Exercises: Changes of state

Copy and complete the diagram with the name of each change of state:

1. a) From solid to liquid: $\dots$
2. b) From liquid to gas: $\dots$
3. c) From gas to liquid: $\dots$
4. d) From liquid to solid: $\dots$
5. e) From solid to gas, without passing through liquid: $\dots$

In a soft drink with ice, the cubes get smaller and smaller until they disappear. What change of state has the ice undergone? State the initial state, the final state and the name of the change.

When boiling water in a kettle, steam comes out of the spout. What is the name of the change of state from liquid to gas? Give another everyday example in which this same change occurs.

When taking a hot shower, the bathroom mirror fogs up and becomes covered in small droplets. What change of state does the water vapour undergo when touching the cold mirror? Write its name and its direction (from which state to which state).

To make ice at home, Leo fills an ice cube tray with water and puts it in the freezer. What change of state does the water undergo? Write its name and its direction (from which state to which state).

Mothballs placed in wardrobes against moths wear away gradually without wetting anything around them: they pass directly from solid to gas. What is this change of state called?

At what temperature does ice melt? And at what temperature does water boil at sea level? Express them in degrees Celsius.

Marc heats a saucepan with ice and water and observes the thermometer: while there is still un-melted ice, the temperature does not rise. The figure shows the apparatus.

What happens to the temperature of a substance while a change of state is taking place? Does it rise, fall or remain constant?

Identify the change of state occurring in each case and reason whether the substance absorbs heat from the surroundings or releases it:

1. a) Morning dew: droplets of water on grass.
2. b) Wet clothes drying in the sun.
3. c) Liquid paraffin of a candle becoming solid when extinguished.

Jimena heats crushed ice coming out of the freezer at $-10\ ^\circ\text{C}$ and measures the temperature every minute: first it rises to $0\ ^\circ\text{C}$; then it remains fixed at $0\ ^\circ\text{C}$ for $6\ \text{min}$ while water appears; finally it rises again. The figure shows the temperature-time graph Jimena obtains.

1. a) In which section is there only ice?
2. b) What is happening during the horizontal section at $0\ ^\circ\text{C}$? In what state or states is the water in this section?

Grandma's pot is boiling and the kitchen thermometer reads $100\ ^\circ\text{C}$. You turn the heat up to maximum and steam comes out more strongly, but the thermometer still reads $100\ ^\circ\text{C}$, as shown in the figure.

1. a) What change of state is occurring?
2. b) If more heat arrives, why does the temperature not rise? What is this energy being used for?

A puddle in the street dries in the sun at $20\ ^\circ\text{C}$ without the water boiling. In contrast, water in a pot only boils when it reaches $100\ ^\circ\text{C}$. In both cases, water changes from liquid to gas. Explain the difference between evaporation and boiling: which part of the liquid is transformed into gas in each case, and at what temperatures can each occur?

After mopping the kitchen floor, you want it to dry as quickly as possible. Will it dry faster on a hot day or a cold day? Does opening the window to let the air circulate help? Give two reasons, thinking about what helps the water particles escape from the floor's surface.

When a wound is disinfected with rubbing alcohol, the skin becomes dry in seconds and you feel a cold sensation in the area.

1. a) What change of state does the alcohol undergo on the skin?
2. b) Vaporisation requires heat to occur: where does the alcohol get this heat from? Why do you feel cold?

On a very cold morning, the windscreen of a car wakes up covered in frost: the water vapour in the air has turned directly into ice, without becoming a liquid first. What change of state is this? How is it similar to and different from the formation of dew?

After a snowfall, a cold, dry week follows, with the temperature not rising above $0\ ^\circ\text{C}$. Even so, the snow on the pavement shrinks day by day without wetting the ground. What change of state explains why the snow disappears without melting? Explain it.

In a laboratory experiment, a saucepan with ice at $-5\ ^\circ\text{C}$ is heated over a constant flame and the temperature is recorded every minute: the temperature rises and reaches $0\ ^\circ\text{C}$ at minute $2$; it stays at $0\ ^\circ\text{C}$ until minute $10$; then it rises again up to $100\ ^\circ\text{C}$, which it reaches at minute $20$, and remains there while the water boils.

1. a) Describe what the temperature-time graph would look like: which rising sections and which horizontal sections (plateaus) it has, and at what temperatures they are.
2. b) What state is the water in at minute $5$? And at minute $15$?
3. c) How many minutes did the melting of the ice last?
4. d) What is happening during each plateau? Why does the temperature not rise during them even though the flame remains on?

When an unknown substance is heated, its temperature-time graph shows two plateaus: one at $50\ ^\circ\text{C}$ and another at $120\ ^\circ\text{C}$, as shown in the figure.

1. a) What does each plateau mean?
2. b) What state is the substance in at $25\ ^\circ\text{C}$? And at $90\ ^\circ\text{C}$?
3. c) Could it be water? Explain why melting and boiling points are used to identify substances.

In a mountain shelter at an altitude of $2000\ \text{m}$, water boils at around $93\ ^\circ\text{C}$, because the air pressure is lower than at sea level.

1. a) Do you think a boiled egg cooks faster or slower there than on the beach? Justify your answer.
2. b) In a pressure cooker, the opposite happens: water boils above $100\ ^\circ\text{C}$. Why does food cook faster in it?

At concerts, dry ice (solid carbon dioxide, which is very cold) is used to create a white cloud covering the stage floor.

1. a) Dry ice turns directly from solid into gas without melting: what is this change of state called?
2. b) The white cloud is not carbon dioxide, which is invisible, but tiny water droplets that form when the water vapour in the air cools down beside it: what change of state does this water vapour undergo? Explain the complete chain of changes that produces the cloud.

A burn from water vapour at $100\ ^\circ\text{C}$ is more severe than a burn from boiling water at $100\ ^\circ\text{C}$, even though both are at the same temperature. Hint: upon touching the skin, the vapour condenses first and turns into liquid. Use the hint to explain why the vapour transfers more heat to the skin than the boiling water.

You put a glass of water at $20\ ^\circ\text{C}$ into the freezer, which is at $-18\ ^\circ\text{C}$.

1. a) Describe what the cooling graph for water would look like: which falling sections and which horizontal section (plateau) it would have, and at what temperature the plateau would be.
2. b) What change of state occurs during that plateau?
3. c) If you repeat the experiment with twice as much water, will the plateau last longer or shorter? Give reasons for your answer.

In winter, Álvaro hangs his wet clothes out to dry inside the house. The clothes take a long time to dry and, after a few hours, the window panes mist up.

1. a) What change of state does the water in the clothes undergo?
2. b) Where do the droplets on the windows come from? Explain the path of the water from the clothes to the pane, naming the two changes of state that occur.
3. c) Propose two ways to speed up the drying of the clothes and justify them.

Ethyl alcohol boils at $78\ ^\circ\text{C}$. In the laboratory, you heat, at the same time and using the same flame, a beaker with water and another with the same amount of alcohol, both starting at the same temperature.

1. a) Which of the two reaches its boiling plateau first?
2. b) When both are boiling, which one is at a higher temperature?
3. c) What characteristic property of each substance does this experiment demonstrate? What can knowing it be useful for?