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Liquids are one of the states of matter – the other states are solids, gases, plasmas and Bose-Einstein condensates. The simplest way to determine if something is a liquid is to ask this question: If I try and move it from one container to another (i.e. by pouring), will it conform to or take on the shape of the new container?

If you have a glass of water and pour it into another glass, it clearly conforms – it takes on the shape of the glass. If you spill the water, it will go everywhere. Because it isn’t in a container, it is conforming to the shape of the floor, making a big puddle!

Keeping this property of a liquid in mind, think about the following list of things and decide if they are liquids – juice, milk, oil, tomato sauce, honey and custard.

  • You regularly pour juice and milk into glasses or bowls – you see them conform to the shape of the container, so you can safely say they are liquids.
  • Oil is also a liquid, but is usually much thicker than water – it has a greater viscosity.
  • Tomato sauce, honey and custard are all special liquids called non-Newtonian fluids. They behave like liquids and, under certain circumstances, like solids. If you turn your tomato sauce bottle upside down and nothing comes out, it behaves like a solid, but if you shake it, the act of applying force or stress to the tomato sauce makes the sauce flow more freely (reduces its viscosity). The same is true for honey. Some types of uncooked custard act like solids if you hit them, and the more force you apply, the harder the custard becomes.

What makes a liquid a liquid?

How particles are arranged and what forces hold them together determines what state of matter something is. Liquids are made up of tiny (invisible) particles that are in constant motion and roll on top of each other. There are cohesive forces that hold the particles together, but they are not rigidly stuck together like in a solid. The particles are touching but can slide past one another. The cohesive force between the particles in the liquid gives liquids their surface tension – the surface of the liquid acts like a thin elastic layer. For example, surface tension causes water to form drops and allows small insects to walk on water.

As well as their ability to conform to containers, liquids also have boiling and freezing points:

  • The boiling point is the temperature at which a liquid becomes a gas.
  • The freezing point is the temperature at which a liquid becomes a solid.

Nature of Science

Things can be done to materials to change some of their properties, but not all materials respond in the same way. (Investigating in science)