Past exam of the medicine course of the University of Oxford / 1867 / First examination / Chemistry / 7

Name some fluids that are heavier and some that are lighter than water.

Examples heavier than water are Mercury, glycerol and concentrated sulfuric acid; examples lighter than water are ethanol and diethyl ether. “Heavier” here means greater mass for the same volume, rather than greater total mass in arbitrarily sized samples.

Representative room-temperature relative densities, with water as unity, are

These are illustrative values, not exact constants independent of temperature or composition. Dilution changes the density of sulfuric acid, glycerol and ethanol solutions.^[ref][ref]

A distinction from simply observing a floating layer is essential: ethanol is miscible with water, so it mixes rather than remaining as a permanent upper layer. Diethyl ether has limited water solubility and can form an upper layer. More broadly, gases are also fluids, but the natural comparison intended here is between liquids; air at ordinary conditions is much less dense than liquid water.