All gases can flow, like liquids. This means the molecules move about independently of each other. Most gases are colourless, like hydrogen. Gas particles will spread about, or diffuse, in order to fill all the space in any container such as a bottle or a room. Compared to liquids and solids, gases have a very low density and viscosity. We cannot directly see most gases since they aren't coloured. However it is possible to measure their density, volume, temperature and pressure.