Why do bubbles form in water

How and why do bubbles form in water?

Son and dad played with some water the other day. First, we blew into a glass of tap water with a straw. A few bubbles formed on the surface and quickly popped.

Why?

Surface tension.

Surface tension arises from the fact that a liquid is held together by cohesion. The molecules that make up the liquid are attracted to all adjacent molecules. Well, at the surface of a liquid, there are fewer adjacent molecules. This means the cohesive forces between the remaining adjacent molecules are enhanced. This is surface tension.

When you blow air through the water, the air displaces the water and we see buoyancy in effect. We also see surface tension take action (yes, even at the bottom of the glass of water). This is because there is a surface at the air-liquid interface, and the water molecules on that surface have fewer adjacent molecules; the forces are enhanced. The tension on the bubble would like to be minimized and the best way to do that is to maximize the surface area over which the force is distributed, giving you a roughly spherical shape to the air bubble.

So, due to surface tension a roughly spherical bubble forms. Due to buoyancy, that bubble moves upward through the water.

Why does a bubble form at the surface? Surface tension again. When the air moves through the top "layer" of water, the water tries to stick together and a bubble forms.

It pops so quickly because the surface tension of water is so great that the water is pulled back to itself quickly. Here's a poor-quality, high-speed-camera video of a soap bubble popping. If you squint and watch closely, you'll see that the bubble does not just collapse. It pulls back upon itself, like a retracting dome. This is due to the surface tension keeping the water together.




So, after that, we put a little dish soap into the water and again blew air through. This caused a lot more bubbles to form on the surface, and most of those bubbles lasted a long time.

Why?

Surface tension.

Dish soap is a detergent, which decreases surface tension. Because the surface tension is decreased, the bubbles on the surface of the water are able to stay together longer; the cohesive forces are not as strong now that the detergent has been mixed in and the water+soap doesn't pull itself back together as quickly. If the surface tension was 0, the liquid would be a gas...

Detergents are used specifically because they lower the surface tension of water, which stops its beading behavior and allows it to soak into clothing or better dissolve junk on dirty dishes. Also, higher temperatures decrease water's surface tension, allowing better cleaning behavior.