Air, Water, Sunlight and the Hydrosphere (13.4.1) — "Our Home: Earth, a Unique Life Sustaining Planet"

1. 13 marks

   Explain, step by step, how oxygen enters the atmosphere and how it is then used, connecting photosynthesis and respiration.

   Hint. This is a two-way cycle between plants and everything that breathes.

   ✦ In the presence of sunlight, plants take carbon dioxide from the air and water from the soil to prepare food by photosynthesis; in this process, oxygen is released. This released oxygen is then what humans, animals, and plants use for respiration — which means the oxygen supply that all life depends on for breathing is continually replenished by the same plants that also need the resulting atmosphere to survive.

2. 23 marks

   Why would Earth become too cold without an atmosphere, according to this chapter?

   Hint. The greenhouse effect, introduced earlier for Venus, reappears here with a specific consequence stated for its absence.

   ✦ Radiation from the Sun heats the Earth, and some of this heat is trapped by the atmosphere due to the greenhouse effect — keeping the temperature just high enough for water to remain in the liquid state. Without an atmosphere, the Earth would lose heat to space and become too cold, since there would be nothing to trap even the mild amount of heat the greenhouse effect currently retains.

3. 32 marks

   Define the hydrosphere, in the chapter's own words, and state what fraction of Earth's surface it covers.

   Hint. A single short definition, plus a specific percentage given directly.

   ✦ Water is found in ponds, lakes, rivers, springs, seas, oceans, and groundwater, covering about 70 per cent of the Earth's surface. All this water together forms the hydrosphere.

4. 43 marks

   List three distinct roles water plays inside animals' bodies, according to the chapter.

   Hint. The chapter names three separate bodily functions water supports.

   ✦ In animals, water regulates body temperature, aids digestion, and ensures hydration — since all three are essential for health and life. (In plants, water also transports nutrients from soil to leaves, a role established in Curiosity Grade 7.)

5. 53 marks

   Explain how water vapour in the air eventually affects farming and water supply on land, tracing the full chain described in the chapter.

   Hint. The chain runs from vapour, through clouds, to rainfall, to its effects on ecosystems and human activity.

   ✦ Water vapour in the air forms clouds and brings rain or snow, which refills rivers, lakes, and underground water. Rainfall affects the kinds of plants and animals that can live in a place, and moving air also shapes weather and rainfall patterns — since this in turn influences farming, water supply, and life on land more broadly.

Solutions written by the tuition.in editorial team and checked against NCERT Curiosity — Textbook of Science for Grade 8, Chapter 13 (hecu113.pdf), Reprint 2026-27, pages 210-228. Questions are referenced from the NCERT textbook for identification.