Liquids (Physics): Definition, Properties & Examples

Liquid is one of the four states of matter, the others being solid, gas and plasma. The study of the physics associated with liquids is a surprisingly large area. But when you consider how much of your life depends on water flowing through pipes, or boats being able to float in the ocean, or even your pancake syrup being able to flow properly from its container, it's easy to see why the study and understanding of liquids is important.

There are four main states of matter: solid state, liquid state, gas state, and plasma. Matter can change from one state to another depending on conditions of pressure and temperature.

In a solid, the molecules of the material are tightly bound, and the material holds its shape. In a liquid, the molecules are less tightly bound and able to slide or flow past each other. In a gas, the molecules become separated from each other. A gas will always fill the container it is in and can easily expand and contract, while liquids and solids cannot (or at least not to the same extent.) A plasma is a state of matter that occurs when gas is heated to the point that it becomes ionized.

When a gas condenses, and the molecules become close enough to affect each other and adhere, it turns into a liquid form. This usually requires cooling, which removes energy from the system.

When something in a solid form melts, it becomes a liquid. This typically requires heating which adds energy to the system. As the temperature of the material increases, the molecular motion increases and overcomes the intermolecular forces trying to hold the molecules rigidly together.

As mentioned previously, liquid is a state of matter. The incompressibility of liquids means that they have a fixed volume (definite volume) and do not expand or contract in any significant way like a gas might.

In a liquid, the molecules are joined together weakly by cohesive forces and can flow freely past each other. Liquids take the shape of the bottom part of whatever container they are in and do not maintain a definite shape like solids do.

Liquids are often categorized as a fluid, which is a broader label applied to both liquids and gases. A fluid is a substance that can flow, and many of the laws of physics that apply to the flow of liquid also apply to the flow of gases.

Examples of liquids can be found all around you. The one you are likely most familiar with is water because it is required for life and covers about 71 percent of the earth's surface. Because water is in liquid form at standard temperatures on Earth, it is believed to be the reason life was able to form and flourish here.

There are, of course, many other substances that are liquid at room temperature including alcohol, gasoline and even mercury.

Substances that exist in a liquid form only at much cooler temperatures include acetylene, carbon dioxide, methane and liquid nitrogen. Substances that exist in a liquid form only at much higher temperatures include aluminum and many other metals, carbon, porcelain and sand.

Liquid crystal is a state of matter between liquid and solid. Some substances have essentially two different melting points: One at which they become a liquid crystal, and another higher point at which they become a regular liquid. Liquid crystals can flow like a liquid but also display symmetries typically associated with crystalline solids. Liquid crystals are used in displays of watches, calculators and TVs.

Pressure is a measure of the force per unit area. In a liquid substance, all of the liquid molecules are pressing against each other and creating an internal pressure. You can imagine the walls of the container feeling this force per unit area as well, and if you were to poke a hole, the pressure would force the liquid out.

Pressure in a liquid is also why you're able to float in a swimming pool. The associated force counteracts gravity.

The value of pressure in a liquid depends on the liquid's density and the depth. The relationship is as follows:

\(P = \rho d g\)

Where P is pressure, ρ is density, d is depth, and g is the acceleration due to gravity.

The fact that pressure increases with depth is why divers have to be careful. They must allow their bodies to acclimate to increases and decreases in pressure to avoid injury.

For liquid in a pipe, differences in pressure along the pipe will cause the liquid to flow. This is because pressure is essentially a force, and an unbalanced force causes change in motion.

As you are likely aware, some objects float, and some objects sink, and even the ones that sink tend to do so slowly. This tells us that there must be a force that the liquid is applying that is counteracting gravity. This force is called the buoyant force. Archimedes' principle describes the buoyant force in a liquid, that is, the force that causes objects to float.

Archimedes states the value of the buoyant force very simply: It is equal to the weight of fluid displaced by the submerged object.This weight is easily calculated as the product of the volume of the object (or the portion of the object) that is submerged, the density of the liquid, and g, the acceleration due to gravity.

Since the force of gravity on an object is the product of its mass and g, and its mass is equal to the product of its volume and density, it is easy to see that in order to float, objects must be less dense than water.

Another property of liquids is viscosity. Viscosity is a measure of how thin or thick a liquid is or its resistance to flow or to objects passing through it. If you compared syrup to water, for example, you would notice that water pours faster and more quickly than the thick syrup. That is because the syrup has a higher viscosity. It is said to be more viscous.

Viscosity is caused by friction between molecules in layers of a flowing fluid. The greater the friction, the greater the viscosity. Factors that determine a liquids viscosity include temperature and molecular shape.