the compounds taking part in the reaction and the product formed, in the form of a chemical equation.
(c) A chemical equation needs to be balanced so as to make the number of atoms of the reactants equal to the number of atoms of the products because a chemical reaction is just a rearrangement of atoms. The atoms themselves are neither created nor destroyed during the course of a chemical reaction.
State four conditions necessary for chemical reactions to take place.
The four conditions necessary for chemical reactions to take place are:
Differentiate between:
(a) Reactants and products
(b) Chemical reaction and chemical equation
(c) A balanced and a skeletal chemical equation
(a) Difference between reactants and products :
| Reactants | Products |
|---|---|
| The substances that undergo chemical change are called reactants. | The new substances formed as a result of reaction are called products. |
| Reactants are written on the left hand side of the equation. | Products are written on the right hand side of the equation. |
| In equation C + O2 ⟶ CO2, C and O2 are reactants. | In equation C + O2 ⟶ CO2, CO2 is product. |
(b) Difference between chemical reaction and chemical equation :
| Chemical reaction | Chemical equation |
|---|---|
| Any chemical change in matter which involves its transformation into one or more new substances is called a chemical reaction. | A chemical reaction can be represented with the help of symbols or the formulae of the elements and the compounds taking part in the reaction in the form of a chemical equation. |
| Example of chemical reaction is burning of candle. | Example — 2H2 + O2 ⟶ 2H2O |
(c) Difference between a balanced and a skeletal chemical equation :
| Balanced chemical equation | Skeletal chemical equation |
|---|---|
| In balanced chemical equation the total number of atoms on the reactant side as well as the product side are equal. | In skeletal chemical equation the total number of atoms on the reactant side and product side are not equal. |
| Example: Zn + H2SO4 ⟶ ZnSO4 + H2 | Example: H2 + O2 ⟶ H2O |
Write word equations for the following skeletal equations:
(a) KClO3 ⟶ KCl + O2
(b) Zn + HCl ⟶ ZnCl2 + H2
(c) FeCl2 + Cl2 ⟶ FeCl3
(d) CO + O2 ⟶ CO2
(e) Ca + O2 ⟶ CaO
(f) Na + O2 ⟶ Na2O
(g) NaOH + H2SO4 ⟶ Na2SO4 + H2O
(h) AgBr ⟶ Ag + Br2
(i) KNO3 ⟶ KNO2 + O2
(j) FeS + 2HCl ⟶ FeCl2 + H2S
(k) Na2CO3 + 2HCl ⟶ 2NaCl + H2O + CO2
(l) 2H2 + O2 ⟶ 2H2O
(m) Na2O + H2O ⟶ 2NaOH
(a) Potassium chlorate ⟶ Potassium chloride + Oxygen (gas)
(b) Zinc + Hydrochloric acid (dil.) ⟶ Zinc chloride + Hydrogen (gas)
(c) Ferrous chloride + Chlorine ⟶ Ferric chloride
(d) Carbon monoxide + Oxygen ⟶ Carbon dioxide
(e) Calcium + Oxygen ⟶ Calcium oxide
(f) Sodium + Oxygen ⟶ Sodium oxide
(g) Sodium hydroxide + Sulphuric acid ⟶ Sodium sulphate + Water
(h) Silver bromide ⟶ Silver + Bromine
(i) Potassium nitrate ⟶ Potassium nitrite + Oxygen (gas)
(j) Iron sulphide + Hydrochloric acid (dil.) ⟶ Iron chloride + Hydrogen sulphide (gas)
(k) Sodium carbonate + Hydrochloric acid (dil.) ⟶ Sodium chloride + Water + Carbon dioxide (gas)
(l) Hydrogen + Oxygen ⟶ Water
(m) Sodium oxide + Water ⟶ Sodium hydroxide
Write your observations for the following chemical reactions and name the products formed:
(a) When sugar is heated.
(b) When manganese dioxide is added to potassium chlorate and heated.
(c) When dilute acetic acid is poured on baking soda.
(d) When an aqueous solution of sodium chloride is mixed with an aqueous solution of silver nitrate.
(e) When ammonium chloride is heated with sodium hydroxide.
(f) When water is added to quick lime.
(a) When sugar is heated it first melts and then gets charred into a black residue. The products formed are charcoal and water vapour.
(b) When manganese dioxide is added to potassium chlorate and heated it decomposes to give potassium chloride and oxygen.
(c) When dilute acetic acid is poured on baking soda, a strong effervescence is observed. The products formed are sodium acetate, water and carbon dioxide.
(d) A white precipitate is formed when an aqueous solution of sodium chloride is mixed with an aqueous solution of silver nitrate. The products formed are silver chloride and sodium nitrate.
(e) A strong pungent smelling gas is evolved when ammonium chloride is heated with sodium hydroxide. The products formed are sodium chloride, water and ammonia gas.
(f) A large amount of heat energy is evolved when water is added to quick lime. The product formed is calcium hydroxide.
What information do you get from the equation
H2 + Cl2 ⟶ 2HCl?
We get the following information from the given chemical equation:
Fill in the blanks:
(a) The substances which undergo chemical change are called ............... .
(b) The substances formed as a result of a chemical reaction are called ............... .
(c) During a chemical reaction, transfer of ............... takes place.
(d) The basic condition necessary for a chemical reaction is ............... .
(e) In some chemical reactions, an insoluble ............... is formed when two solutions are mixed.
(a) Reactants
(b) Products
(c) Energy
(d) close contact
(e) precipitate
Write True or False for the following statements:
(a) No new substance is formed during a chemical reaction.
(b) Hydrogen sulphide has rotten egg smell.
(c) When potassium iodide solution is added to lead acetate solution, a red precipitate is formed.
(d) A black residue is formed when sugar is heated.
(e) When iron and sulphur are heated together a grey mass is formed which is attracted by a magnet.
(a) False
Correct Statement — New substance is formed during a chemical reaction.
(b) True
(c) False
Correct Statement — When potassium iodide solution is added to lead acetate solution, a yellow precipitate is formed.
(d) True
(e) False
Correct Statement — When iron and sulphur are heated together a grey mass is formed which is not attracted by a magnet.
A chemical equation is a statement that describes a chemical change in terms of:
Symbols and formulae
Reason — A chemical reaction can be represented with the help of symbols or the formulae of the elements and the compounds taking part in the reaction in the form of a chemical equation.
Balancing a chemical equation is based on:
Law of conservation of mass
Reason — Balancing a chemical equation is based on Law of conservation of mass i.e. the atoms themselves are neither created nor destroyed during the course of a chemical reaction.
Copper carbonate when heated turns:
Black
Reason — Copper carbonate when heated produces copper oxide which is black in colour and carbon dioxide gas.
When lead acetate solution is added to potassium iodide solution a precipitate is formed which is:
Yellow
Reason — When lead acetate solution is added to potassium iodide solution a yellow precipitate of lead iodide is formed.
This gas has a rotten egg smell:
Hydrogen sulphide
Reason — Hydrogen sulphide has rotten egg smell.
When sodium carbonate reacts with dilute hydrochloric acid, the gas evolved is:
Carbon dioxide
Reason — When sodium carbonate reacts with dilute hydrochloric acid, sodium chloride, water and carbon dioxide gas is produced.