Thinking about things

We were all taught in school that water boil at 100 degrees Celsius? But why does this happen? And what is the difference between evaporation and boiling?

When any substance is heated, its molecules are supplied with kinetic energy. In solids, this energy causes them vibrate in place, whereas in liquids and gases, they are free to move around. When a molecule in a pool of liquid moves fast enough, it breaks free from the pool and escapes away as gas. That is how evaporation occurs. The higher the temperature, the greater the kinetic energy of the liquid molecules and greater the rate of evaporation.

However, another force opposes this tendency for liquids to turn into vapour – atmospheric pressure. Gas molecules from the atmosphere push down on the liquid molecules, keeping them in place and preventing them from turning into water vapour. This dance between heat (measured as temperature), that causes water molecules to evaporate and atmospheric pressure that prevents this from happening continues until a critical point is reached.

That critical point, in the case of water, happens to be 100 degrees Celsius. At this temperature, all the molecules in a pool of water have enough kinetic energy to overcome atmospheric pressure and they vapourize instantly. This point is what we call the boiling point.

Once you understand this, you also know why water boils at lower temperatures in regions of higher altitude. Since the air is thinner as we go higher, the air pressure that keeps water from vapourising is lower.

Such accessible explanations to all manners of scientific phenomena around us are provided by Richard Feynman. Feynman mentions how he likes to think about things and explain them in terms that anybody can understand. Unfortunately, our science textbooks don’t work this way. Even a Wikipedia entry on boiling point uses terms such as vapour pressure and standard boiling point, which causes our interest on the topic to vapourize faster than water at 100 degrees Celsius.

School has filled our heads with facts, but doesn’t teach us to think about things. Learning to do that is up to each one of us.

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