Matter has three states: gas, liquid and solid.* A solid “melts” into a liquid and vaporizes into a gas at the boiling point. Gases “condense” back into liquids and “freeze” back into solids.

The temperatures at which different molecules change state are quite different. Here’s a sample:

Water is an unique molecule. It’s melting and boiling points are within temperatures that occur around us pretty regularly. That is not true of most other molecules.

Another very strange thing about water is that it’s solid state, ice, is less dense than its liquid state. Most molecules contract when chilled and have solid states denser than their liquid states. Water, in liquid form, does contract at first when chilled but as as it approaches freezing it actually begins to expand.  By the time it is solid ice it is almost 10% more voluminous than it is as a liquid.  “With almost any other substance, parking molecules on a regular grid would make them sit closer together than when they’re allowed to roam free. But water’s not like that. [Water’s] growing crystal is less dense than the water around it, and so it floats.”**

Related IFOD on Density!. There are some other substances where the solid state is less dense than liquid state, such as silicon, but it’s very common.

If water didn’t have that quality life on earth would be very different and some speculate that life wouldn’t exist at all as during the ice age the oceans would have frozen solid.  This might occur because if ice sank then lakes and oceans would freeze from the bottom up.  Without surface ice to hold heat in, the water’s warmth would radiate away, leaving it even chillier and creating even more ice.  The oceans and lakes would freeze and possibly stay that way. Especially during the ice age. Life is thought to have originated in the oceans. If they were frozen solid, then life would be quite different. Or would not have happened at all.

Related IFOD on the Anthropic Principle (dealing with how amazing it is that life on earth exists at all!).

Here’s another interesting question: if you have a glass with water and ice in it, as the ice melts what happens to the level of the water in the cup? Does it go up or down or stay the same? Answer: “If you put some water in a transparent glass and add some largish lumps of ice, the ice floats so that most of it is below the surface but about 10 percent sticks up above the liquid level. You can mark the liquid level on the outside of the glass with a marker pen. Once it’s melted, all those water molecules that are now sticking up above the water level will have to join the rest of the drink. Does this mean that the water level will rise? This is proper cocktail party physics, if you’re patient enough (or bored enough) at a party to spend time watching ice melt. The answer is straightforward, and you should test it for yourself if you don’t believe me. The water level will stay in exactly the same place. Once the molecules in the ice become liquid again, they can fit together more closely. This means that they’ll fit perfectly in the hole that the submerged part of the ice was taking up. That bit of the ice cube that’s sticking up above the water line is exactly the size of the extra volume that the ice cube has because it expanded as it froze.”**

A related question is this: Imagine you are floating in a canoe in a small pond. On your lap is a decent sized rock that weighs 50 pounds.  What happens to the level of the water in the pond if you throw the rock overboard? Does  the water level rise, fall or stay the same?  Answer: How do big steel ships float?

*There is a fourth state of matter: plasma

**From: Czerski, Helen. Storm in a Teacup: The Physics of Everyday Life