Скачать или смотреть When water boils, where do the bubbles come from?

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The best way to answer this question is by examining what exactly is happening when water is boiling. The diagram you see in front of you is what is known as a phase diagram and displays how water changes phases under different temperatures and pressures. So in this diagram we see that the boiling point of water is 100 degrees Celsius. At this temperature we can see that the water is in equilibrium between its liquid and gas phase. In other words the liquid is at equilibrium between evaporating into vapor and vapor condensing into liquid. Since the pot is the hottest at the bottom when you see these bubbles forming it is actually water that is being vaporized. The bubbles form at the bottom because this is the hottest part of the system compared to the top where the water would be coolest. At the bottom enough heat is supplied to allow the water to escape as vapor or in the case of water steam.
• However there is also a second aspect to this. When you initial start heating water you will notice that there are bubbles forming on the sides of the pot. This is not to be confused with water molecules evaporating. These bubbles are simply air leaving the liquid . As the temperature rises the solubility of air decreases and it escapes the water through these bubbles. The bubbles of water that escape are only seen when water reaches this boiling temperature of 100 degrees Celsius.

Picture : http://apchemcyhs.wikispaces.com/Phas...