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Characteristics of Fluids

by Ron Kurtus (revised 26 March 2007)

The states of matter are solid, liquid, gas and plasma. A fluid is a subset of the states of matter, consisting of liquids, gases and plasmas. This is because they have common properties that are distinct from solids. A fluid does not have a specific shape as does a solid. Instead, fluids take the shape of their containers. They also will flow or pour when under the influence of a force such as gravity.

Questions you may have include:

• What is the natural shape of fluids?
• How do fluids take the shape of their containers?
• How do fluids flow?

This lesson will answer those questions. There is a mini-quiz near the end of the lesson.

Useful tools: Metric-English Conversion | Scientific Calculator.

Natural shape

Solids have specific shapes because the molecular forces holding particles in place are stronger than the kinetic energy of the molecules. Usually, the molecules just vibrate in place, with little or no other movement.

On the other hand, fluids exist at higher temperatures and thus their particles have greater kinetic energy. The shape of a fluid adapts to its environment or container.

Liquids

A liquid in space will form the natural shape of a sphere. This is because the attraction between its atoms or molecules is greater than the forces from their kinetic energy moving outward. A sphere is a shape with the smallest surface area for a given volume of material.

A liquid sphere or drop of liquid—such as water—that is falling toward the Earth through the atmosphere will be a slightly flattened sphere, due to the air resistance.

If you spill some water on the floor, it will splash and spread out on the floor. Liquids like thin oil will spread out even more than water on the floor.

Gases

The molecules in a gas have more energy than when the material is in the liquid state, such that they overcome the molecular forces. A gas in space or in the atmosphere will continually spread in a shapeless form.

A gas that is heavier than air may gravitate toward the floor, where it then spreads out.

The rate that the gas expands is a function of its temperature or kinetic energy of its particles.

Plasmas

A plasma is an ionized gas, usually at extremely high temperatures. That means some of its electrons have been stripped off. Plasmas have most of the same properties as gases.

Shape in container

Under the influence of gravity, a fluid will take the shape of its container, provided the volume of the container is greater than or equal to the volume of the fluid.

Liquids

If you pour a liquid into a container, it will take the shape of the container, provided none overflows. Under the influence of gravity, a liquid will stay in an open container, such as a cup.

If the container is filled to the top, the volume of the liquid will equal the volume of the container. This fact has been used to measure the volume of an irregularly shaped container or flask.

Gases

A gas will take the shape of its container too. If the container is open and the gas is heavier than air, it will stay in the container for a while.

For example, Chlorine gas (Cl₂) is one of the few gases that is colored. If you pour it into a container, you will see the light green gas take the shape of the container. But the high energy of the gas molecules will result in it slowly dissipating into the air.

Usually, gases are put in closed containers. Since gases tend to spread, and since the rate of spreading is proportional to the temperature of the gas or the kinetic energy of its particles, there is a constant pressure on the walls of the container in all directions. This pressure increases with increased temperature or reduced volume of the container.

Plasmas

Because of their high temperature, plasmas are seldom placed where they could take the shape of the container.

Flowing

The major feature of a fluid is that it flows when acted upon by some force. This makes a fluid different than a solid, which may be distorted by a force but will not start to flow. Typically, the force is that of gravity, but other forces can also apply.

Gravity

Fluids under the influence of gravity will flow or can be poured. You certainly have poured liquids from one container to another. Gases also can be poured. Since plasmas are typically very hot, they are seldom poured.

Although, you cannot see carbon dioxide (CO₂), you can demonstrate pouring it from one jar to another. This is shown by using dry ice to fill a jar with CO₂ and then pouring it into a jar containing a burning candle. The candle flame will be snuffed out as the invisible CO₂ is poured into the jar.

Other forces

The forces caused by the acceleration, deceleration or change in direction of a moving container can cause the fluid to flow or change its shape.

The force of wind on a body of water will cause the water to flow, as well as to create surface waves.

Summary

A fluid is a subset of the states of matter, consisting of liquids, gases and plasmas. They have common properties that are distinct from solids. Fluids do not have a specific shape as do solids. Instead, fluids take the shape of their containers. They also will pour when under the influence of a force such as gravity.

Answers to Readers' Questions

Walk with fluid grace

Resources

The following resources provide information on this subject:

Websites

Physical Science Resources

Books

Fluid Mechanics by Ira M. Cohen and Pijush K. Kundu, Academic Press (2004) $74.95

Vectors, Tensors and the Basic Equations of Fluid Mechanics by Rutherford Aris, Dover Publications (1990) $14.95

Fundamentals of Fluid Mechanics by Bruce R. Munson, Donald F. Young, Theodore H. Okiishi; Wiley (2001) $37.95

Miscellaneous

Mini-quiz to check your understanding

1. What is the shape of a falling water drop?

2. What eventually happens to a gas placed in an open container?

3. Why does an exhaust fan in your kitchen work to get rid of smells?

If you got all three correct, you are on your way to becoming a Champion in Physical Science. If you had problems, you had better look over the material again.

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