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X Chemistry Sample Question Chemistry for Term I (September 2012)
Q.1 Give an example of an exothermic reaction.
Ans. Burning of natural gas is an exothermic reaction.
CH4 + 2O2
CO2 + 2H2O
Q.2. A compound which is prepared from gypsum has a property of hardening when mixed with water.
Identify the compound. Write the chemical equation of its preparation. Mention two uses of the
compound.
Ans. The compound is plaster of Paris (CaSO4. 1/2H2O) and its chemical name is calcium sulphate
hemihydrate.
Preparation: It is prepared by heating gypsum (calcium sulphate dehydrate) at 373 K
CaSO4.2H2O ---273k--- CaSO4.1/2 H2O
Gypsum
+
3/2 H2O
Plaster of Paris
water
Uses of plaster of Paris are:
1. It is used for plastering the fractured bones in hospitals.
2. It is used for making toys and statues.
Q.3 Name a metal which is a good conductor of heat and a metal which is a poor conductor of heat.
Ans. Silver is a good conductor of heat and mercury is a poor conductor of heat.
Q.4 Give the composition and one use of following alloys:
(a) Solder
Ans.
(b) Bronze
Alloy
Composition
(a) Solder
(b) Bronze
Use
Lead and Tin
For welding electrical wires.
Copper and tin
For making statues.
Q.5 Arrange the following substances in order of increasing acidity:
A with pH 1;
B with pH 11;
C with pH 7;
D with pH 4
Ans. Following is the sequence of given substances in order of increasing acidity: B < C < D < A
Q.6 Give the reaction and colour change observed when:
(i) Copper is heated in the presence of air.
(ii) Silver articles are exposed to air for sometime.
(iii) Carbon dioxide gas is passed through lime water.
Ans. (i) Copper oxidises to black coloured copper oxide. The reaction takes place as:
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2Cu + O2- 2CuO
Copper oxide (black)
(ii) Silver reacts with sulphur to give a black coating of silver sulphide. The reaction takes place as:
2Ag + S
- Ag2S
Silver sulphide (black)
(iii) White precipitate of calcium carbonate is formed. The reaction takes place as:
Ca(OH)2 + CO2 -CaCO3 + H2O
Calcium carbonate (white)
Q.7 See the reaction below and answer which of the substance is getting reduced and which of the
substance is getting oxidised?
2PbO + C
2Pb + CO2
Ans. Oxidation is the process of involving the loss of elctrons and reduction is the process of involving
the gain of electrons . Hence in the given equation, PbO is getting reduced and C is getting oxidised.
Q8. Give reactions with water to show that? magnesium is less reactive than sodium
Ans. Sodium reacts vigorously with cold water to form sodium hydroxide whereas magnesium cannot
react with cold water; it reacts with hot water to form magnesium hydroxide. Reactions involved are as
follows:
2Na
+
Sodium
2H2O -------- 2NaOH + H2
Cold water
Mg
+
2H2O ------ Mg(OH)2 + H2
Magnesium
Hot water
Q.9. Give reasons for (i) Decomposition reactions are called endothermic reactions. (ii) Antioxidants are
added to foods containing fats and oil. (iii) Sodium and potassium are kept in kerosene oil.
Ans. (i) Decomposition reactions require energy either in the form of heat, light or electricity for breaking
down the reactants. Hence they are called endothermic reactions.
(ii) When fats and oil are oxidized, they become rancid and their smell and taste change. Thus in orders to
prevent the oxidation of fats and oil, antioxidants are added to the foods containing fats and oil.
(iii) Sodium and potassium are very reactive and reacts so vigorously with oxygen that they catch fire, if
kept in air. Hence these are kept in kerosene to prevent accidental fires.
Q.10. Why does the colour of the copper sulphate solution changes when iron nails are dipped in it?
Ans. Iron is more reactive than copper due to low reduction potential and hence displaces copper from its
salt solution (copper sulphate). Thus, colour of the copper sulphate solution changes blue to dirty green..
The reaction takes place as following:
CuSO4
Copper sulphate
+
Fe
--------------
Iron
FeSO4
Ferrous sulphate
+
Cu
copper
Q.11. Why does dry HCl gas not turn dry blue litmus red although it is an acid?
Ans. Acids produce H+ ions only in the presence of water and thus can show acidic behaviour only in the
presence of water. Therefore dry HCl gas does not show any change on dry litmus paper.
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Q.12. What should be added to a cake to make it soft and fluffy? What is the composition of the
substance added?
Ans. Baking powder should be added to a cake to make it soft and fluffy. It consists of sodium carbonate
and tartaric acid (a mild acid).
Q.13. Write balanced chemical equations for following reactions and identify the type of reaction:
(a) Zinc oxide reacts with carbon to form zinc metal and carbon monoxide.
(b) Barium chloride reacts with sodium sulphate to give barium sulphate and sodium chloride.
Ans. (a) ZnO + C
Zn + CO (Oxidation-Reduction reaction)
(b) BaCl2 + Na2SO4
BaSO4 + 2NaCl (Double displacement reaction)
Q.14. What is chlor-alkali process? Which product of this process is used to make bleaching powder?
Ans. Decomposition of an aqueous solution of sodium chloride by passing electricity through it to
produce chlorine (chlor) and sodium hydroxide (alkali) is called chlor-alkali process.
The chlorine evolved in this reaction is used for making bleaching powder.
Q.15. What happens when: (a) Copper oxide is mixed with dilute hydrochloric acid. (b) Copper
sulphate crystals are heated. (c) A small piece of sodium is dropped into the ethanol.
