Kitchen chemistry: Melting and boiling

Previous kitchen chemistry posts have discussed solids, liquids, and gases. Molecules in a solid are fixed in place within a structure, like the carbon dioxide molecules in this crystal of “dry ice”:

These molecules vibrate in place, and as the temperature increases, they vibrate faster (indeed, that’s what temperature means). In most solids (“dry ice” is an exception), there comes a point where the molecules break loose from the solid structure (though still being held down by gravity) – this is called melting, and produces a liquid. As the temperature increases even further, the molecules vibrate even faster. Eventually, gravity no longer holds them down, and they fly around in all directions – this is called boiling, and produces a gas.

Generally speaking, the heavier molecules are, the harder it is to get them to break loose and fly away. Heavier molecules therefore tend to have higher melting and boiling points. Among hydrocarbons, for example, propane is a gas at room temperature (room temperature is marked by a dashed line in the chart below). Octane (C8H18), found in petrol (gasoline), is a liquid at room temperature. Hentriacontane, with 31 carbon atoms (C31H64), is a waxy solid:

Melting and boiling points

The chart shows some exceptions, however. Water molecules and ethanol (alcohol) molecules are very light, but they tend to “stick” to each other, and this means that water and ethanol are liquids at room temperature.

In water, for example, the oxygen atom (red in the model below) carries a slight negative electrical charge, while the hydrogen atoms (white below) carry a slight positive electrical charge. Because of the same phenomenon that allows static electricity on a recently used comb to attract small scraps of paper, the oxygen atom on one water molecule is attracted to the hydrogen atoms on other water molecules. This “stickiness” means that it takes more heat to make the water molecules fly away.

Armed with a good thermometer, the home experimenter can verify that water boils at 100 °C (place the thermometer in a saucepan of boiling water) and freezes at 0 °C (place the thermometer in a cup of water which is in turn placed in a bucket of salt and crushed ice, and see at what temperature the water starts to freeze – the same method can be used to find the freezing point of different kinds of oil, and even to make home-made ice cream). Alternatively, put the thermometer in a cup of ice, and see at what temperature the ice starts to melt.

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One thought on “Kitchen chemistry: Melting and boiling

  1. Pingback: Kitchen chemistry: three books | Scientific Gems

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