1. 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 take place. Mention the physical states of all the reactants and products involved in the chemical equation.

a) Lead nitrate

b) Lead iodide

c) Formation of a precipitate

d) Pb(NO₃)₂(aq) + 2Kl (aq) → Pbl₂(s) + 2KNO₃(aq)

2. 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?

a) Calcium oxide, CaO

b) Calcium hydroxide, Ca(OH)₂

c) Lime water

d) CaO + H₂O → Ca(OH)₂

e) Change in temperature

3. 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.

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)

4. 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.

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

5. A silvery-white metal X taken in the form of ribbon, when ignited, burns in air with a dazzling white flame to form a white powder Y. When water is added to powder Y, it dissolves partially to form another substance Z.

a) What could metal X be?

b) What is powder Y?

c) With which substance metal X combines to form powder Y?

d) What is substance Z? Name one domestic use of substance Z.

e) Write a balanced chemical equation of the reaction which takes place when metal X burns in air to form powder Y.

a) Magnesium, Mg

b) Magnesium oxide, MgO

c) Oxygen (of air), O₂

d) Magnesium hydroxide, Mg(OH)₂; Used as antacid to relieve indigestion

e) 2Mg + O₂ →2MgO

6. A metal X forms a salt XSO₄. The salt XSO₄ forms a clear solution in water which reacts with sodium hydroxide solution to form a blue precipitate Y. Metal X is used in making electric wires and alloys like brass.

a) What do you think metal X could be?

b) Write the name, formula and colour of salt XSO₄

c) What is the blue precipitate Y?

d) Write a chemical equation of the reaction which takes place when salt XSO₄ reacts with sodium hydroxide solution. Give the state symbols of all the reactants and products which occur in the above equation.

a) Copper, Cu

b) Copper sulphate, CuSO₄, Blue colour

c) Copper hydroxide, Cu(OH)₂

d) CuSO₄(aq) + 2NaOH(aq) → Cu(OH)₂(s) + Na₂SO₄(aq)

7. The metal M reacts vigorously with water to form a solution S and a gas G. The solution S turns red litmus to blue whereas gas G, which is lighter than air, burns with a pop sound. Metal M has a low melting point and it is used as a coolant in nuclear reactors.

a) What is metal M?

b) What is solution S? Is it acidic or alkaline?

c) What is gas G?

d) Write a balanced chemical equation for the reaction which takes place when metal M reacts with water.

e) Is this reaction exothermic or endothermic?

a) Sodium, Na

b) Sodium hydroxide solution (NaOH solution), Alkaline

c) Hydrogen, H₂

d) 2Na + 2H₂O → 2NaOH + H₂

e) Exothermic

8. When a mixture of gases X and Y is compressed to 300 atm pressure and then passed over a catalyst consisting of a mixture of zinc oxide and chromium oxide (heated to a temperature of 300⁰C), then an organic compound Z having the molecular formula CH₄O is formed. X is a highly poisonous gas which is formed in appreciable amounts when a fuel burns in a limited supply of air; Y is a gas which can be made by the action of a dilute acid on an active metal; and Z is a liquid organic compound which can react with sodium metal to produce hydrogen gas.

a) What are X, Y and Z?

b) Write a balanced chemical equation of the reaction which takes place when X and Y combine to form Z. Indicate the conditions under which the reaction occurs.

a) X is carbon monoxide gas(CO); Y is hydrogen gas ( H₂); Z is methanol (or methyl alcohol) (CH₃OH)

Note. The molecular formula  CH₄O for compound Z tells us that one molecule of this compound contains 1 carbon atom, 4 hydrogen atoms and 1 oxygen atom (CH₄O = CH₃OH)

b) For chemical equation,

  CO (g) + 2H₂ (g) → CH₃OH(l)

9. The white solid compound A decomposes quite rapidly on heating in the presence of a black substance X to form a solid compound B and a gas C. When an aqueous solution of compound B is reacted with silver nitrate solution, then a white precipitate of silver chloride is obtained along with potassium nitrate solution. Gas C does not burn itself but helps burn other things.

a) What is compound A?

b) What is compound B?

c) What is gas C?

d) What do you think is the black substance X? What is its function?

e) What is the general name of substances like X?

a) Potassium chlorate, KClO₃

b) Potassium chloride, KCl

c) Oxygen, O₂

d) Manganese dioxide, MnO₂; It acts as a catalyst in the decomposition of potassium chlorate to form oxygen gas

e) Catalysts

10. Gas A, which is the major cause of global warming, combines with hydrogen oxide B in nature in the presence of an environmental factor C and a green material D to form a six carbon organic compound E and a gas F. The gas F is necessary for breathing.

a) What is gas A?

b) What is the common name of B?

c) What do you think could be C?

d) What is material D? Where is it found?

e) Name the organic compound E.

f) What is gas F? Name the natural process during which it is released.

a) Carbon dioxide, CO₂

b) Water, H₂O

c) Sunlight

d) Chlorophyll; Green leaves of plants

e) Glucose, C₆H₁₂O₆

f) Oxygen; Photosynthesis