Ch 15 explains what air is made of and why it is essential. Students learn the composition of air, how to demonstrate the presence of various components, and the role of air for living organisms and burning.
Air is a mixture of gases: ~78% nitrogen, ~21% oxygen, ~0.04% CO₂, plus water vapour, dust, and traces of other gases. Air occupies space, has weight, and is present everywhere (even dissolved in water and soil).
Oxygen is needed for breathing (respiration) and burning (combustion). Plants use CO₂ for photosynthesis and release O₂. Nitrogen is used by plants (through bacteria) to make proteins. Wind (moving air) generates energy, helps in pollination, and drives the water cycle.
Download: https://ncert.nic.in/textbook/pdf/fesc115.pdf | Complete book: https://ncert.nic.in/textbook/pdf/fesc1ps.zip
Aquatic animals like fish breathe using gills. Gills absorb dissolved oxygen from water. That is why it is important that water bodies are not polluted — polluted water has less dissolved oxygen, which harms aquatic life.
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