Melting and freezing

A beaker of ice warmed by room air sat at exactly 0 °C for more than half an hour while it melted.

Put a beaker of ice at −12 °C on the bench. OpenStax shows what happens. After ten minutes the ice has warmed to 0 °C and a little has melted. Thirty minutes later more has melted, but the thermometer still reads 0 °C. Only when the last of the ice is gone does the temperature start to climb, reaching 22.2 °C. The ice kept absorbing heat from the room the whole time. During the melting, none of that heat raised the temperature.

Where the heat goes

In a crystal each molecule is held in place by its neighbours. Heating a solid makes the molecules vibrate harder, and the temperature rises. At the melting point the vibrations are strong enough to pull molecules out of their fixed places. From then on the added energy goes into breaking those holds, not into faster motion, so the temperature stays flat until the whole solid has become liquid. The energy needed to melt one mole of a solid is its enthalpy of fusion. For ice it is 6.01 kJ/mol, so melting 18.02 g of ice takes 6.01 kJ.

How big is 6.01 kJ/mol? Compare it with warming. Liquid water needs 4.18 J to warm 1 g by 1 °C, so 18.02 g needs about 75.3 J per degree. The 6.01 kJ that melts that mole of ice would instead warm the meltwater by 6010 ÷ 75.3 ≈ 80 °C. That is why a drink with ice in it stays cold for so long: each cube soaks up a lot of heat before it is gone, and the drink stays near 0 °C until then.

Freezing runs the same process backwards. As liquid water at 0 °C turns to ice it gives out 6.01 kJ for every mole. Melting takes in heat (it is endothermic); freezing releases heat (it is exothermic). Melting and freezing happen at the same temperature, so the melting point of ice and the freezing point of water are one number. In a perfectly insulated flask a mixture of ice and water stays at that temperature, with melting and freezing going on at equal rates.

Every substance has its own melting point

How hot a solid must get before it melts depends on how strongly its particles attract one another. The values below are at a pressure of 1 atm, from the NIST Chemistry WebBook. Mercury is the only metal in the list that is liquid at room temperature; it freezes at −38.84 °C, so a mercury thermometer stops working below that temperature. Nitrogen and oxygen, held together only by weak forces between their molecules, melt more than 200 degrees below zero.

Because each pure substance melts at a fixed temperature, chemists use the melting point as a quick identity check. A sample that starts to melt below the expected value is probably not pure, because dissolved impurities lower the freezing point. Salt spread on an icy road works the same way: a salt solution freezes below 0 °C, the freezing point of pure water, so the ice melts even though the air is below 0 °C (OpenStax 11.4).