If you heat a pot of water on the stove ‘till it boils, what do you see? More to the point, what can you see through? The liquid water is clear, and the steam is clear. And you can tell the two apart.
Liquid water and steam (gaseous water) are two phases of the same substance. To boil water is to cause a phase transition because we start with one phase (liquid) and end up with another. There’s another special thing about this phase transition – there is both liquid and gaseous water in the pot at the same time (and again, we can clearly distinguish liquid water from gaseous steam). If we went in the opposite direction – cooling the steam to liquid – we’d see droplets of water condense bit by bit, just like we see bubbles of steam appear. Either way, the transition doesn't happen everywhere all at once. It happens in bits and pieces, and takes some time and a certain amount of energy (in the form of heat) before all the water has turned to gas.
Look at the phase diagram above; it shows you at what temperatures and pressures water exists as a solid (ice), as a liquid, and as a gas (steam). A red line has been drawn to show the boiling transition on the phase diagram.
Ask yourself:
- What are some phase transitions you see all the time? What do they have in common?
- Here are a few: rain, snow, seeing your breath on a cold day, turning on a fluorescent light or a plasma TV, dissolving sugar into tea... You might even think about car traffic as having different phases – standstill, slow, fast...
- Why do we see the bubbles in the boiling water if liquid and gaseous water are both see-through?
- Molecules in liquid water are much closer to each other than in gaseous water. So the density of liquid water is greater than that of gaseous water. Very often, light travels slower through denser media than through less dense media. At the boundary between “slow” (dense) and “fast” (less dense) media, light bends – this is called refraction. This is what causes haze or mirages on a hot day, and what makes glasses and telescopes work. And, except in a few special circumstances, not all of the light goes through the boundary – a part of it (sometimes all of it) bounces back, or is reflected. This is why you can see your reflection in a window while still seeing through the window. And because some light bounces off the steam bubbles in the liquid water and into our eyes, we can see the bubbles in boiling water.
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Quizlet:
- Is an ice cube melting in a drink a phase transition?
- Is blending strawberries, bananas, and milk to make a smoothie a phase transition?
- Is burning a piece of wood a phase transition?
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