# Water cycles and Changes in State

# Factors Affecting Rate of Evaporation

This science revision note aims to help your child understand how to explain evaporation concepts clearly.

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**MUST-KNOW** about evaporation:

**Factors affecting evaporation**

- Wind: As the amount of wind increases, the rate of evaporation increases
- Exposed surface area: As the exposed surface area of the water increases, the rate of evaporation increases.
- Temperature: As the temperature increases, the rate of evaporation increases

**Watch this video**

__Question__

The diagram below shows two cups with equal amount of water hanged on a lever balance. Cup A is filled with water of 25°C while cup B is filled with water of 70°C.

After 15 minutes, what will happen to the lever balance? Explain your answer. (2m)

__STEP 1__

Identify the correct factor and how it affects evaporation.

Based on the experiment above, the factor is the temperature of water.

__STEP 2 __

Identify the evidence from the diagram and show how this affects the mass of water in both cups at the end of the experiment.

__FINALLY, put your answers together.__

After 15 minutes, what will happen to the lever balance? Explain your answer. (2m)

__Answer:__

The lever balance will tilt towards cup A. The water in cup A is of a lower temperature than in cup B. The water at 25°C evaporates slower than the water at 70°C.

Hence, more water will be left in cup A, resulting in cup A having a greater mass and being heavier than cup B.

**Let’s try out another question!**

Rachel conducted an experiment. She had three different containers with an equal volume of water in each, as shown in the diagram below. She left the containers open for 20 minutes under the hot sun.

After 20 minutes, which of the containers (A, B or C) would contain the least volume of water? Explain your answer.

__STEP 1__

Identify the **factor affecting the rate of evaporation** that is being **changed** here.

In the lesson notes, your child has learnt 3 factors affecting rate of evaporation.

- Presence of
**W**ind **E**xposed surface area**T**emperature of the liquid

We use the acronym **WET**.

Now, we have to identify which is the relevant factor for the above question.

- The
**temperature**of water: As all containers were placed in the same place, this factor is not changed. - Presence of
**wind**: As all containers were placed in the same place, this factor is not changed. **Exposed****surface****area**of water: The shapes of the containers are different and their openings are of different sizes, as seen by the diagram. Hence, the**exposed surface area of water is the factor being changed**.

__STEP 2__

Explain how the factor changed affects the rate of evaporation.

How does exposed surface area affect rate of evaporation? **The greater the exposed surface area of water, the faster the rate of evaporation, the ****lower the volume of water left at the end of the experiment.**

__Step 3__

Identify which container will contain the least volume of water after 20 minutes.

Let’s work backwards: **Least water left the**** fastest rate of evaporation the**** greatest exposed surface area**

Which container has the greatest exposed surface area?

Container B

Hence, **Container B will contain the least volume of water after 20 minutes.**

__FINALLY, put your answers together.__

Question: After 20 minutes, which of the containers (A, B or C) would contain the least volume of water? Explain your answer.

**Answer: **

Container B will contain the least volume of water after 20 minutes. Container B has the greatest exposed surface area, so the water in Container B has the fastest rate of evaporation. Hence, Container B will have the least volume of water left after 20 minutes.

**Note**: Your answer must state the factor affecting the rate of evaporation. For this question, the relevant factor is exposed surface area.

**Common mistakes**

**Wrong** answer: Container B will contain the least volume of water after 20 minutes. Container B has a greater exposed surface area, so the water in Container B has a faster rate of evaporation. Hence, Container B will have less water left after 20 minutes.

Since you are comparing **more than two containers**, you have to use **superlatives** (eg. great__est__, fast__est__) to explain your answer!

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