Have you ever seen a thick glass cup crack when filled with hot water?
Let’s watch this video together:
https://youtu.be/avzyhROCCSI?si=60QXVEa_MKaKjntx
Recall an important P4 Heat concept- when matter gains heat, it expands.
This is because the particles in matter gain heat and move about more, thus taking up more space. Its volume increases. The space occupied increases. However, the mass stays the same.
This is why train tracks need a gap for hot days. Without a gap, the metal tracks will gain heat and expand, bending out of shape!
Conversely, when matter loses heat, it contracts.
This is because the particles in matter lose heat and slow down in movement, thus taking up less space. Its volume decreases. The space occupied decreases. However, the mass stays the same.
This is why wires hanging from electric poles have to be loose, so that on cold days, the wires can lose heat and contract without snapping.
So why did our thick glass break when hot water was poured in?
The reason is simple- uneven expansion!
Imagine that there are two layers of glass. An inner layer of glass, and an outer layer of glass.
The inner layer of glass is in direct contact with the hot water.
When we pour hot water, the inner layer of glass is in direct contact with the hot water. (evidence 👀)
The inner layer of glass gains more heat than the outer layer of glass, and expands more than the outer layer, causing a crack. (concept)
(Do take note that the comparison word “more” is important!)
The outer layer of glass also gains heat. However, it gains less heat/ gains heat slower than the inner layer. This is because the outer layer is not in direct contact with the hot water.
Therefore, there is uneven expansion, where the inner layer of glass expands more than the outer layer, causing a crack.
Let’s practice this 2019 PSLE question together!
Q17b) Two glasses, A and B, are stacked together as shown. They became stuck and Ahmad was unable to separate them with his hands.
For safety reasons, Ahmad is not allowed to use lubricants (e.g. oil) on the glass and not to force the glasses apart with his hands.
(i) describe two ways that Ahmad can separate the two glasses. You may use a diagram in your answer. (2 marks)
First method: place the stuck glasses in a basin of hot water so that only glass B is in contact with the hot water.
Second method: place ice cubes into glass A
(ii) explain why one of the ways in (i) works. (1 mark)
Explanation for the first method:
Glass B is in direct contact with the hot water and gains more heat from the hot water, expanding more than A. This increases the space between B and A, making it easier to pull A out of B.
Explanation for the second method:
Glass A is in direct contact with ice, so A loses heat to the ice and contracts more than B. This increases the space between A and B, making it easier to pull A out of B.
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2023 PSLE Science Paper Analysis
