NCERT Solutions for Class 9th Science Chapter 13 Earth as a System: Energy, Matter, and Life
Updated on 2026-09-19
About this chapter
The Earth is a single system of five interacting spheres — geosphere (rock, soil, interior), hydrosphere (liquid water), cryosphere (ice and snow), atmosphere (air) and biosphere (all life). A disturbance in one sphere shows up as a change in the others. Solar radiation reaches us as electromagnetic waves travelling at 3 × 10 8 m s –1 through vacuum. About 99 per cent of the Sun's energy lies in the UV, visible and infrared bands. The solar constant is 1.4 kW m –2 (1400 J s –1 m –2 ); after absorption and scattering, the maximum insolation at the surface is about 1 kW m –2 . Heating is uneven because of albedo (snow reflects 0.80 – 0.90, black soil and ocean water reflect very little), because of latitude (the same beam is spread over a larger area near the poles) and because of the tilt o
- Chapter opener
- Introduction
- Lead-in to 13.1 Uneven Heating of the Earth
- .1 Uneven Heating of the Earth
- .1.1 Interaction of solar radiation on the Earth's surface
- .1.3 Role of the atmosphere
- Why is the ozone layer so important?
- .2.1 Local winds / 13.2.3 Ocean currents
- .3.2 Carbon cycle
- .3.4 Oxygen cycle
Quick revision
| Term | What it means | Value / example from the chapter | Where it appears |
|---|---|---|---|
| Solar constant | Average solar energy received per unit time on unit area held perpendicular to the Sun's rays at the top of the atmosphere | 1.4 kW m–2 = 1400 J s–1 m–2 | Section 13.1 |
| Insolation | The solar radiation that actually reaches the Earth's surface | About 1 kW m–2 under a clear sky | Section 13.1 |
| Albedo | Fraction of the solar radiation a surface reflects (Latin albedo = whiteness) | Snow 0.80 – 0.90; ice 0.50 – 0.70; crushed rock 0.25 – 0.30 | Table 13.1 |
| Troposphere | Lowest layer, heated from below by the Earth's surface; carries almost all weather | 0 – 12 km; temperature falls at about 6.5 °C km–1 | Table 13.2 |
| Stratosphere | Holds the ozone layer; UV absorption warms it, so temperature rises with height and the layer stays calm | 12 – 50 km | Table 13.2 |
| Greenhouse gases | Gases that absorb the infrared radiated back by the warm Earth and stop it escaping to space | CO2, CH4, water vapour | Section 13.1.3 |
| Valley breeze | Daytime wind — sunlit slope heats, air rises, cool valley air flows up the slope | Shimla, Dehradun and other Himalayan valleys | Fig. 13.8(a) |
| Mountain breeze | Night wind — slope cools fast, dense cold air slides down into the valley | Shimla, Dehradun and other Himalayan valleys | Fig. 13.8(b) |
| Pressure belts | Bands of high and low pressure set up by uneven heating between equator and poles | Equatorial low, sub-tropical high (30°), sub-polar low (60°), polar high (90°) | Fig. 13.9(b) |
| Gyre | Large circular pattern of ocean currents made by the Earth's rotation deflecting moving water | Clockwise in the Northern Hemisphere, anticlockwise in the Southern | Fig. 13.10(a) |
| Biogeochemical cycle | Cyclic movement of matter and energy between the abiotic and biotic components of the Earth | Water, carbon, nitrogen and oxygen cycles | Section 13.3 |
| Nitrogen fixation | Converting unreactive N2 into ammonia that living things can use | Rhizobium in root nodules, Azotobacter in soil, lightning, Haber–Bosch process | Section 13.3.3 |
| Nitrification / Denitrification | NH3 → NO2– → NO3–, and nitrate back to N2 gas | Nitrosomonas, Nitrobacter; Pseudomonas | Fig. 13.15 |
| Eutrophication | Excess nitrate causes an algal bloom, the algae use up dissolved oxygen and fish die | Fig. 13.17 | Section 13.4 |
| Keeling curve | Graph of atmospheric CO2 since 1960; the saw-teeth are the yearly plant growth in the Northern Hemisphere | 315 ppm (1960) → about 420 ppm (2025) | Fig. 13.14 |
| Urban heat island | Cities stay warmer than the countryside because concrete, brick and asphalt store and re-radiate heat | Higher air-conditioning demand in cities | Threads of Curiosity, p. 257 |
Exercises
- Chapter opener — Think It Over Page 252
- Introduction — Activity 13.1: Let us explore Page 253
- Lead-in to 13.1 Uneven Heating of the Earth — In-text Questions Page 253
- .1 Uneven Heating of the Earth — Think as a Scientist Page 25513
- Lead-in to 13.1.1 Interaction of solar radiation on the Earth's surface — In-text Questions Page 256
- .1.1 Interaction of solar radiation on the Earth's surface — Activity 13.2: Let us find out Page 25613
- .1.3 Role of the atmosphere — Pause and Ponder Page 25813
- Why is the ozone layer so important? — Threads of Curiosity Page 259
- .2.1 Local winds / 13.2.3 Ocean currents — Pause and Ponder Page 26113
- .3.2 Carbon cycle — Pause and Ponder Page 26313
- .3.4 Oxygen cycle — Pause and Ponder Page 26513
- .4 Human Impact on Earth's Processes — What if … Page 26513
- .4 Human Impact on Earth's Processes — Pause and Ponder Page 26613
- End-of-chapter questions — Revise, Reflect, Refine Page 267 – 268
- Project work — The Journey Beyond Page 268
- An open question to carry forward — The Quest Continues … Page 268