ICSE Class 7 Chemistry · Selina Solutions Concise Chemistry Class 7 ICSE · 33 solved questions
A chemical equation is a statement that describes a chemical change in terms of:
Symbols and formulae
Reason — A chemical equation is a symbolic representation of a chemical reaction. It shows the reactants and products using their chemical symbols and formulae, along with their relative amounts.
Balancing a chemical equation is based on:
Law of conservation of mass
Reason — Balancing a chemical equation is based on the Law of Conservation of Mass, because atoms are neither created nor destroyed, only rearranged. We don’t balance equations by measuring mass; we balance atoms. Symbols and formulae are used to write equations, not to balance them.
Copper carbonate when heated, turns :
Black
Reason — When copper carbonate is heated, it turns black due to the formation of copper(II) oxide (CuO).
$\underset{\text{green solid}}{\text{CuCO}_3}\xrightarrow{\text{heat}}\underset{\text{black solid}}{\text{CuO}}+ \underset{\text{gas}}{\text{CO}_2}$
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, lead iodide is formed as an insoluble yellow solid, so it separates out as a yellow precipitate.
$\text{Lead acetate} + \text{Potassium iodide} \rightarrow \text{Lead iodide} + \text{Potassium acetate}$
$\text{Pb(CH}_3\text{COO)}_2 + 2\text{KI} \rightarrow \text{PbI}_2 \downarrow + \space 2\text{CH}_3\text{COOK}$
This gas has a rotten egg smell:
Hydrogen sulphide
Reason — Hydrogen sulphide has a rotten egg smell.
Ammonia has a sharp, pungent smell. Hydrogen and oxygen are odourless.
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.
Sodium carbonate + Hydrochloric acid (dil.) ⟶ Sodium chloride + Water + Carbon dioxide
One of the conditions necessary for a chemical reaction to occur is :
Physical contact between reactants
Reason — For a chemical reaction to take place, the reactants must come into close contact, i.e., they should be mixed.
Others are outcomes of a reaction not necessary conditions for a chemical reaction. Gas may or may not be produced in some reactions. Some reactions show colour change. But many reactions occur without any visible colour change. A precipitate is formed only in certain reactions not in all.
What happens when a chemical reaction takes place ?
All of the above
Reason — When a chemical reaction takes place, the original substances (reactants) are transformed into new substances (products). This transformation can be accompanied by observable changes, such as formation of a precipitate, evolution of a gas, or change in colour.
Assertion (A): A chemical reaction can be written in the form of a chemical equation with the help of symbols and formulae of the substances involved.
Reason (R): In a chemical equation, the reactants are written on the right hand side and products are written on the left hand side of the arrow.
A is true but R is false.
Reason — A is true as a chemical reaction is written using symbols and formulae.
R is false as reactants are written on the left and products are written on the right side.
Therefore, A is true but R is false is the correct option.
Assertion (A): There is a need to balance a chemical equation.
Reason (R): Matter can neither be created nor be destroyed, it can only be converted from one form to another.
Both A and R are true and R is the correct explanation of A.
Reason — A is true as we need to balance a chemical equation so that the number of atoms of each element is the same on both sides.
R is true as the law of conservation of mass states that mass can neither be created nor destroyed, it can only be transformed from one form to another.
Therefore, Both A and R are true and R is the correct explanation of A is the correct option.
Assertion (A): When a few drops of an aqueous solution of silver nitrate are added to the aqueous solution of sodium chloride, a white precipitate of silver chloride is formed.
Reason (R): Some substances react only when they are heated together.
Both A and R are true but R is not the correct explanation of A.
Reason — A is true as when silver nitrate solution is added to sodium chloride solution, a white precipitate of silver chloride is formed.
R is true as some reactants need to be heated to undergo a chemical change.
Silver nitrate + Sodium chloride ⟶ Silver chloride + Sodium nitrate
But in this reaction no heating was required. Therefore, Both A and R are true but R is not the correct explanation of A is the correct option.
Assertion (A): The correct formula of magnesium chloride is Mg2Cl.
Reason (R): The combining capacity of magnesium is two, while that of chlorine is one.
A is false but R is true.
Reason — A is false as the correct formula of magnesium chloride is MgCl2, not Mg2Cl.
