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Various familiar liquids, from left to right: vegetable oil, honey, water, wine, and ketchup.
Hydraulic jack on a truck
Hydraulic jack on a truck.
Oil is more viscuous than water
Oil is more viscuous than water, as can be seen when each is poured.
The state of matter intermediate between a solid and a gas, in which the molecules are relatively free to move with respect to each other but are restricted by cohesive forces to the extent that the liquid maintains a fixed volume. Liquids assume the shape of the vessel containing them, but are only slightly compressible.

Liquids have much higher densities than gases – comparing liquids with common gases under ordinary conditions the factor is of the order of 103. Their compressibility is low. They have no rigidity but their viscosity is of the order of 102 times greater than that of ordinary gases.

In liquids, the molecules are packed quite closely together so that each one is bonded to a number of neighbors. Given the position of one molecules it is possible to state how many more molecules should be found in contact with it. But still, as in the case of a gas, the pattern as a whole is a disordered one. The molecules are moving with just the same order of velocity as in a gas at the same temperature, though the motion is now partly in the form of rapid vibrations and partly translational. The configuration is therefore constantly changing.

This picture can be correlated with the macroscopic properties – the high density from the large number of molecules per unit volume, the lack of rigidity from the lack of order and the continual alteration of the arrangement. The comparatively close packing explains the low compressibility. The fairly high viscosity arises from the fact that the molecules have to wriggle past one another in this irregular but closely packed arrangement, rather like people moving past one another in a dense crowd, where slow relative movements are easy but rapid ones are difficult.

Water, wine, and other familiar liquids

Various liquids differ in their physical properties such as boiling point and viscosity (stickiness). Water and wine have roughly the same boiling point and low viscosity. Oil is more viscous and boils at a higher temperature whereas exceptionally viscous liquids such as honey and tomato ketchup have extremely high boiling temperatures.

Motor oil is more viscous than water, as can be seen when each is poured; the water flows much more easily then oil because layers of water molecules slide over each other more easily than do layers of oil molecules. Normally, the viscosity of a liquid decreases with rise in temperature. Oil for a particular application must have the right lubricating properties and much research has been done to produce the correct oils for car engines and gear boxes, including oils whose viscosities change only a little when they got hot. When a liquid flows in a pipe in streamlined motion, the region in contact with the pipe is still and that near the axis has the greatest velocity.

Incompressibility of liquids

Liquids are almost incompressible and as a result can transmit pressure. This important principle finds many applications in the branch of engineering called hydraulics. Many trucks have an hydraulic jack in which pressure transmitted by means of oil is used to tilt the load. A pump is used to provide the pressure and provision has to be made for the oil to run back when the pressure is released.

Surface tension

A water drop at the end of a glass tube takes its shape because of surface tension. The attraction between its molecules leads to a spherical shape as the "skin" effect caused by inward-acting forces in the surface holds the bulk of the liquid back. But a water droplet becomes a distorted sphere because gravitational as well as surface tension forces are acting on it.

water drop

Thermal expansion

mercury and alcohol thermometers
The most common method of measuring temperature makes use of the expansion of a liquid on heating. A mercury thermometer has a wide range (-39°C to 360°C), but needs a large bulb (reservoir) and a narrow stem if small temperature changes are to be detected. Alcohol can be used in thermometers for measuring lower temperature; it expands more, but boils at 78C. The great advantage of liquid-in-glass thermometers is that they can be read directly and carried easily.

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