Gas bubbles

It may seem strange to talk about a water drop in a topic on gas bubbles but the two share something in common. They both stay round if they can.

When a drop of water lands on a surface, it does not stay round. It becomes a little flattened but if you look closely at its surface, it is curved round the edge. The drop behaves as if it has a skin. The reason for this is due to the particles inside the water drop. The particles pull on each other. Those particles in the centre of the drop are pulled by the particles all around them. The particles on the surface are pulled by those underneath them and those by their sides. The pulls on the particles at the surface make them try to shape the drop into a ball. When the drop lands on the surface the particles make the surface as round as possible. The way that the particles pull on each other, to make a liquid surface behave like a skin, is called surface tension.

The same thing happens when a bubble forms. A bubble is a quantity of gas surrounded by a liquid. Some of the most familiar bubbles are the bubbles you can form by squashing a sponge underwater. When you put the sponge under water, air is trapped inside it. You cannot see the air because it stays in place but when you squash the sponge, the air moves. It shoots out into the water. Each cavity in the sponge sends out its own quantity of air. The water surrounds it and makes a round shape we call a bubble. The shape is due to the way the particles in the water surface of the bubble are being held by surface tension.

The air in the bubble is a mixture of gases. It is much lighter in weight than the liquid around it. This difference in weight makes the air bubble float upwards. When it reaches the surface the top of the bubble may stay attached to the skin-like water surface. There is a simple experiment you can do to show the strength of the water skin. Lay a needle on a fork and gently lower the fork into the water. As the tines of the fork go below the water the needle stays on the water surface. The strength of surface tension holds it up. If you want to sink the needle just add a drop of liquid soap. It makes the surface tension so weak that it can no longer hold up the needle and the needle sinks.

You can use soap solution to make water fit for blowing bubbles. The soap solution makes the surface tension of the water so weak that it can make a thin sheet on a bubble frame. If you hold up the sheet in the bubble frame and blow into it the sheet will break off from the frame and form a round bubble. Although the surface tension is weak it still makes the water particles hold onto each other to make a round shape.

What happens if you add soap solution to a drop of water?
The surface tension holds the drop of water as near to a circle as it can. This means that the edges of the drop are raised up in a curve and the smallest amount of water is actually in contact with the surface underneath it. When you add soap solution to the drop the surface tension becomes weaker. This makes the edges of the drop collapse and the water drop spreads out further over the surface it is resting on.

Why do we add soap to water when we are washing something?
The soap weakens the surface tension of water and lets it get closer to the surface of whatever you are washing. This allows the water to get closer to dissolve any soluble dirt. It also allows the soap to get close to the surface too. The soap can lift off lumps of greasy dirt and make it into tiny balls suspended in the water. When the water is allowed to run away it carries away both soluble and greasy dirt leaving the surface clean.

Why can you see lots of different colours in soap bubbles?
This is due to the way the walls of the bubble reflect light. When you look at a soap bubble you see light reflected from the outside surface nearest you and the inside surface farthest from you. When light coming from the inside surface mixes with the light coming from the outside surface it makes the range and pattern of colours you see.

You can sometimes see bubbles coming off the leaves of water plants. How are they formed?
These can be seen best when the water has been lit by bright sunlight for awhile. The plant uses some of the energy in sunlight to make food. At the same time as it makes food it also makes oxygen. It makes more oxygen than it needs so it lets the oxygen go. The bubbles coming from the plant contain the extra oxygen that the plant does not need,

What happens to the bubbles of oxygen?
They burst at the surface and the oxygen mixes with the air. Plants have produced all the oxygen you breathe. You cannot see the oxygen that land plants produce because they are not surrounded by water and so do not make bubbles on their leaves.

Some tablets make bubbles when you put them in water. How do they do it?
The tablets contain chemicals. When they are dry the chemicals cannot reach each other. When they are put in water and the tablets dissolve, the chemicals can reach each other. The chemicals cause each other to change. One of the things that is produced in this change is a gas called carbon dioxide. It forms in the water and the water encloses it in bubbles.

Where do the bubbles come from in a fizzy drink?
A gas called carbon dioxide makes the bubbles. It dissolves in water. When a large amount of carbon dioxide is dissolved in water it gives the water a refreshing taste. Carbon dioxide is not simply stirred into water the way you may stir sugar. It is squashed up like the air you pump into a bicycle tyre, then released into the water. Perhaps you have seen a fizzy drink maker at use in the home. A strong metal canister containing the squashed up carbon dioxide is attached to the fizzy drink maker. When you want to make a drink you put a beaker of water in the machine and the carbon dioxide squirts into it. In a bottle or can the carbon dioxide is kept squashed. When you open them the squashing push or pressure is released and the dissolved carbon dioxide makes bubbles, which rise out of the drink.