Page 60 - Ripples SCIENCE 7 - TEJPUR Edition 2024 Answer Key

Exothermic reactions release energy to their surroundings. The surroundings get

warmer because these reactions release heat. For example, burning of wood,
respiration in living organisms.

2. Why is magnesium ribbon rubbed with sand paper before igniting it?
Ans. Magnesium ribbon is rubbed with sandpaper before igniting it to remove the layer

of magnesium oxide that forms on its surface when it reacts with oxygen in the air.
This layers acts as a barrier, preventing the magnesium metal underneath from easily

catching fire. By rubbing the ribbon with sandpaper, this layer is removed allowing
the magnesium underneath to ignite more easily and burn brightly when heated.

3. Why does spoilt food produce foul smell?
Ans. Spoilt food produces a foul smell because bacteria, moulds and other microorganisms

break down the food and release various by products. These include some gases and
compounds, which have strong, unpleasant odours.

4. Why is a combustion reaction also called an exothermic reaction?
Ans. A combustion reaction is also called an exothermic reaction because it releases

energy in the form of heat & light. During combustion, a significant amount of energy
is released, which increases the temperature of the surroundings.

5. Why is crystallisation better than evaporation?
Ans. Crystallisation is better than evaporation for the following reasons:

• Some solids decompose or some, like sugar, get charred on heating due to
dryness.

• Some impurities remain dissolved even after filtration. On evaporation, these
contaminate the solid.

6. What is an endothermic reaction? Give one example.
Ans. Endothermic reactions : An endothermic reaction is a reaction that absorbs heat

from the environment. The absorbed heat energy provides energy for the reaction
to take place.

Reactants + Heat  Products
One example of endothermic reaction is the reaction between water and ammonium

nitrate to form ammonium hydroxide and nitric acid.
7. Give the answer in one word.

(a) Metal employed in galvanisation of iron.
Ans. Zinc

(b) Metals mixed with iron to make stainless steel.
Ans. Chromium, Nickel, Manganese and varying percentage of carbon.

(c) Process by which common salt is extracted commercially from seawater.

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