Imagine having a bath with volcanic foam. Weird? Not really, because lots of people use it everyday - as pumice stones.
Pumice is the name given to the hardened foam of lava when it comes out of a volcano. The inside of a volcano has very high pressure, and can be as hot as 1600oC. When the lava comes out of the volcano, it meets the cool air (and sometimes sea water), which is around 25oC.
Two things happen immediately. Air and water that were mixed in the lava bubble out, as the pressure falls. The huge difference in temperature gives the lava a 'cold shock', so it freezes almost immediately. So the bubbles get trapped in the freezing stone, giving it a very spongy appearance. In some kinds of pumice, the bubbles can make upto 90% of the volume.
Why does pumice float?
If you visit a volcanic island (like Barren Island in the Andamans, or Anak Krakatau in Indonesia), you can see several acres of pumice around the shore, remnants of past eruptions. You may see some pumice stones that have broken off due to erosion by the sea. Amazingly, they will actually float on the water for some time.
Dense stones, of course, do not float on water. But because pumice is full of air bubbles, it is less dense than water. So in the beginning it will float. But in time, water will enter the bubbles driving out the air. Slowly, the pumice sinks.
If you have a pumice stone at home, try this out. Drop it in a glass bowl full of water. You'll see it float first. And as it sinks, you'll see lots of bubbles escaping, just like in an open soda bottle. The chemistry word for this is exsolution.
What is it used for?
Well, some folks use it while bathing to remove dead skin, persistent dirt and calluses, especially around the knees and elbows. Crushed pumice is also added in cosmetics called exfoliants, and a kind of soap called lava soap (called so because pumice comes from lava!) It is also used for sharpening industrial tools.
Another use for pumice is to make light-weight bricks. These are called breezer blocks or cement blocks. These bricks are stronger than ordinary red bricks, but lighter because pumice has such low density. Often the important walls of a building (which bear weight) are made of these blocks, and red bricks are used for the rest.