Lakhmir Singh & Manjit Kaur Solutions: Chemical Reactions & Equations - 2 - Science Class 10

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Question 22:
(a) Explain, with an example, how the physical states of the reactants and products can be shown in a chemical equation.
(b) Balance the following equation and add state symbols :
Zn + HCl ——– > ZnCl₂ + H₂
(c) Convey the following information in the form of a balanced chemical equation :
“An aqueous solution of ferrous sulphate reacts with an aqueous solution of sodium hydroxide to form a precipitate of ferrous hydroxide and sodium sulphate remains in solution.”
Solution :
(a) The physical states of the reactants and products are shown by putting the “state symbols”
in an equation.
For example: Zn (s) + H₂SO₄ (aq) —–> ZnSO₄ (aq) + H₂(g)Showing physical state of reactants and productsb) Zn (s) + 2HCl(aq) —–> ZnCl₂ (aq) + H₂ (g)
(c) FeSO₄(aq)+ 2NaOH (aq) —–> Fe(OH)₂ (s) + Na₂SO₄(aq)

Question 23:
Write any two observations in an activity that may suggest that a chemical reaction has taken place. Give an example in support of your answer.
Solution :

1.  Evolution of gas.
   For example: When sodium carbonate reacts with dilute hydrochloric acid, carbon dioxide gas is evolved.
2.  Formation of a precipitate.
   For example: When potassium iodide solution is added to a solution of lead nitrate, a yellow precipitate of lead iodide is formed.

Question 24:
(a) Aluminium hydroxide reacts with sulphuric acid to form aluminium sulphate and water. Write a balanced equation for this reaction.
(b) Balance the following chemical equation :
MnO₂ + HCl —–> MnCl₂ + Cl₂ + H₂O
Solution :
(a) 2Al(OH)₃+ 3H₂SO₄ —–> Al₂(SO₄)₃+ 6H₂O
(b) MnO2+ 4HCl —–> MnCl₂+ Cl₂ + 2H₂O

Question 25:
 Write the balanced equations for the following reactions, and add the state symbols (a)  Magnesium carbonate reacts with hydrochloric acid to produce magnesium chloride, carbon dioxide, and water.
(b) Sodium hydroxide reacts with sulphuric acid to produce sodium sulphate and water.
Solution :
(a) MgCO₃ (s) + 2HCl (aq) MgCl₂ (aq) + CO₂ (g) + H₂O (l)
(b) 2NaOH (aq) + H₂SO₄ (aq) Na₂SO₄ (aq) + 2H₂O (l)

Question 26:
Carbon monoxide reacts with hydrogen under certain conditions to form methanol (CH₃OH). Write a balanced chemical equation for this reaction indicating the physical states of reactants and product as well as the conditions under which this reaction takes place.
Solution :

The conditions for this reaction to take place are a pressure of 300 atmospheres (written as 300 atm), a temperature of 300oC, and a catalyst which is a mixture of zinc oxide and chromium oxide (ZnO + CrO₃).

Question 27:
(a) Potassium chlorate (KClO₃) on heating forms potassium chloride and oxygen. Write a balanced equation
for this reaction and indicate the evolution of gas.
(b) Rewrite the following information in the form of a balanced chemical equation :
Magnesium burns in carbon dioxide to form magnesium oxide and carbon.
Solution :
(a) 2KClO₃
(s) 2KCl (s) + 3O₂ (g)
(b) 2Mg + CO₂ —–> 2MgO + C

Question 28:
(a) Substitute formulae for names and balance the following equation :
Calcium carbonate reacts with hydrochloric acid to produce calcium chloride, water, and carbon dioxide gas.
(b) Write a balanced chemical equation with state symbols for the following reaction: Sodium hydroxide solution reacts with a hydrochloric acid solution to produce sodium chloride solution and water.
Solution :
(a) CaCO₃+ 2HCl —–> CaCl₂ + H₂O + CO₂
(b) NaOH (aq) + HCl(aq) —–> NaCl (aq) + H₂O (l)

Question 29:
Ammonia reacts with oxygen to form nitrogen and water. Write a balanced chemical equation for this reaction. Add the state symbols for all the reactants and products.
Solution :
4NH₃ (g)+ 3O₂ (g) —–> 2N₂ (g) + 6H₂O (l)

Question 30:
Write a balanced chemical equation for the process of photosynthesis giving the physical states of all the substances involved and the conditions of the reaction.
Solution :
6CO₂ (g) + 6H₂OC₆ —–> H₁₂O₆ (aq) + 6O₂(g)
Carbon
dioxide Water
Glucose Oxygen

Question 31:
Translate the following statement into a chemical equation and then balance it :
Barium chloride solution reacts with aluminium sulphate solution to form a precipitate of barium sulphate and aluminium chloride solution.
Solution :
3BaCl₂(aq) + Al₂(SO₄)₃ (aq) —–> 3BaSO₄ (s) + 2AlCl₃(aq)

Question 32:
When potassium nitrate is heated, it decomposes into potassium nitrite and oxygen. Write a balanced equation for this reaction and add the state symbols of the reactants and products.
Solution :
2KNO₃ (s) —–> 2KNO₂ (s) + O₂(g)

Question 33:
(a) What is meant by a chemical reaction? Explain with the help of an example.
(b) Give one example each of a chemical reaction characterized by :

1. evolution of a gas
2. change in colour
3. formation of a precipitate
4. change in temperature
5. change in state.

