Natural Resources — Air, Water, Pressure

Overview

Natural resources form the foundation of Environmental Science in MAHA TET Paper II, connecting physical science concepts with real-world applications. This topic specifically addresses the composition and properties of air, the concept of atmospheric pressure, and water as a vital resource — all essential for upper-primary science teaching.

For MAHA TET, expect questions that test your understanding of air composition percentages, demonstrations of atmospheric pressure (like the inverted glass experiment), and the water cycle. Questions often link scientific concepts to everyday phenomena that teachers must explain to students. Mastery requires knowing exact figures (nitrogen 78%, oxygen 21%), understanding pressure concepts qualitatively, and connecting water resource management to environmental awareness.

This topic bridges physics (pressure) and environmental science (resources and conservation), making it a favourite for integrated questions in the exam.

Key Concepts

• **Air is a mixture, not a compound**: Air contains nitrogen (78%), oxygen (21%), and other gases including carbon dioxide, argon, water vapour and trace gases (together about 1%). These gases retain their individual properties and can be separated by physical means.

• **Atmospheric pressure is the weight of air above us**: The column of air extending from the earth's surface to the upper atmosphere exerts pressure in all directions. At sea level, this equals approximately 101.3 kPa or 760 mm of mercury.

• **Pressure decreases with altitude**: As we go higher, there is less air above us, so atmospheric pressure falls. This is why ears pop during flight and why cooking takes longer at hill stations.

• **Water exists in three states and cycles continuously**: The water cycle (hydrological cycle) involves evaporation, condensation, precipitation and collection. Solar energy drives this cycle, making it a natural purification process.

• **Only 3% of Earth's water is fresh**: Of this, about 68% is locked in glaciers and ice caps, 30% is groundwater, and less than 1% is easily accessible surface water in rivers and lakes.

• **Air exerts pressure equally in all directions**: This is why a filled balloon expands uniformly and why atmospheric pressure does not crush us — internal body pressure balances it.

• **Water pressure increases with depth**: Unlike atmospheric pressure, water pressure increases as we go deeper because more water weight presses down from above.

Formulas / Key Facts

| Concept | Key Fact | |---------|----------| | Nitrogen in air | 78.09% (approximately 78%) | | Oxygen in air | 20.95% (approximately 21%) | | Carbon dioxide in air | 0.04% (but vital for photosynthesis and climate) | | Standard atmospheric pressure | 101.325 kPa = 1 atm = 760 mm Hg = 1013.25 millibar | | Pressure formula | Pressure = Force ÷ Area (P = F/A) | | Unit of pressure | Pascal (Pa); 1 Pa = 1 N/m² | | Fresh water on Earth | About 3% of total water | | Accessible fresh water | Less than 1% of total fresh water | | Water cycle stages | Evaporation → Condensation → Precipitation → Collection | | Boiling point variation | Water boils below 100°C at high altitudes due to lower pressure |

Worked Examples

**Example 1: Demonstrating Atmospheric Pressure**

*Question*: A student fills a glass with water, covers it with a cardboard, and inverts it. The water does not fall. Explain why.

*Solution*:

• Step 1: When the glass is inverted, the cardboard has water pushing down on it from inside.
• Step 2: Atmospheric pressure acts upward on the cardboard from outside (from below in this position).
• Step 3: Atmospheric pressure (equivalent to about 10 metres of water column) is much greater than the pressure exerted by the small column of water in the glass.
• Step 4: The upward atmospheric pressure exceeds the downward water pressure, holding the cardboard in place.

**Example 2: Calculating Pressure**

*Question*: A book weighing 5 N rests on a table. Its base area is 0.02 m². Calculate the pressure exerted.

*Solution*:

• Pressure = Force ÷ Area
• Pressure = 5 N ÷ 0.02 m²
• Pressure = 250 Pa

The book exerts a pressure of 250 pascals on the table.

**Example 3: Water Cycle Application**

*Question*: Why does water collected as rain or snow eventually become fresh, even though most evaporation occurs from salty oceans?

*Solution*:

• Step 1: When ocean water evaporates, only water molecules escape into the air.
• Step 2: Dissolved salts and impurities are too heavy to evaporate and remain in the ocean.
• Step 3: Water vapour rises, cools, and condenses into clouds.
• Step 4: Precipitation (rain/snow) is therefore naturally distilled and fresh.

This explains how the water cycle acts as a natural purification system.

Common Mistakes

• **Confusing air composition percentages**: Students often swap nitrogen and oxygen values or think oxygen is the major component because we breathe it. *Correct thinking*: Nitrogen is 78%, oxygen is 21% — remember "NO" (Nitrogen-Oxygen) in decreasing order.

• **Thinking atmospheric pressure acts only downward**: Since we associate pressure with weight, students assume it pushes only from above. *Correct thinking*: Air molecules move randomly in all directions, so atmospheric pressure acts equally in all directions — up, down and sideways.

• **Believing all water on Earth is usable**: Students may think the 71% water coverage of Earth means abundant drinking water. *Correct thinking*: 97% is saline ocean water; of the 3% fresh water, most is frozen or underground. Accessible fresh water is extremely limited.

• **Confusing evaporation and boiling**: Both involve liquid turning to gas, so students mix them up. *Correct thinking*: Evaporation occurs at any temperature and only at the surface; boiling occurs at a specific temperature (boiling point) throughout the liquid.

• **Ignoring the role of carbon dioxide**: Because CO₂ is only 0.04%, students think it is unimportant. *Correct thinking*: Despite its small percentage, CO₂ is crucial for photosynthesis and acts as a greenhouse gas affecting climate.

Quick Reference

• Air composition: N₂ (78%) > O₂ (21%) > Ar + CO₂ + others (1%)

• Atmospheric pressure at sea level: 760 mm Hg or 101.3 kPa

• Pressure = Force ÷ Area; unit is Pascal (Pa)

• Fresh water is only 3% of Earth's total water; most is frozen

• Water cycle: Evaporation → Condensation → Precipitation → Collection

• Atmospheric pressure decreases with altitude; water pressure increases with depth