Hot and Cold
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flagWhat you'll discover
- arrow_forwardDescribe melting and freezing using the particle picture
- arrow_forwardMatch each substance to its real melting point
- arrow_forwardExplain why the thermometer pauses while something melts
- arrow_forwardShow that freezing is just melting in reverse, at the same temperature
Solid or liquid? Ask the particles
Everything is made of tiny particles, far too small to see. In a solid like ice or a chocolate bar, the particles hold hands in a neat pattern and only wiggle in place. That is why solids keep their shape.
Heat the solid up and the particles wiggle harder and harder — until they break free and start sliding past each other. Now it flows: it has melted into a liquid!
Every substance has its own magic number
Each substance melts at its own special temperature, called its melting point. Ice melts at exactly 0 °C. Chocolate melts at about 34 °C and butter at about 35 °C — just below your body temperature of 37 °C. Candle wax needs about 60 °C.
That is why chocolate melts in your hand and on your tongue, but a candle stays solid on a warm day. Try all four substances in the simulation and check their numbers!
The thermometer takes a break
Heat ice steadily and watch the thermometer climb: −10, −5, 0… and then it stops! While the ice is melting, the temperature stays stuck at 0 °C even though you keep adding heat.
Why? All that energy is busy breaking the particles out of their neat solid pattern. Only when the very last bit has melted does the temperature start climbing again. Scientists call this hidden energy latent heat.
Freezing: the movie in reverse
Cooling does exactly the opposite. Slow the particles down and, at the very same magic number, they snap back into their neat pattern — the liquid freezes solid. Water freezes at 0 °C, the same temperature ice melts.
Melting and freezing are the same doorway, just walked through in opposite directions. Melted chocolate left on the counter hardens again because the room is colder than 34 °C.