R is true as magnesium has valency 2, while chlorine has valency 1.
Therefore, A is false but R is true is the correct option.
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.
Match the following:
| Column A | Column B |
|---|---|
| (a) The substance undergoing a chemical change | (i) Product |
| (b) The reaction in which energy is released | (ii) Photosynthesis |
| (c) The substance that increases the speed of a chemical reaction | (iii) Reactant |
| (d) A photochemical reaction | (iv) Exothermic reaction |
| (e) Represented on the right side of the arrow in a chemical equation | (v) Catalyst |
| Column A | Column B |
|---|---|
| (a) The substance undergoing a chemical change | (iii) Reactant |
| (b) The reaction in which energy is released | (iv) Exothermic reaction |
| (c) The substance that increases the speed of a chemical reaction | (v) Catalyst |
| (d) A photochemical reaction | (ii) Photosynthesis |
| (e) Represented on the right side of the arrow in a chemical equation | (i) Product |
Name the gas evolved when :
(a) Calcium carbonate is heated
(b) Potassium chlorate is heated
(c) Zinc reacts with dilute hydrochloric acid
(d) Iron sulphide is heated with dilute hydrochloric acid.
(a) Carbon dioxide (CO2)
(b) Oxygen (O2)
(c) Hydrogen (H2)
(d) Hydrogen sulphide (H2S)
Name the type of energy involved in the given reactions from the following list:
[Light energy, heat energy and electrical energy]
(a) Decomposition of water
(b) Photosynthesis
(c) Magnesium and oxygen
(a) Decomposition of water ⟶ Electrical energy
(b) Photosynthesis ⟶ Light energy
(c) Magnesium and oxygen ⟶ Heat energy
Define:
(a) Chemical reaction
(b) Catalyst.
(a) Any chemical change in matter which involves its transformation into one or more new substances is called a chemical reaction.
(b) A catalyst is a substance which changes the rate of a chemical reaction without itself undergoing any chemical change.
Differentiate between Reactants and products.
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 the product. |
State two ways in which a chemical reaction can be represented by showing the reactants and products.
Two ways in which a chemical reaction can be represented are:
What happens to the atoms of the reacting molecules during a reaction? Do they get destroyed?
According to the Law of Conservation of matter, the atoms themselves are neither created nor destroyed during the course of a chemical reaction. They only rearrange themselves to form new substances.
Write your observations when the following reactions take place:
(a) Sugar is heated.
(b) Dilute acetic acid is poured on baking soda.
(c) An aqueous solution of sodium chloride is mixed with an aqueous solution of silver nitrate.
(d) Ammonium chloride is heated with sodium hydroxide.
(e) Water is added to quick lime.
(f) A glass rod dipped in ammonia solution is brought near a test tube containing concentrated hydrochloric acid.
(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 dilute acetic acid is poured on baking soda, a strong effervescence is observed. The products formed are sodium acetate, water and carbon dioxide.
(c) 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.
(d) 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.
(e) A large amount of heat energy is evolved when water is added to quick lime. The product formed is calcium hydroxide.
(f) Dense white fumes are observed due to the production of ammonium chloride (which is a solid).
Write word equations for the following reactions:
(a) Magnesium is burnt in oxygen.
(b) Copper carbonate is heated.
(c) Hydrogen gas is burnt in oxygen.
(d) Sodium reacts with water.
(e) Iron is heated with sulphur.
(a) Magnesium + Oxygen $\xrightarrow{\text{heat}}$ Magnesium oxide
(b) Copper carbonate (green solid) $\xrightarrow{\text{heat}}$ Copper oxide (black solid) + Carbon dioxide
(c) Hydrogen + Oxygen $\xrightarrow{\text{heat}}$ Water
(d) Sodium + Water ⟶ Sodium hydroxide + Hydrogen
(e) Iron + Sulphur $\xrightarrow{\text{heat}}$ Iron sulphide
Identify the reactants and products in the following equations and represent them by word equations.
