States of matter and density: a science glossary

Plumbing science leans on a handful of physical terms about matter, density and pressure. These plain definitions are for revision; confirm any technical value against the current guidance.

• Matter: anything that has mass and takes up space; in plumbing the substance we move and heat is mostly water and air.
• Solid, liquid and gas: the three common states of matter. Water appears as ice, as liquid water, and as steam or vapour, changing state as heat is added or removed.
• Change of state: the switch from one state to another, such as melting, freezing, boiling and condensing, which involves heat without a change in temperature at the change point.
• Mass: the amount of matter in an object, measured in kilograms, and the same wherever the object is.
• Weight: the downward force gravity exerts on a mass; weight is a force, while mass is the quantity of matter.
• Force: a push or pull, measured in newtons, that can start or change movement, or change the shape of an object.
• Density: mass divided by volume, commonly expressed in kilograms per cubic metre. Liquid water is about 1 kilogram per litre under ordinary conditions.
• Relative density (specific gravity): density divided by the density of a stated reference under specified conditions; it has no units. Water is a common reference for liquids and solids, while fuel-gas vapour is compared with air at the same temperature and pressure. Keep the substance, physical state and reference explicit.
• Pressure: force spread over an area, force divided by area; in a liquid it increases with depth, which is why a head of water produces pressure.
• Atmospheric pressure: the pressure the air exerts on everything around us, the baseline that gauge pressure is measured above.

These ideas join up: density and depth give pressure, mass and gravity give weight, and changes of state explain why water expands and why steam carries so much heat. Confirm technical detail against the current guidance.