Ans. (a) Copper oxide dissolves in the acid to form blue-green coloured copper chloride and hence the
solution turns blue-green.
CuO + 2HCl → CuCl2 +H2O
(b) When copper sulphate crystals are heated, it loses its water of crystallization and turns white.
CuSO4.5H2O ---heat CuSO4 + H2O
(c) Sodium reacts with alcohol to produce sodium ethoxide and the hydrogen gas is evolved
2Na + 2CH3COOH --- 2CH3COONa + H2
Q.16. Give reasons for:
(a) While mixing an acid and water, acid must be added slowly to the water.
(b) Toothpastes are basic in nature. (c) Plaster of Paris should be stored in a moisture proof container.
Ans. (a) Reaction between water and acid is a highly exothermic reaction in which a large amount of
heat is generated that can cause splashing of the mixture and burns if water is added to a concentrated
acid. In order to prevent this, acid is added to the water slowly with constant stirring.
(b) Bacteria present in the mouth produce acids by degradation of sugar and food particles remaining in
the mouth after eating which can corrode teeth enamel. Thus in order to neutralise excess acid in the
mouth and prevent tooth decay, toothpastes are made basic.
(c) Plaster of Paris absorbs water molecule to form a hard solid mass of gypsum and becomes useless to
be used for required purposes. Thus it should be stored in a water proof container.
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Q.17. Explain the process of electrolytic refining.
Ans. The process of refining impure metals with the help of electricity to get pure metals is called
electrolytic refining. In this process, impure metal is made anode, a thin strip of pure metal is made the
cathode and a solution of the respective metal salt is used as electrolyte. On passing the current through
the electrolyte, the pure metal (in the form of ions) from the anode dissolves into the electrolyte and an
equal amount of pure metal from the electrolyte is deposited on the cathode.
Q.18 (a) Differentiate between addition reaction and substitution reaction with an example of each.
(b) Name element along with its symbol and electronic classification which has:
(i) a total of three shells, with one electron in its valence shell?
(ii) four shells and has as many electrons in its valence shell as in its first shell?
(iii) the oxide of formula X2O3 and belongs to third period of the modern Periodic table?
Ans. (a)
Addition reaction
(i) These reactions involve addition of an atom or a group of atoms to an unsaturated carbon compound.
Substitution reaction
(i) These reactions involve substitution of an atom or a group of atoms in place of hydrogen in a saturated
carbon compound.
(ii) CH2=CH2 + H2
CH3-CH3
(ii) CH4 + H2
CH3Cl + HCl
(b) (i) Sodium (Na, Electronic configuration=2, 8, 1) (ii) Calcium (Ca, Electronic configuration=2, 8, 8,
2)
(iii) Aluminium (Al, Electronic configuration=2, 8, 3).
Q.19. What is water of crystallisation?
Ans. The fixed number of water molecules present in one formula unit of a salt is called water of
crystallization. Washing soda has 10 molecules of water of crystallization.
Q.20. Write the chemical name of bleaching powder. How is it prepared? Write chemical equation
involved. Give its two uses.
Ans. Bleaching powder is Calcium oxychloride (CaOCl2).
Preparation: It is prepared by passing chlorine gas over dry slaked lime
Ca (OH)2
+
Calcium hydroxide
Cl2(g) →
Chlorine
(Slaked lime)
CaOCl2
+ H2O
Calcium oxychloride Water
(Bleaching powder)
Uses of bleaching powder:
1. Bleaching powder is used for bleaching cotton and linen in the textile industry and for bleaching wood
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pulp in paper industry.
2. It is used for disinfecting drinking water to make it free of germs.
Q. 21. Give one example of each: (a) Metal that can displace hydrogen from dilute acids. (b) Metal that
cannot displace hydrogen from dilute acids.
Ans. (a) Zinc can displace hydrogen from dilute acids.
(b) Copper cannot displace hydrogen from dilute acids.
Q.22. Why should curd and sour substances not be kept in brass and copper vessels?
Ans. Curd and sour substances contain acids which can react with the metal of the vessel to form
poisonous metal compounds. These compounds can cause food poisoning and damage our health.
Q.23 What is denatured alcohol?
Ans. Ethanol mixed with certain poisonous substances like methanol, etc is called denatured alcohol.
Q.24 Give an example of each (a) A non-metal that is lustrous.
(b) A non-metal that is liquid at room temperature.
Ans. (a) Iodine, (b) Bromine
Q.25. Metal compound X reacts with dilute sulphuric acid to evolve a gas which produces a pop sound
when a burning splinter is brought near it. Identify the gas and write the relevant balanced equation if one
of the compounds is zinc sulphate.
Ans. Since the gas evolved supports burning, it is hydrogen.
The equation can be represented as follows:
Zn
Zinc
+ H2SO4
→
ZnSO4
Sulphuric acid
Zinc sulphate
+
H2
Hydrogen
Q.26. What do you mean by corrosion of metals? What are the required conditions for the corrosion?
Suggest two methods for preventing corrosion.
Ans. (a) The process of deterioration of metals due to chemical reaction with moist air is known as
corrosion. Conditions required for corrosion are:
Presence of air
Presence of water/ moisture
Methods of prevention of corrosion are:
Painting or applying grease or oil – By painting or applying grease or oil on the metal surface, they are
prevented from coming in contact with air directly.
Alloying- Combining metals with other metals or non-metals increase their resistance to corrosion.
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Q. 27.(b) What is the colour of the compound formed by corrosion of silver and copper?
Ans: Silver forms black coloured coating and copper forms green colour coating on getting corroded.
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