Liquid

A liquid is a state of matter in which particles are still close together but have enough energy to slide past each other. A liquid has a fixed volume but no fixed shape: it takes the shape of whatever container holds it. Pour water into a glass and you get a glass-shaped water. Pour the same water into a bowl and you get a bowl-shaped water. Pour it onto a flat table and it spreads out into a puddle. Liquids flow, fill, drip, splash and form drops, and life depends on them.

• ShapeTakes container shapeFlat surface on top
• VolumeFixedDoes not change with pressure
• ParticlesClose but mobileSlide past each other
• Flow rateDepends on viscosityWater flows, honey crawls
• Densest common liquidMercury13.5 g/cm3
• Least dense commonLiquid helium0.125 g/cm3

What makes a liquid?

In a liquid, the particles are still close together (about as close as in a solid) but they are moving fast enough to slip around each other. The forces holding the particles together are still real, but they are not strong enough to fix the particles in place.

This is why liquids are nearly impossible to squash (the particles are already touching) but easy to pour (the particles can flow past one another).

Properties of liquids

• Fixed volume: 1 litre of water stays 1 litre whether it is in a tall glass or a shallow tray.
• No fixed shape: takes the shape of its container.
• Flat surface: gravity pulls liquids into a flat top, except for the small curved meniscus at the edges due to surface tension.
• Flowing: liquids can be poured. How easily they pour depends on viscosity. Water has low viscosity; honey has high viscosity.
• Surface tension: at the surface, liquid molecules pull on each other more than on the air above, creating a stretchy "skin" that lets small insects walk on water.
• Mostly incompressible: squeezing a liquid hardly changes its volume. Hydraulic systems (car brakes, cranes, lifts) rely on this.

Fact Liquid mercury is so dense that solid iron will float on it. The dense, silvery metal stays liquid at room temperature (it freezes at -39 degrees Celsius), which is why old thermometers used it. Mercury is poisonous, so modern thermometers use coloured alcohol or digital sensors instead.

Viscosity

The "thickness" of a liquid (how easily it flows) is called viscosity. Low-viscosity liquids flow easily; high-viscosity ones move slowly.

• Very low viscosity: liquid helium (so low it can flow uphill)
• Low viscosity: water, milk, tea, petrol
• Medium viscosity: cooking oil, shampoo
• High viscosity: honey, syrup, ketchup
• Very high viscosity: tar, lava, glass at room temperature

Viscosity drops as you heat a liquid: warm honey pours easily, cold honey is stiff. Engineers carefully match oil viscosities to engine designs to keep moving parts well lubricated at all temperatures.

Surface tension

Liquid molecules in the middle of a body of liquid are pulled in all directions by the molecules around them. Molecules at the surface only have other liquid molecules below them, so the pull is inward and sideways. This makes the surface act like a stretched elastic sheet, a phenomenon called surface tension.

Surface tension is why:

• Water droplets are round (the surface tension pulls them into the smallest shape).
• You can fill a glass slightly above the brim without it overflowing.
• Small insects can walk on water.
• Water beads up on a waxed car or oily plate.

Soaps and detergents reduce surface tension, which is why soapy water spreads out flatter on a dish and gets into smaller cracks.

How liquids change state

Cool a liquid enough and the particles slow down until their attractions lock them in place. The liquid freezes into a solid. Heat a liquid enough and the particles fly apart. The liquid boils into a gas.

For pure water at normal atmospheric pressure, these temperatures are 0 degrees Celsius (freezing) and 100 degrees Celsius (boiling). For other liquids they vary enormously:

• Liquid nitrogen boils at -196 degrees Celsius.
• Ethanol boils at 78 degrees Celsius.
• Mercury boils at 357 degrees Celsius.

Even below boiling, some liquid molecules at the surface have enough energy to escape into the air. This slow loss is called evaporation. A puddle slowly dries out even on a cool day because of evaporation.

Did you know? When you sweat and the sweat evaporates, the fastest-moving water molecules leave first, carrying their energy with them. The remaining sweat (and the skin under it) is therefore cooler. That is why evaporation is such an effective cooling mechanism, and why a desert dweller wearing a damp cloth on their head feels cooler in the dry heat.

Liquids in everyday life

Liquids are essential to almost every part of life:

• Water: drinking, washing, cooking, the basis of life.
• Blood and other body fluids: carry nutrients, oxygen and signals around your body.
• Petrol and diesel: liquid fuels for cars, lorries, ships and planes.
• Oil: lubricates moving parts in engines and machines.
• Paint: liquid until it dries into a hard solid coating.
• Hot lava: molten rock flowing out of volcanoes.
• Cooking oil and stocks: the basis of most meals.
• Liquid medicines: easier to swallow and absorb than pills.

Try this Pour different liquids slowly onto a flat plate and watch what happens. Water spreads out in a wide thin puddle. Honey forms a steep dome that slowly creeps outward. Cooking oil falls between the two. Try adding a drop of washing-up liquid to the water puddle and watch it spread out further (lower surface tension). You are watching viscosity and surface tension at work side by side.

Deeper dive: why does water behave so strangely?

Water (H2O) is one of the most ordinary liquids you can imagine, and yet it has so many unusual properties that scientists still study it today.

• Ice floats on water: for almost every other substance, the solid is denser than the liquid. Water is the opposite. Ice has an open hexagonal structure that takes up more space than liquid water, so ice floats. This is vital for life: frozen ponds and lakes form ice on top, insulating the water below and letting fish survive winter.
• Water expands when it freezes: water actually contracts as it cools all the way down to 4 degrees Celsius (its densest point), then expands again as it freezes. This is why frozen pipes can burst.
• Very high boiling point: based on its molecular weight, water should be a gas at room temperature. It is a liquid only because of strong hydrogen bonds between molecules.
• Excellent solvent: more substances dissolve in water than in any other common liquid. This makes water the perfect medium for the chemistry of life.
• High heat capacity: it takes a lot of energy to warm water by even a degree. This is why oceans moderate climate, and why kettles take so long to boil.

All these unusual properties trace back to one thing: hydrogen bonds. The slightly negative oxygen end of each water molecule attracts the slightly positive hydrogen ends of nearby molecules. The result is a network of bonds that makes water behave like a much bigger, more organised substance than its simple H2O formula would suggest. Without these quirks, the world would be unrecognisable, and life as we know it could probably not exist.

For more, see solid and gas.