[ELI5] How does a glass of water standing for a while create small bubbles on edge of the glass?

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[ELI5] How does a glass of water standing for a while create small bubbles on edge of the glass?

In: Chemistry

Bubbles mean something in the water is ~~changing from a liquid into a gas~~ escaping the water as a gas.

Tap water contains atmospheric gases, such as nitrogen and oxygen, dissolved in it (dissolved means microscopic bubbles of gas get ‘trapped’ in the water by any change in pressure, such as water in a pipe or the wind blowing against the top of the ocean). As the glass filled with water sits out for a few hours, its temperature rises slightly (water gets warmer), which causes the dissolved gases in it to come out of the water and form bubbles along the inside of the glass.

This is a phenomenon called Nucleation. When you have something dissolved in water, and you introduce the water to a rough surface, the dissolved things come out of the water.

Air is always mixed with water in a small amount, because air is all around us.

Normally, air would leave the water through the top of the glass.

There are tiny nicks or rough spots you can’t see in the glass, where the air wants to come out, and it makes a tiny bubble. Then the bubble covers the nick, and no more air comes out.

This is best demonstrated with making sugar candy. You take hot water, dissolve a ton of sugar in it, then let it cool. Now the water has more sugar than it should be able to dissolve. When you put a string into the water, the sugar nucleates onto the string. Voila, you now have sugar crystal rock candy.

To add, it’s also simply due to Impurities in the water. Most commonly soap residue will push bubbles to edges due to weaker surface tension. But any salts or minerals will cause additional gas to separate out. Distilled water in sterile beaker will have few bubbles compared to your tap or dirty glass. Also look up Nucleation