Solution :
(a)Chemical reactions are the processes in which new substances with new properties are formed.
For example: When magnesium ribbon is heated, it burns in air to form a white powder called magnesium oxide.
(b)The chemical reaction between zinc and dilute sulphuric acid.

1. The chemical reaction between citric acid and purple coloured potassium permanganate solution is characterized by change in colour (from purple to
   colourless).
2. The chemical reaction between potassium iodide and lead nitrate is characterized by the formation of a yellow precipitate of lead iodide.
3. The reaction between quick lime and water to form slaked lime is characterized by a change in temperature.

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Question 34:
(a) State the various characteristics of chemical reactions.
(b) State one characteristic each of the chemical reaction which takes place when :

1. dilute hydrochloric acid is added to sodium carbonate
2. lemon juice is added gradually to potassium permanganate solution
3. dilute sulphuric acid is added to barium chloride solution
4. quicklime is treated with water
5. wax is burned in the form of a candle

Solution :
(a) The various characteristics of chemical reactions are:

1. Evolution of a gas
2. Formation of a precipitate
3. Change in colour
4. Change in temperature
5. Change in state.

(b) Evolution of carbon dioxide gas

1. Change in colour from purple to colourless
2. Formation of white precipitate of barium sulphate
3. Change in temperature
4. Change in state from solid to liquid and gas.

Question 35:
(a) What do you understand by exothermic and endothermic reactions ?
(b) Give one example of an exothermic reaction and one of an endothermic reaction.
(c) Which of the following are endothermic reactions and which are exothermic reactions ?

1. Burning of natural gas
2. Photosynthesis
3. Electrolysis of water
4. Respiration
5. Decomposition of calcium carbonate

Solution :
(a) Those reactions in which heat is evolved are known as exothermic reactions.
The reactions in which heat is absorbed are known as endothermic reactions.
(b) Example of exothermic reaction:
C (s) + O₂ (g) —–> CO₂ + Heat
Example of endothermic reaction:
N₂ (g) + O₂ (g) + Heat —–> 2NO (g)
(c) Endothermic reactions: Photosynthesis, Electrolysis of water, Decomposition of calcium carbonate.
Exothermic reactions: Burning of natural gas, Respiration.

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Question 46:
When the solution of substance X is added to a solution of potassium iodide, then a yellow solid separates out from the solution.
(a) What do you think substance X is likely to be ?
(b) Name the substance which the yellow solid consists of.
(c) Which characteristic of chemical reactions is illustrated by this example ?
(d) Write a balanced chemical equation for the reaction which takes place. Mention the physical states of all the reactants and products involved in the chemical equation.
Solution :
(a) Lead nitrate.
(b) Lead iodide.
(c) Formation of a precipitate.
(d) Pb(NO₃)₂ (aq) + 2KI (aq) —–> PbI₂ (s) + 2KNO₃ (aq)

Question 47:
When water is added gradually to a white solid X, a hissing sound is heard and a lot of heat is produced forming a product Y. A suspension of Y in water is applied to the walls of a house during white washing. A clear solution of Y is also used for testing carbon dioxide gas in the laboratory.
(a) What could be solid X ? Write its chemical formula.
(b) What could be product Y ? Write its chemical formula.
(c) What is the common name of the solution of Y which is used for testing carbon dioxide gas ?
(d) Write chemical equation of the reaction which takes place on adding water to solid X.
(e) Which characteristic of chemical reactions is illustrated by this example ?
Solution :
(a) Calcium oxide, CaO.
(b) Calcium hydroxide, Ca(OH)₂
(c) Lime water.
(d) CaO + H₂O —–> Ca(OH)₂
(e) Change in temperature.

Question 48:
When metal X is treated with a dilute acid Y, then a gas Z is evolved which burns readily by making a little explosion.
(a) Name any two metals which can behave like metal X.
(b) Name any two acids which can behave like acid Y.
(c) Name the gas Z.
(d) Is the gas Z lighter than or heavier than air ?
(e)Is the reaction between metal X and dilute acid Y exothermic or endothermic ?
(f) By taking a specific example of metal X and dilute acid Y, write a balanced chemical equation for the reaction which takes place. Also indicate physical states of all the reactants and products.
Solution :
(a) Zinc and Iron.
(b) Dilute hydrochloric acid and dilute sulphuric acid.
(c) Hydrogen.
(d) Lighter than air.
(e) Exothermic.
(f) Suppose metal X is zinc (Zn) and acid Y is dilute hydrochloric acid (HCl) ;
Zn (s) + 2HCl (aq) ZnCl₂ (aq) + H2 (g)

Question 49:
A solid substance P which is very hard is used in the construction of many buildings, especially flooring. When substance P is heated strongly, it decomposes to form another solid Q and a gas R is given out. Solid Q reacts with water with the release of a lot of heat to form a substance S. When gas R is passed into a clear solution of substance S, then a white precipitate of substance T is formed. The substance T has the same chemical composition as starting substance P.
(a) What is substance P ? Write its common name as well as chemical formula.
(b) What is substance Q ?
(c) What is gas R ?
(d) What is substance S ? What is its clear solution known as ?
(e) What is substance T ? Name any two natural forms in which substance T occurs in nature.
Solution :
(a) Calcium carbonate (limestone), CaCO₃
(b) Calcium oxide, CaO
(c) Carbon dioxide, CO₂
(d) Calcium hydroxide, Ca(OH)₂; Lime water.
(e) Calcium carbonate; Limestone and Marble.