Water

Water contracts and gets denser as it cools, like most substances, until about 4 °C, where it reaches its maximum density.

Below 4 °C its density falls as it approaches freezing. From 4 °C to 0 °C the drop is only about one part in 10,000, but it is enough for the colder water to rise to the top.

Water expands on freezing, so a pond freezes from the top down and ice floats.

The hexagonal lattice of ice contains more space than the liquid, which is why ice is less dense than water.

Water sets the standard for density at 1 gram per cubic centimetre.

Hydrogen is found in the Sun and most stars, and Jupiter is composed mostly of hydrogen.

Water is H2O, with a molecular weight of 18.0153.

Hydrogen is present in water and in almost all the molecules in living things.

Oxygen combines with hydrogen to form H2O. The element and its compounds make up 49.2% by mass of the Earth's crust.

Joseph Priestley published the first account of oxygen in 1774. Carl Wilhelm Scheele had made it in 1771, but his account was not published until 1777.

Priestley, working in England, made the gas by heating mercuric oxide with sunlight. Scheele worked in Sweden.

Antoine Lavoisier later proposed the name oxygène for the element. Oxygen gas is fairly soluble in water, which supports aquatic life.

Photosynthesis splits water into oxygen and hydrogen.

Earlier, Paracelsus in the early 1500s and Robert Boyle in 1671 noted the flammable gas now called hydrogen but did not pursue it. Antoine Lavoisier later named the element.

Oxygen and hydrogen together make up the molecule: two hydrogen atoms bond to one oxygen atom in H2O.

The boiling point of water is listed at 373.17 K with an uncertainty of 0.04 K, the average of 7 values.

Its critical temperature is listed at 647 K with an uncertainty of 2 K, the average of 7 values.

The pressure at its triple point is 0.0061 bar.

About 97% of Earth's water is in the ocean, which covers more than 70% of the planet's surface.

Less than 1% of all Earth's water is fresh. A tiny fraction exists as water vapour in the atmosphere.

The US Geological Survey puts the planet's total at over 1,386,000,000 cubic kilometres of water. The National Geophysical Data Center estimates the ocean holds 1,335,000,000 cubic kilometres of it.