The answers given here of Science class 7 holiday homework of a private school in Dwarka New Delhi are for helping children in learning and verifying that their answers are right and not for copying.
Q 1 a. Which bacterium can take atmospheric nitrogen and convert it into a soluble form?
A 1 a). Rhizobium bacteria converts atmospheric nitrogen into soluble form.
Extra: Azotobacter bacteria can also take atmospheric nitrogen and convert it into soluble form that can be used by plants.
Q 1 b. What do crops require usually to make protein?
A 1 b). Plants require nitrogen to make protein.
Q 2. Give one word/two words for the following statements:
a. The pores through which leaves take in carbon dioxide from the air.
A 2 a). Stomata
Q 2 b. A plant that has both autotrophic and heterotrophic modes of nutrition.
A 2 b). Pitcher
Extra: Venus Flytrap
Q 2 c. Movement of air from land towards ocean at night.
A 2 c). Land breeze
Q 2d . The phenomenon of increase in the atmospheric temperature due to increase of carbon dioxide concentration.
A 2 d). Global warming
Q 3. How do saprophytes help in cleaning the environment?
A 3: Saprophytes eat dead and decaying matter and thus help in keeping the environmental clean. In their absence the dead and decaying matter won’t be decomposed and converted into useful matter.
Examples of saprophyte are some fungi and some bacteria.
Q 4. Why does ship suffer a lot of damage from rusting in spite of being painted?
A 4: Ships have large surface area and are mostly in contact with salty water that strikes the painted surface with high speed (when the ship is moving fast).
Due to this the paint is worn away from the ship’s surface that is in contact with salty water.
Also in addition to water, there is availability of moisture in the air near the sea water. Thus in presence of water, oxygen and moisture ships exposed metallic part gets rusted despite of paint.
Q 5. Which would you prefer to use during winter, two thin woollen blankets or a thick woollen blanket? Give reasons for your choice.
A 5: In winter we should use two thin blankets rather than one thick blanket because two thin blankets have more layers and thus will have more air trapped in them compared to a single thick blanket. As air is a bad conductor of heat so it will prevent heat to flow from our body. More heat trapped in two blankets compared to one blanket will keep us warmer.
Q 6. Why does mercury level start falling as soon as a laboratory thermometer is taken out of a beaker containing boiling water?
A 6: The mercury level start falling as soon as a laboratory thermometer is taken out of a beaker containing boiling water because it does not have a kink or bend to stop mercury level from falling down.
Q 7. Name the elements present in the following compound.
i. Magnesium hydroxide,
ii. Calcium carbonate
iii. Sulphuric acid
iv. Sodium carbonate
v. Sodium chloride
A 7: i). The elements present in magnesium hydroxide Mg(OH)2) are magnesium, hydrogen and oxygen.
A 7 ii). The formula for calcium carbonate is CaCO3, so the elements in it are calcium (Ca), carbon (C) and oxygen (O).
A 7 iii). The formula of sulphuric acid is H2SO4. So the elements present in sulphuric acid are hydrogen (H), oxygen (O), and sulphur (S).
A 7 iv). The elements present in sodium carbonate Na2CO3 are sodium (Na), Carbon ( C ) and Oxygen (O).
A 7 v). The elements present in sodium chloride (NaCl) are sodium (Na) and Chlorine (Cl).
Q 8. How are plants and animals interdependent on each other in the forests?
A 8: Plants provide food and shelter for animals, and provide oxygen by converting carbon dioxide to oxygen through photosynthesis.
Plants also regulate the balance between carbon dioxide and oxygen for animals.
Food and Shelter: Plants are eaten by herbivorous animals, which in turn become food for the omnivorous (flesh and plant eating) and carnivorous (flesh eating) animals.
Plants are also the homes of many animals, small and big. Plants provide shelter from predators and harsh factors of the environment, like the hot sun, cold snow and heavy rains.
Plants dependant on animals: On the other hand, plants depend on animals for nutrients, pollination and seed dispersal, and as the animals consume plants, they control the numbers of different species of plants preventing from over burden of too many plants.
Animals also provide carbon dioxide to plants during exhalation. When animals die, they decompose and become manure for plants. Plants are not able to make movements, so they depend on animals to pollinate them for reproduction. And when the fertilized plants eventually produce seeds encased in fruits, animals eat them or carry them along on their fur, dispersing seeds to far and wide places ensuring the continuation of the plant species.
Q 9. Why do buffaloes enjoy a dip in a river or in a lake on a hot summer afternoon?
A 9: The black colour of the skin of buffaloes absorbs more heat in summer. The water of river or lake absorbs their body heat and helps them in lowering down their body temperature and feeling cool. So, as buffaloes feel cooler in river or lake water, they enjoy a dip in water on a hot summer afternoon.
Q 10. Classify as Physical or Chemical change:
a. Burning of Magnesium ribbon
b. Reaction between vinegar and baking soda.
c. Dipping of iron nail in copper sulphate solution
d. Stretching of rubber band
A 10 a): Burning of magnesium ribbon is a chemical change as new substance magnesium oxide is formed.
A 10 b). Reaction between vinegar and baking soda is a chemical change as new substances carbon dioxide, salt and water are formed.
A 10 c). Dipping of iron nail in copper sulphate solution is a chemical reaction as new substances ferrous sulphate and copper are formed.
A 10 d). Stretching of a rubber band is a physical change as there is temporary change in its shape and no new substances are being formed.
