Solution

HALF YEARLY EXAM(CHEMISTRY)

Class 10 - Science
Section A
1.
(b) 2NaOH + H2SO4 ⟶ Na2SO4 + 2H2O
Explanation:
It is a neutralisation or double displacement reaction.

2.
(b) A and D
Explanation:
The condition produced by aerial oxidation of fats and oils in foods marked by unpleasant smell and test is called rancidity.
Fatty foods become rancid because of oxidation means reaction with oxygen.

3.
(d) 2:1
Explanation:
1 Mole of water has 2 moles of Hydrogen and 1 mole of oxygen. Hence mole ration between hydrogen and oxygen is 2:1.

4.
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(b) NaCl, CaCO3, NH3
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Explanation:
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In this process a concentrated solution of sodium chloride (brine) is saturated with ammonia, carbon dioxide is passed through
it, and the product is calcined by calcium carbonate.
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5. (a) mild non-corrosive base

Explanation:
Baking soda(sodium bicarbonate) is a mild non-corrosive base because it is a weak base and do not cause burns or damage to
living tissue and is used in the preparation of cakes. When it is heated, it decomposes to sodium carbonate carbon dioxide and
water molecules.
6.
(d) Turmeric and Litmus
Explanation:
Turmeric and Litmus

7. (a) orange, blue

Explanation:
The colour of pH paper is orange in acidic medium, i.e. in oxalic acid while it is blue in basic medium, i.e. in sodium carbonate
solution.
8.
(d) Copper and Tin
Explanation:
Bronze is a solid alloy composed primarily of 88% copper and 12% tin. It is used in the production of hardware, utensils
musical instruments, medals, and other items. Due to its resistance to corrosion caused by seawater, it is used in submerged
bearings and ship propellants.

9.
(b) It reacts with both acid as well as base to form salt and water.

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 Explanation:
It reacts with both acid as well as base to form salt and water.

10.
(c) Oxygen
Explanation:
Oxygen

Section B
11. In Decomposition reaction a single substance decomposes to form two or more substances which is exact opposite of
Combination reaction in which two or more reactants combine to form a single product that is why decomposition reactions called
the opposite of combination reactions. For examples
△

Pb(NO3)2(s) −
→ 2PbO(s) + 4NO2 (g) +O2(g)

Ba(s) + F2(g) −
→ BaF2(s)

12. i. 'X' = Na2SO4 i.e. sodium sulphate & 'Y' = BaSO4 i.e. barium sulphate
ii. Double displacement reaction.
13. i. Reaction with Metals: Acids react with metals to release hydrogen gas.
Acid + Metal → Salt + hydrogen
H2SO4 + Fe → FeSO4 + H2
ii. Reaction with metal oxides: Acids react with Metal oxides to form salt and water. These are basic in nature.
Metal oxide + Acid → Salt + water ir
CuO + 2HCl → CuCl2 + H2O
iii. The reaction of an acid and base to form salt and water is called Neutralisation reaction.
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14. Litmus paper is extracted from lichens and lichens have the characteristic of changing colour. It is a natural indicator. In distilled
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water it remains in purple (mauve) colour. When added to acidic solution it changes its colour to blue and when added to basic
solution it changes its colour to red.
OR
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When plaster of paris reacts with water it produce gypsum as shown in the following chemical reaction
1 3
C aSO4 ⋅ ⋅ H2 O + H2 O → C aSO4 ⋅ 2H2 O
2 2
Gypsum
P laster of P aris

15. i. Sodium metal is very reactive.

ii. It reacts readily with air and water.
iii. The reaction is so vigorous that it catches fire even if it is kept in open air.
Therefore, to protect it, sodium is stored or kept in kerosene oil.
16. Copper is not affected by dry air at ordinary temperature. On exposure to moist air, it gets covered with a beautiful green coating
of either basic carbonate or basic sulphate.
2Cu + H2O + CO2 + O2 → CuCO3.Cu(OH)2
8Cu + 6H2+2SO2 + 5O2→ 2[CuSO4.3Cu(OH)2]
Copper present in bronze or in utensils is corroded by moist air containing acidic oxides like carbon dioxide, sulphur dioxide etc.
The greenish layer formed is of basic copper carbonate or basic copper sulphate. This phenomenon is called ‘corrosion of metals’
OR
i. Sodium
ii. White phosphorus
Section C
Δ

17. i. F e2 O3 (s) + 2Al(s) ⟶ 2F e(l) + Al2 O3(s)

Iron(III)oxide Aluminium Molten iron Aluminium oxide

The thermite reaction is a displacement reaction.

Δ

ii. 3M g(s) + N2 (g) −

→ M g3 N2 (s)
Magnesium N itrogen
Magnesium nitride
The formation of magnesium nitride as above is a combination reaction.

