Matter and its three states

Your child learnt these properties of matter:

1) Solid has fixed shape and volume

2) Liquid has no fixed shape but it has a fixed volume

3) Air has no fixed shape and volume.

Air can be compressed.

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Let’s apply the science knowledge into this question.

Study this diagram

The capacity of the container below is 400cm³. It contained 100cm³ of water.

Please follow the sequence in each question.

Question 1

David added 150cm³ of rocks into the container. What was the final volume of the container?

The final volume is 400cm³

Why?

The rocks are in solid state. They have a fixed volume and cannot be compressed.

Water has a fixed volume and they cannot be compressed.

When the rocks are added, the rocks displace the air inside the container. As the air cannot escape, the air is compressed.

Question 2

After David has added the rocks, he pumps in 60cm³ of air into the container using the air pump.

What is the final volume of air?

We have to get the answer using mathematics.

Volume of container = 400cm³

Volume of rocks = 150cm³

Volume of water = 100cm³

Volume of air = 400 – 150 – 100 = 150cm³

We do not add in the 60cm³ of air.

Why?
Air has no fixed volume. The air can be compressed and will occupy the remaining space inside the container.

Question 3A

Next, David then removes 50cm³ of water by turning on the tap. What is the final volume of water?

100cm³ – 50cm³ = 50cm³

Question 3B

What is the final volume of air after David removes 50cm³ of water based on Question 3A?

Answer: Take 150cm³ of air from question 2, add 50cm³.

150 + 50 = 200cm³ of air.

Why do we add the volume of air?

Take note:

You’ve learnt this in school: Air does not have a fixed volume. Air can be compressed.

Many students have the assumption that no fixed volume means air can only be compressed.  That is wrong!

Actually, no fixed volume means the air will occupy the remaining space in the container!

Add this Science Knowledge today: Air will always occupy the remaining space inside the container.

For this question, after the rocks and water have occupied the space inside the container, whatever remaining space inside the container belongs to air.

Hence, if you remove 50cm³ of water, the volume of water removed belongs to the volume of air inside the container.

The same concept applies when you remove rocks. If you remove 50cm³ of rocks, that volume of rocks removed belongs to the volume of air inside the container.

Question 4

Finally, David decides to remove 30cm³ of air. What is the final volume of air?

Answer: 200cm³ (same answer as question 3B)

Why?

Air has no fixed volume. Air will always occupy the remaining space inside the container.

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Talk again soon!

Andy Ling

Simplifying science concepts for your child

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