Changes of state
You put some ice on to heat and, while it is melting, the thermometer stays stuck at $0\ \celsius$ for quite a while, even though the heat is still on. Where does that heat go if the temperature does not rise?
The six changes of state
You take an ice cube out of the freezer, after a while it turns into a puddle of liquid water and, if you leave it in the sun for several days, the glass ends up dry: how can matter change from one form to another without ceasing to be the same substance?
Copy and complete the diagram with the name of each change of state: a) From solid to liquid: $\dots$ b) From liquid to gas: $\dots$ c) From gas to liquid: $\dots$ d)…
Copy and complete the diagram with the name of each change of state:
- a) From solid to liquid: $\dots$
- b) From liquid to gas: $\dots$
- c) From gas to liquid: $\dots$
- d) From liquid to solid: $\dots$
- 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…
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.
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…
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?
Melting and freezing: the melting point
Move the slider past $0\ \celsius$: you will see the particles change from vibrating in place to sliding freely past one another. That temperature value is the melting point of this substance.
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…
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).
At what temperature does ice melt? And at what temperature does water boil at sea level? Express them in degrees Celsius.
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.…
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?
Vaporisation and condensation: evaporation and boiling
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…
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.
- a) What change of state is occurring?
- 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 $100\ ^\circ\text{C}$.…
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?
When a wound is disinfected with rubbing alcohol, the skin becomes dry in seconds and you feel a cold sensation in the area. a) What change of state does the alcohol…
When a wound is disinfected with rubbing alcohol, the skin becomes dry in seconds and you feel a cold sensation in the area.
- a) What change of state does the alcohol undergo on the skin?
- b) Vaporisation requires heat to occur: where does the alcohol get this heat from? Why do you feel cold?
Sublimation and deposition
A toilet air-freshener block gets smaller every week until it disappears completely, but it never leaves any liquid around it: how can a solid evaporate without passing through the liquid state?
Some substances, such as carbon dioxide, cannot exist as a liquid at normal air pressure: when heated they change directly from solid to gas. That is why they do not have a separate melting point and boiling point, but a single sublimation temperature ($-78\ \celsius$ for CO₂). Others, such as camphor, sublime slowly at room temperature, just as a puddle evaporates without boiling.
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…
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…
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).
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.…
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?
Heating curves
You heat ice with a sensor that records the temperature every minute: you would expect a continuous rise, but you get a graph with completely flat steps where time keeps running and the temperature stands still: why does the thermometer stop if the heat is still on?
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…
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.
- a) In which section is there only ice?
- b) What is happening during the horizontal section at $0\ ^\circ\text{C}$? In what state or states is the water in this section?
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…
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.
- 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.
- b) What state is the water in at minute $5$? And at minute $15$?
- c) How many minutes did the melting of the ice last?
- 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…
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.
- a) What does each plateau mean?
- b) What state is the substance in at $25\ ^\circ\text{C}$? And at $90\ ^\circ\text{C}$?
- c) Could it be water? Explain why melting and boiling points are used to identify substances.
Back to the beginning: The heat supplied while the thermometer reads $0\ \celsius$ does not raise the temperature because it does not speed up the particles, but is used entirely to break the bonds that hold them fixed in the solid lattice of the ice.