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18. a. Double decomposition reaction [An exchange of ions took place]
b. Displacement reaction [A more reactive non-metal displaces a less reactive non-metal from its salt solution.]
c. Decomposition reaction/Reduction reaction [A compound decomposes to form two or more products./CuO is reduced to Cu.]
19. The steps are:
i. Take about 2 mL of dilute NaOH solution in a test tube and add two drops of phenolphthalein solution.
ii. Add dilute HCl solution to the above solution drop by drop.
iii. Now add a few drops of NaOH to the above mixture.
Observation: On adding Phenolphthalein to NaOH, the colour becomes pink. On adding dilute HCl solution dropwise, finally, the
pink colour disappears and the solution becomes colourless. On adding NaOH, pink colour again appears because the medium
becomes basic.
HCl + NaOH ⟶ NaCl + H2O
Acid + Base ⟶ Salt + Water
The reaction is known as Neutralisation Reaction. It can be defined as the reaction of an acid and a base to form salt and water.
OR
I. Uses of acid:
i. Hydrochloric acid is used in many industries like cleaning boilers from the inside, cleaning sinks and sanitary, etc.
ii. Nitric acid is used for making fertilizers, and cleansing of ornaments of gold and silver.
iii. Sulphuric acid is used in cells, car batteries etc. It is known as the King of Chemicals.
II. Uses of bases:
i. Sodium hydroxide is used to make soaps, in paper industry and in the textile industry.
ii. Calcium hydroxide is used to remove the acidity of soil. It is an important constituent of lime and pesticides.
iii. Milk of magnesia (Magnesium hydroxide) is used as an antacid.
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III. Uses of salts:
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i. Calcium carbonate is used to make floors.
ii. In metallurgy for extraction of iron and in the making of cement.
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iii. Silver nitrate is used in photography, Ammonium nitrate is used as an explosive and in making fertilizers.

20. Name of the solution Colour change with phenolphthalein Colour change with blue litmus
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(a) Sodium carbonate (Na2CO3) Turns pink No change

(b) Hydrochloric acid (HCl) No change Turns red

(c) Sodium chloride (NaCl) No change No change

Sodium carbonate is basic in nature. Hydrochloric acid is acidic in nature. Sodium chloride is neutral and has no effect on the
indicators.
21. i. Sulphur is a non-metal because of the following reason:
a. It is a poor conductor of electricity.
b. Sulphur is neither malleable nor ductile.
c. Sulphur forms acidic oxide.
S + O2 ⟶ SO2
SO2 + H2O ⟶ H2SO4
ii. Magnesium is metal because of the following reason.
a. It is a good conductor of electricity.
b. Magnesium is malleable nor ductile.
c. It forms basic oxides
2Mg + O2 ⟶ 2MgO
OR
a. i. No action.
ii. it turns moist litmus paper red and then bleaches it.
b. S + O2= SO2

22. i. B is most reactive as it displaces all other metals from their respective salt solutions.

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 ii. B will displace copper from copper(II) sulphate, being a more reactive metal than it.
iii. B > A > C > D.
Section D
23. i. Carbonic acid does not form an acidic salt.
ii. Sodium bicarbonate, commonly known as baking soda or bicarbonate of soda, is a chemical compound with the formula
NaHCO3.
iii. Ca(OH)2 treatment with chlorine to obtain bleaching powder.
OR
Ca(OH)2 + Cl2 ⟶ CaOCl2 + H2O
Washing soda is used for removing the permanent hardness of the water.
OR
i. i. Groups A and B - less than 7
ii. Group C - greater than 7
ii. pH paper and universal indicator.
iii. a. Copper vessel is tarnished due to formation of basic copper oxide.
b. Lemon juice being acidic react with copper oxide and the salt formed is washed away.
OR
a. An optimal pH is required for digestion.
b. Change in pH can cause tooth decay.
c. Animals and plants defend themselves through change in pH.
d. Survival of aquatic life becomes difficult when pH of river water becomes low.
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24. i. Metals are malleable and ductile. Metals are good conductors of heat and electricity. Metals are lustrous (shiny) and can be
polished. Metals are solids at room temperature (except mercury, which is liquid).
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ii. Metals Non-metals
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These are solids at room temperature except mercury These exist in all three states

These are very hard except sodium These are soft except diamond
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These are malleable and ductile These are brittle and can break down into pieces

These are shiny These are non-lustrous except iodine

Electropositive in nature Electronegative in nature

Have high densities Have low densities

iii. Exceptions of Metals on the basis of their Physical Properties are given below:
Malleability: Zinc, Arsenic, Mercury, Antimony are non-malleable and non-ductile metals.
Conductivity: Aluminium and Titanium are poor conductors of Heat and Electricity.
Lustre: Sodium is a non-lustrous metal.
Hardness: Cesium, Mercury (Liquid Metal), Gallium are soft Metals.
OR
Sodium is stored in kerosene because sodium reacts vigourously with oxygen and moisture due to its high reactivity so
keeping it in kerosene will prevent sodium from coming in contact with oxygen and moisture.

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