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Our Environment - Water Cycle

Grade 4ICSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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The Water Cycle, also known as the Hydrological Cycle, is the continuous movement of water between the Earth's surface and the atmosphere.

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Evaporation: The process where the Sun heats up the water in rivers, lakes, and oceans, turning liquid H2OH_{2}O into water vapor (H2O(g)H_{2}O_{(g)}).

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Condensation: As water vapor rises, it cools down and changes back into tiny liquid H2OH_{2}O droplets, which cluster together to form clouds.

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Precipitation: When the water droplets in clouds become too heavy, they fall back to Earth in the form of rain, snow, sleet, or hail.

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Transpiration: The process by which plants lose water in the form of vapor through small pores in their leaves.

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Collection: The process where fallen precipitation gathers in oceans, lakes, and underground reservoirs.

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The Sun is the primary source of energy that drives the entire water cycle.

📐Formulae

H2O(l)+Heat→H2O(g) (Evaporation)H_{2}O_{(l)} + \text{Heat} \rightarrow H_{2}O_{(g)} \text{ (Evaporation)}

H2O(g)−Heat→H2O(l) (Condensation)H_{2}O_{(g)} - \text{Heat} \rightarrow H_{2}O_{(l)} \text{ (Condensation)}

Total H2O on Earth≈Constant\text{Total } H_{2}O \text{ on Earth} \approx \text{Constant}

100∘C (Boiling point of water)100^{\circ}C \text{ (Boiling point of water)}

0∘C (Freezing point of water)0^{\circ}C \text{ (Freezing point of water)}

💡Examples

Problem 1:

During a hot summer afternoon, Rahul noticed that a small puddle of water on the pavement disappeared after an hour. Which process of the water cycle is responsible for this?

Solution:

The process is Evaporation.

Explanation:

The heat from the Sun provides energy to the H2OH_{2}O molecules in the puddle, changing them from a liquid state to a gaseous state (water vapor), which then mixes with the air.

Problem 2:

Why do we see tiny water droplets on the outer surface of a glass containing ice-cold juice?

Solution:

This is due to Condensation.

Explanation:

The water vapor (H2O(g)H_{2}O_{(g)}) present in the surrounding air touches the cold surface of the glass, loses heat, and turns back into liquid H2OH_{2}O droplets.

Problem 3:

What would happen to the water cycle if the temperature of the Earth stayed below 0∘C0^{\circ}C everywhere?

Solution:

Most water would remain in the form of ice or snow, and the rate of evaporation would decrease significantly.

Explanation:

Since 0∘C0^{\circ}C is the freezing point of H2OH_{2}O, liquid water would turn to ice. Without enough heat to cause evaporation, the cycle would slow down, and precipitation would mostly occur as snow.