Primary 6 · Science · Cycles: Boiling, Melting & Freezing
Cycles: Boiling, Melting & Freezing questions for Primary 6 Science
9 questions to work through online, 2 of them written out the way the paper asks. Answer and you get the working straight away, then a score at the end. No sign-up, and you can retake it as often as you like.
- Reading a heating curve
- What happens at a plateau
- Impurities
- Reasoning about mass
- Evaporation against boiling
- Naming a temperature
- Naming a change
- Applying the ideas
Same picture as the question above.
| Liquid | Freezing point (°C) | Boiling point (°C) |
|---|---|---|
| pure water | 0 | 100 |
| salty water | -4 | 103 |
| very salty water | -11 | 107 |
Answers and explanations for all 9 questions
Every answer with the reasoning behind it. Worth reading after you have tried the set — or on its own, if you are a parent marking it.
In the graph above, what is present during the first flat section?
Answer: a mixture of ice and water
The flat section is the melting itself. While any ice remains, ice and water sit together at 0 °C, and only when the last of it melts does the temperature rise.
According to the graph above, at what temperature does the second flat section occur?
Answer: 100 °C
The second plateau is the boiling, which for water is at 100 °C.
Which of these are true while a substance is melting at its melting point?
Answer: A, B, C and D
All four hold at once. Heat keeps arriving and is used to change the state rather than raise the temperature, both states coexist, and nothing is gained or lost.
Based on the table above, what does dissolving salt in water do?
Answer: It lowers the freezing point and raises the boiling point.
Salt widens the range over which water stays liquid: it freezes later and boils later, which is why salt is spread on icy roads.
A sealed flask of ice is weighed, left until the ice has melted, and weighed again. What is found?
Answer: The mass is unchanged.
Melting rearranges the same matter. The volume shrinks slightly, which is why ice floats, but the sealed flask contains exactly what it did before.
Which two statements distinguish evaporation from boiling?
Answer: A and B only
Evaporation is a surface process at any temperature; boiling is a whole-liquid process at one temperature. The other options reverse each of those.
The second flat part of the graph above is where the liquid turns into a gas throughout. The temperature there is called the ______ point.
Answer: boiling
The temperature at which a liquid boils throughout is its boiling point.
Frost on a cold morning forms directly from water vapour in the air, without ever being liquid. The reverse change — a solid turning straight into a gas — happens to dry ice. What state does dry ice turn into?
Answer: gas
Dry ice turns straight from a solid into a gas without becoming liquid.
Why does a pan of water on a hot stove stay at 100 °C however much longer it is heated?
Answer: The extra heat is used to turn water into steam rather than raise the temperature.
At the boiling point all the incoming heat goes into the change of state. The pan boils dry rather than getting hotter, which is why the temperature holds.