What Is a Liquid?
A liquid is a state of matter that has a fixed volume but no fixed shape. When you pour water into a glass it takes the shape of the glass, but the amount of water stays the same. Pour it into a bowl and it takes the shape of the bowl.
Water, orange juice, mercury, and molten lava are all examples of liquids.
Particles in a Liquid
The particles in a liquid are:
- Close together — the volume is fixed because there is little empty space, similar to a solid.
- Not in a regular arrangement — unlike a solid, the particles are not locked into a crystal lattice.
- Free to move past each other — this is why a liquid can flow and change shape.
- Moving with moderate energy — faster than in a solid, but slower than in a gas.
The particles are still attracted to each other, but the attractions are weak enough that they can slide around one another.
Mercury is the only metal that is a liquid at room temperature (20 °C). It is very dense and shiny, and its particles, like all liquids, can flow freely. Mercury is toxic, so it is handled carefully in laboratories.
Physical Properties of Liquids
| Property | Description |
|---|---|
| Shape | Variable — takes the shape of the container |
| Volume | Fixed — stays the same |
| Compressibility | Very difficult to compress |
| Flow | Flows easily |
| Surface tension | Particles at the surface are pulled inward, creating a "skin" |
Viscosity describes how easily a liquid flows. Water has low viscosity (flows quickly), while honey has high viscosity (flows slowly). Thicker liquids have stronger forces between their particles.
Surface Tension
At the surface of a liquid, particles are only pulled sideways and downward (not upward), so the surface acts like a stretched elastic sheet. This is called surface tension. It is strong enough to support the weight of a small insect like a pond skater walking on water.
To see surface tension in action, carefully place a paper clip flat on the surface of still water. Despite being denser than water, it floats because of surface tension. Adding a drop of washing-up liquid breaks the surface tension and the clip sinks.
Changing State
- Evaporation / Boiling: When a liquid is heated, particles gain energy and escape from the surface as gas. At the boiling point, bubbles form throughout the liquid.
- Freezing: When a liquid loses heat energy, particles slow down. At the freezing point, they lock into fixed positions and the liquid becomes a solid.
Real world: Cooking with water
When you fill a pot with water and put it on the stove, the water fills the shape of the pot perfectly. When you pour it into a differently shaped pan, it takes the new shape but the volume stays the same. This is a classic demonstration that liquids have a fixed volume but no fixed shape.
Real world: Honey vs. water — viscosity
Pouring honey is much slower than pouring water because honey has a much higher viscosity. The particles in honey have stronger attractions between them, resisting flow. Warming honey reduces its viscosity, just as raising temperature generally makes liquids flow more easily.
Real world: Water strider insects
Pond-skater insects can walk on the surface of water because of surface tension. The water molecules at the surface pull inward, creating a thin "skin" strong enough to support a small insect. This is why a needle can also float on still water if placed flat.
Where do you see this in real life? Every time you pour a drink, notice how it takes the shape of the container while keeping the same volume. Watch how washing-up liquid breaks the surface tension of water.
Practice Problems
A liquid is poured from a tall narrow bottle into a wide flat dish. What happens to its shape and volume?
Show answer
Why can liquids flow but solids cannot?