Effects of Heat on Matter

What is thermal expansion? Explain it in your own words and give an everyday example where it can be observed, other than a thermometer.

Between the rails of a train track there are small gaps every few meters. Observe the figure.

1. a) What are these gaps for?
2. b) What would happen to the rails on a very hot day if the track did not have these gaps?

If you suddenly pour boiling water into a thick glass, it may crack; in a thin glass it is less likely. Taking into account that the inner surface of the glass heats up and expands before the outer surface, explain why a thick glass breaks more easily.

At Noa's house, the wooden door to the terrace rubs against the frame and is difficult to close in August, but in January it closes easily.

1. a) Explain this using thermal expansion.
2. b) At what time of year will the door be slightly more "shrunk"?

A metal bridge lengthens by $1\ \text{cm}$ for every degree its temperature rises. Look at the diagram of the bridge.

1. a) Between a winter night at $0\ ^\circ\text{C}$ and a summer afternoon at $35\ ^\circ\text{C}$, by how many degrees does the bridge's temperature rise?
2. b) How many centimeters does the bridge lengthen between those two days?
3. c) Will the expansion joints of the bridge be more open in winter or in summer? Give reasons for your answer.

When ice is heated, it turns into liquid water and, if heating continues, into steam.

1. a) What is the change from solid to liquid called?
2. b) What about the change from liquid to gas?
3. c) What are the two reverse changes called, those that occur when cooling?

After the rain, puddles in the park disappear as soon as the Sun comes out.

1. a) What change of state does the water in the puddles undergo?
2. b) Where does the water get the heat needed for this change?

When a candle is blown out, the melted wax next to the wick turns hard in a few minutes.

1. a) What is the change of state that the wax undergoes called?
2. b) In this change, does the wax absorb or release heat?

When cooking pasta, the underside of the pot lid fills with small droplets of water. Look at the diagram.

1. a) What happens to the steam when it touches the lid, which is cooler?
2. b) What is this change of state called?
3. c) When turning into droplets, does the steam absorb or release heat?

When getting out of the shower, Nico feels cold even though the bathroom is warm.

1. a) What change of state does the water remaining on his skin undergo?
2. b) Where does this water get the heat it needs for the change?
3. c) Use the previous answers to explain why it is advisable to dry oneself with a towel straight away.

Seawater evaporates with the heat of the Sun; the steam rises, cools in the upper layers of the atmosphere and forms clouds; finally, rain falls.

1. a) Name the change of state that seawater undergoes when it turns into steam.
2. b) Name the change of state that occurs when the cloud droplets form.
3. c) In each of these two changes, does the water absorb or release heat? Who supplies it or takes it away?

An ice cube comes out of the freezer at $-10\ ^\circ\text{C}$ and we leave it on a plate in the kitchen.

1. a) How high does the temperature of the ice rise before it starts to melt?
2. b) What is the name of the change of state that it undergoes next?
3. c) In this change of state, does the water absorb or release heat?

Water freezes at $0\ ^\circ\text{C}$ and boils at $100\ ^\circ\text{C}$. State the state of matter of the water in each case and justify your answer. The diagram places the three cases on a temperature scale:

1. a) The water on the ice rink, which is at $-3\ ^\circ\text{C}$.
2. b) The water in a glass, which is at $25\ ^\circ\text{C}$.
3. c) The water coming out of a pot, which is at $110\ ^\circ\text{C}$.

We slowly heat ice that is at $-10\ ^\circ\text{C}$ until it turns into steam at $110\ ^\circ\text{C}$, measuring the temperature the whole time. The setup is shown in the diagram.

1. a) Make a list of the five stages of the process, indicating what is present in each stage (ice, water, steam or a mixture) and what the temperature is doing.
2. b) In which two stages does the temperature not rise even though we continue to supply heat? At what temperatures do they occur?
3. c) What is the heat being used for during those two stages?

We put an ice cube tray filled with water into the freezer and measure its temperature every five minutes: $20\ ^\circ\text{C}$, $12\ ^\circ\text{C}$, $5\ ^\circ\text{C}$, $0\ ^\circ\text{C}$… and for a long time it continues to read $0\ ^\circ\text{C}$ while the ice is forming. The graph shows the measurements.

1. a) Your brother says that the water should go from $0\ ^\circ\text{C}$ to $-5\ ^\circ\text{C}$ continuously. Who is right, him or you? Why?
2. b) While ice is forming, does the water continue to lose heat or not?
3. c) What is that heat lost by the water during the change of state used for?

In the mountains, rainwater seeps into cracks in the rocks. At night it freezes and, little by little, the cracks get bigger until the rock eventually breaks. Look at the diagram.

Knowing that water increases in volume when it freezes, explain how it manages to break the rock.

Water is an unusual substance: when it freezes its volume increases, so ice is less dense than liquid water.

1. a) Explain why ice cubes float in a glass.
2. b) In a lake in winter, does the surface freeze first or the bottom? Justify your answer.
3. c) What advantage does this have for the fish living in the lake?

At a scientific base in Antarctica, the thermometer reads $-50\ ^\circ\text{C}$. Mercury freezes at $-39\ ^\circ\text{C}$ and alcohol at $-114\ ^\circ\text{C}$. The diagram summarizes the data on a temperature scale.

1. a) Which thermometer works there, the mercury one or the alcohol one?
2. b) What happens to the liquid in the other thermometer, making it unusable?
3. c) What is the name of the change of state that this liquid undergoes?