A Simple Experiment to Explain Density

Heating or Cooling Substances Causes Changes in Volume but not Mass

Jul 29, 2009

This easy experiment helps students visualize changes in density due to heating and cooling of gases and apply their understanding to studies of weather and other topics.

Many fundamental processes are driven by differences in density. For example, a hot air balloon rises because the hot air inside the balloon is less dense than the surrounding cooler air. Volcanoes form because hot magma is less dense than surrounding rock and therefore rises. However, density can be a difficult concept for students to grasp.

Materials for the Density Experiment

Safety Issues When Doing the Experiment

This experiment involves the use of very hot water. Depending on the age of the students, this experiment should be done as a demonstration (for younger students) or with the supervision of an adult (for older students). Tongs should be used to hold the bottle in the pan of hot water in order to prevent burns.

Experimental Procedure

What Students Should Observe While Doing the Experiment

During Step 3, the balloon should inflate. During Step 4, the balloon should deflate. The weight of the bottle measured during Step 5 should be the same as it was in Step 2. If individual observations are different, check to make sure that the seal on the mouth of the bottle is good and that the balloon doesn’t have a hole.

If desired, teachers can repeat Steps 3 and 4 more than once to demonstrate that the process is reversible. Young students actually enjoy seeing this done over and over!

Explaining the Observations

In Step 2, the balloon inflates because the heat from the water in the pan is conducted through the bottle and warms the air inside. Temperature is actually a measure of the intensity of motion of particles in a substance. When air is heated, the gas molecules absorb the heat energy causing them to move faster.

The net effect is that the molecules move further apart from each other. When the particles spread out, they occupy more space. Because the bottle is made of rigid plastic, the easiest course is for the warmed air to spread out of the bottle and into the balloon. This exerts outward pressure on the balloon and causes it to inflate.

When the bottle is placed in the ice water bath, conduction causes cooling of the air inside. The particles in the cooled air move less quickly and therefore exert less pressure against the balloon, causing it to deflate. If the experiment is repeated, the balloon will inflate or deflate in response to the change in motion of the gas molecules inside.

How Gas Expansion Affects Density

Density is defined as mass divided by volume. In the experiment, the total mass of the gas remains the same whether it is warmed or cooled. However, the change in volume results in a change in density.

The expansion of hot air causes it to be less dense than surrounding cooler air because there are fewer molecules in a given volume (or the same number of molecules occupying a larger volume). Similarly, the contraction of cooled air resulting from a constant mass of gas occupying a smaller volume gives rise to an increase in density.

How Density Affects Weather

The phenomena collectively known as weather are driven by unequal heating of air at the surface of the Earth. Warm air masses are less dense than surrounding cooler air masses, causing them to be buoyant and rise. This results in low barometric pressure at the Earth's surface. Cold air masses subside relative to surrounding warmer air, causing high surface air pressure. Surface pressure differences produce movement of air (from high to low pressure regions) known as winds.

A wide range of processes, from deep ocean circulation, to heating and cooling a house, to convection of the Earth’s mantle depend on differences in density to drive them. However, the fundamental principles remain the same.

The copyright of the article A Simple Experiment to Explain Density in Curricula/Lesson Plans is owned by Adrienne Larocque. Permission to republish A Simple Experiment to Explain Density in print or online must be granted by the author in writing.