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A Journey through States of Water - Cooling Effect

Grade 6CBSE

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

🔑Concepts

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Evaporation is the process where a liquid changes into its vapor state at any temperature below its boiling point (e.g., below 100∘C100^\circ \text{C} for water).

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Evaporation causes cooling because the liquid particles absorb energy (latent heat) from the surface or surroundings to overcome the forces of attraction and change into vapor.

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The rate of evaporation increases with an increase in Surface Area. For example, spreading wet clothes allows them to dry faster.

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The rate of evaporation increases with an increase in Temperature and Wind Speed, but decreases with an increase in Humidity.

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In earthen pots (matkasmatkas), water seeps through microscopic pores to the outer surface and evaporates, taking heat from the water inside, thus keeping it cool.

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Transpiration is a biological process in plants where water evaporates from the leaves, providing a cooling effect to the plant during hot weather.

📐Formulae

Rate of Evaporation∝Surface Area\text{Rate of Evaporation} \propto \text{Surface Area}

Rate of Evaporation∝Temperature\text{Rate of Evaporation} \propto \text{Temperature}

Rate of Evaporation∝1Humidity\text{Rate of Evaporation} \propto \frac{1}{\text{Humidity}}

ΔT=Tinitial−Tfinal\Delta T = T_{\text{initial}} - T_{\text{final}}

💡Examples

Problem 1:

In a science experiment, the initial temperature of water in an earthen pot was 42∘C42^\circ \text{C}. After 33 hours of evaporation, the temperature dropped to 25∘C25^\circ \text{C}. Calculate the total reduction in temperature.

Solution:

42−2517\begin{array}{r} 42 \\ - 25 \\ \hline 17 \end{array}

Explanation:

The reduction in temperature is calculated by subtracting the final temperature (25∘C25^\circ \text{C}) from the initial temperature (42∘C42^\circ \text{C}). The resulting drop is 17∘C17^\circ \text{C} due to the cooling effect of evaporation.

Problem 2:

Why do we feel cooler when we sit under a fan after sweating?

Solution:

The wind from the fan increases the rate of evaporation of sweat from our skin. Since evaporation requires heat, it takes this heat from our body, leading to a cooling sensation.

Explanation:

This follows the principle that Rate of Evaporation∝Wind Speed\text{Rate of Evaporation} \propto \text{Wind Speed}. As the sweat evaporates faster, the body loses heat more rapidly.

Problem 3:

If 500 ml500 \text{ ml} of water is kept in a narrow bottle and 500 ml500 \text{ ml} is kept in a wide tray, which one will cool faster through evaporation?

Solution:

The water in the wide tray will cool faster.

Explanation:

Evaporation is a surface phenomenon. The tray has a larger surface area compared to the narrow bottle. Since Evaporation∝Surface Area\text{Evaporation} \propto \text{Surface Area}, more water molecules can escape into the air from the tray, leading to a faster cooling effect.