KS3 · Chemistry

Melting points and freezing points

Silver freezes at 962 °C. Freezes, not melts. Something can be 'frozen' and still be far hotter than any room you've stood in. By the end, that will feel normal.

One scale, one question

Which side of its point are you on?

Room temperature is 20 °C. Drag each tag to its temperature in °C: the melting/freezing point of water (0), paraffin (67), Substance A (15) and Substance B (30), plus room temperature itself (20). Press check, then look at which side of the Room tag each point lands.

Predict, then check

You just used one tag for a 'melting and freezing' point. Was that cheating?

Water melts at 0 °C and silver melts at 962 °C. Where do you think each one freezes?

Warm it up, cool it down

Solid state (ice)

Temperature of the water: -10 °C. State: Solid state (ice). static

Temperature of the water-10 °C

Below 0 °C: the particles are in fixed positions in a regular arrangement. They can only vibrate.

Water melts and freezes at 0 °C. Slide the temperature up past 0 °C, then back down again, and watch what the particles do.

One change, two directions

Melting, freezing, or both?

Sort each statement. Does it belong to melting, to freezing, or to both? If it belongs to neither, leave it outside.

  • A Melting
  • B Freezing
  1. Solid state to liquid state
  2. Liquid state to solid state
  3. Happens when a liquid is cooled
  4. Particles become fixed in position
  5. Particles start sliding past each other
  6. Happens at 0 °C for water
  7. Happens at 962 °C for silver
  8. Happens at 20 °C for water

Check your own thinking

Is it solid or liquid?

A tub of Substance B has a freezing point of 30 °C. It is sitting in a room at 20 °C.

Which idea is closest to what you think right now?
How sure are you?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Put a temperature next to a substance's own point and the answer is sitting right there.

What you need to know

  • Melting is the change from the solid state to the liquid state. Freezing is the change from the liquid state to the solid state. Freezing is melting in reverse.
  • The melting point is the temperature at which a substance changes from the solid state to the liquid state. The freezing point is the temperature at which it changes from the liquid state to the solid state.
  • For a pure substance these are the same temperature. Water melts and freezes at 0 °C, and silver melts and freezes at 962 °C.
  • Below its point a substance is in the solid state; above its point it is in the liquid state. Compare the temperature with that substance's own point.

The big picture

A pure substance melts and freezes at the same temperature, so one number, its melting/freezing point, tells you its state at any temperature: the solid state below it, the liquid state above it. Freezing is melting in reverse, and the particles go from fixed and regular to sliding and random when a substance melts, and back again when it freezes.

Key points

1Melting: solid state to liquid state. Freezing: liquid state to solid state. They are opposite changes.
2Melting point: where a substance changes from the solid state to the liquid state. Freezing point: where it changes from the liquid state to the solid state.
3For a pure substance, the melting point and the freezing point are the same temperature (water 0 °C, silver 962 °C).
4Below the point: solid state. Above the point: liquid state. In a 20 °C room, paraffin (67 °C) is in the solid state, a substance with a freezing point of 15 °C is in the liquid state and one with a freezing point of 30 °C is in the solid state.
5When a substance melts, its particles go from fixed positions in a regular arrangement to sliding past each other in a random arrangement. When it freezes, they go back.

Worked example

Problem

Substance C is a pure substance with a melting point of 45 °C. Is it a solid or a liquid at 20 °C, and at 60 °C?

⚠ Watch out

Thinking 'frozen' means cold, or that freezing always happens at 0 °C. 0 °C is only water's freezing point. Every pure substance has its own, and a substance can be in the solid state at temperatures much higher than 0 °C.

🧠

Memory hook

One point, both ways. Below the point, solid. Above the point, liquid. And 'frozen' just means in the solid state, not cold.

✓

Check yourself

A pure substance has a freezing point of 40 °C. Without looking back: what is its melting point, and which state is it in at 25 °C and at 55 °C?

Flashcards

(12)
Which change of state is melting?
The solid state changing to the liquid state.
What happens when a substance freezes, and how does it relate to melting?
It changes from the liquid state to the solid state. Freezing is the reverse of melting.
What is a melting point?
The temperature at which a substance turns from the solid state into the liquid state (0 °C for water, for example).
What is a freezing point?
The temperature at which a substance turns from the liquid state into the solid state (962 °C for silver, for example).
How do the melting point and freezing point of a pure substance compare?
They are the same temperature. Water melts and freezes at 0 °C; silver melts and freezes at 962 °C.
The temperature is lower than a substance's melting/freezing point. Solid or liquid?
Solid. Higher than the point gives the liquid state.
Paraffin has a melting point of 67 °C. Which state is it in at room temperature, 20 °C?
The solid state, because 20 °C is below 67 °C.
Can something be frozen and hotter than 0 °C?
Yes. Frozen means in the solid state. Silver is frozen at any temperature below 962 °C.
What are the particles doing in the solid state?
They are in fixed positions and can only vibrate.
What are the particles doing in the liquid state?
They slide past their neighbours, which is why a liquid takes the shape of its container.
What happens to the particles when a substance melts?
They go from fixed positions in a regular arrangement to sliding past each other in a random arrangement.
What happens to the particles when a substance freezes?
They go from sliding past each other in a random arrangement to fixed positions in a regular arrangement.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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