The reference freeze/boil points, water's densest temperature, and why ice and steam each take up more room.
Freezing and boiling points
At normal atmospheric pressure, plain water freezes at about 0 °C and boils at about 100 °C. These two points are the standard reference temperatures used throughout plumbing and heating work, from frost protection to hot water safety.
Water is densest at about 4 °C
Unlike most liquids, water does not simply keep shrinking as it cools. It contracts steadily down to about 4 °C, where it reaches its maximum density, then expands slightly again as it cools further towards freezing. That is why the coldest water in a pond sits near the surface while slightly warmer, denser water stays lower down.
Ice takes up more room than water
For the same mass, freezing increases water’s volume by about 9%: a litre of liquid becomes roughly 1.09 litres of ice. Density is mass divided by volume, so the ice has about 92% of the liquid’s density, a decrease of about 8%. That lower density lets it float. If expansion is confined, for example by an ice blockage and a closed outlet, pressure in the trapped water can split a pipe or joint. The failure can be away from the ice itself, and the leak may appear on thawing.
Steam takes up far more room than water
Boiling is a change of state, not just a temperature rise: the liquid turns into a gas that is far less dense, so the same mass of water takes up a much bigger volume as steam. The mass does not change, only how much space it fills, which is part of why an uncontrolled steam release is so dangerous.
Pressure moves the boiling point
Water boils when its vapour pressure matches the pressure acting on it. At 100 °C the vapour pressure of water is about one atmosphere, which is why open water boils there. Under more pressure the water has to be hotter before its vapour pressure catches up, so the boiling point rises; at lower pressure water boils at a lower temperature. That is how water in a pressurised system can be above 100 °C without boiling.