Melting is particles gaining enough energy to leave their fixed positions.
Step 1: Something to Notice
Watch first. The explanation comes later.
Put a thermometer in a pan of ice and water and heat it hard. The reading sits at 0 °C and will not move until the last ice has gone.
Where is all that energy going?
Step 2: Find Out
Add energy steadily and find where the temperature refuses to move.
Step 1 — Predict
You keep heating ice-water at 0 °C. What does the thermometer do?
Choose what you think will happen. You cannot see the experiment until you do — guessing first is what makes it worth watching.
Step 3: So Here Is Why
Now the explanation, after you have seen it happen.
The temperature holds still
While a state is changing, the reading does not move even though energy is still arriving.
The energy goes into separating
Pulling particles apart takes energy and does not make the rest move faster.
Boiling takes far more than melting
Loosening particles is one thing; separating them completely takes about seven times as much.
Why the thermometer stops
During melting or boiling the temperature holds steady even though heat is still going in. The energy is breaking the forces between particles rather than speeding them up.
Changes of state are physical
The substance is the same before and after — ice, water and steam are all water. No new substance appears, and the change reverses completely on cooling.
Step 4: A Common Mistake
Lots of people think
“If you keep heating something, its temperature keeps rising.”
Step 5: Why Sweating Cools You
The same idea somewhere new.
Evaporating takes a great deal of energy, and that energy comes from whatever the liquid is touching. Sweat leaving your skin as vapour carries that energy away with it, which is why you feel cold when you get out of a bath and why a fan helps — it removes the vapour so more can evaporate.
Step 6: Think It Through
Practice makes it stick.
The Stuck Thermometer
Problem 1 of 2
A thermometer in melting ice will not rise. Where is the energy going?
Which Takes More
Problem 2 of 2
Which takes more energy for the same mass of water: melting it, or boiling it away?
The Flat Parts
1 of 4
What happens to the temperature during melting?
2 of 4
Where is the energy going during a change of state?
3 of 4
Which flat section is wider on the curve?
4 of 4
Why does sweating cool you?
Step 7: Quick Check
Show what you know.
Question 1 of 1
Why does the temperature stop rising while ice melts?
Step 8: Explain It in Writing
Claim, evidence, then reasoning.
The question
Explain why a thermometer in melting ice will not rise however hard you heat it.
Words to useseparatingarrangementaverage speedstill arrivingflatmotionnot faster
Fill in all three boxes. The reasoning box is the one that matters most.
What You Found Out
- Melting and boiling are particles gaining enough energy to break their arrangement.
- The temperature holds still during a change of state, even while heating continues.
- The energy goes into separating particles, which a thermometer cannot see.
- Boiling takes about seven times the energy melting does, for the same mass.