(a) Zn + HCl ⟶ ZnCl2 + H2
(b) CO + O2 ⟶ CO2
(c) Ca + O2 ⟶ CaO
(d) BaCl2 + Na2SO4 ⟶ BaSO4 + NaCl
(e) FeS + HCl ⟶ FeCl2 + H2S
(a) Zn + HCl ⟶ ZnCl2 + H2
Word Equation:
Zinc + Hydrochloric acid ⟶ Zinc chloride + Hydrogen gas
(b) CO + O2 ⟶ CO2
Word Equation:
Carbon monoxide + Oxygen ⟶ Carbon dioxide
(c) Ca + O2 ⟶ CaO
Word Equation:
Calcium + Oxygen ⟶ Calcium oxide
(d) BaCl2 + Na2SO4 ⟶ BaSO4 + NaCl
Word Equation:
Barium chloride + Sodium sulphate ⟶ Barium sulphate + Sodium chloride
(e) FeS + HCl ⟶ FeCl2 + H2S
Word Equation:
Iron(II) sulphide + Hydrochloric acid ⟶ Iron(II) chloride + Hydrogen sulphide
Write chemical equations for the following word equations using the symbols and formulae of the reactants and products.
(a) Calcium carbonate ⟶ Calcium oxide + Carbon dioxide
(b) Zinc oxide + Carbon ⟶ Zinc + Carbon monoxide
(c) Iron + Sulphur ⟶ Iron sulphide
(d) Calcium oxide + Water ⟶ Calcium hydroxide
(e) Carbon + Oxygen ⟶ Carbon dioxide
(f) Sulphur + Oxygen ⟶ Sulphur dioxide
(a) CaCO3 $\xrightarrow{\text{heat}}$ CaO + CO2
(b) ZnO + C ⟶ Zn + CO
(c) Fe + S ⟶ FeS
(d) CaO + H2O ⟶ Ca(OH)2
(e) C + O2 ⟶ CO2
(f) S + O2 ⟶ SO2
State the law of conservation of matter.
According to the law of conservation of matter, "Matter can neither be created nor be destroyed. It can only be transformed from one form to another."
State four conditions necessary for chemical reactions to take place.
The four conditions necessary for chemical reactions to take place are:
(a) What is a chemical equation?
(b) Why do we need to balance chemical equations?
(a) 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.
(b) 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.
Differentiate between chemical reaction and chemical equation by giving suitable examples.
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 |
Differentiate between exothermic and endothermic reaction by giving suitable examples.
Difference between exothermic and endothermic reaction:
| Exothermic reaction | Endothermic reaction |
|---|---|
| The chemical reactions in which energy is released are called exothermic reactions. | The chemical reactions in which energy is absorbed are called endothermic reactions. |
| Example: Calcium oxide + Water ⟶ Calcium hydroxide + Heat | Example: Calcium carbonate $\xrightarrow{heat}$ Calcium oxide + Carbon dioxide |
Differentiate between skeletal and balanced chemical equation by giving suitable examples.
Difference between skeletal and balanced chemical equation:
| Skeletal chemical equation | Balanced chemical equation |
|---|---|
| In a skeletal chemical equation, the total number of atoms on the reactant side and product side are not equal. | In a balanced chemical equation, the total number of atoms on the reactant side as well as the product side are equal. |
| Example: H2 + O2 ⟶ H2O | Example: Zn + H2SO4 ⟶ ZnSO4 + H2 |
A chemical reaction is represented as follows:
H2 + Cl2 ⟶ 2HCl
(a) What information do you get from this equation ?
(b) Write word equation for the above equation.
(a) The given equation gives us the following information:
(b) Hydrogen + Chlorine ⟶ Hydrogen chloride
A teacher demonstrated an experiment in the class to show the evolution of a gas during a chemical reaction. She took some baking soda in a test tube and added some vinegar to it.
(a) What do you observe when vinegar is added to baking soda?
(b) Name the gas evolved.
(c) Give the chemical names of baking soda and vinegar.
(d) Why is baking soda used while baking cakes?
(e) In which food item can vinegar be used in place of lemon?
(a) When vinegar is added to baking soda, strong effervescence (bubbles) is seen.
(b) The gas evolved is carbon dioxide.
(c) The chemical name of baking soda is Sodium hydrogen carbonate and Vinegar is dilute acetic acid.
(d) Baking soda is used while baking cakes because, on heating or on reacting with acidic ingredients, it releases carbon dioxide gas. This gas gets trapped in the batter and makes the cake soft, spongy and fluffy.
(e) Vinegar can be used in salads in place of lemon.
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