Teacher Demonstration 8
Hot air expands
Materials:
Glass bottle / two large jars* / balloon / hot water and cold water **
* one jar should be suitable for hot water – large coffee plungers with handle are ideal
** if possible, use boiling water from a kettle – colouring the water is optional
Brief description: this demonstration provides clear visual evidence that air expands when it is heated.
It can be used to explain why hot air rises and how wind is produced by heat energy from the Sun.
Instructions:
y To avoid spills, check how much hot water you will need by
submerging the bottle and mark the jar before the demonstration.
y Show the class that the bottle is completely empty and stretch a
balloon over the neck of the bottle so that it hangs limp (ie there
should be no pressure in the balloon)
y Discuss that air cannot escape from or enter the bottle
y Pour cold water into one of the jars (add blue food colouring if
desired)
y Pour hot water into the second jar (add red food colouring if
desired)
y Ask the class to consider what will happen to the air inside the
bottle when it is submerged in hot water (the air will heat up)
y Submerge the bottle in the hot water and step away to prove you
are not manipulating the bottle in any way – the balloon will
partially inflate so that it stands upright (expect a few laughs)
y Ask the class to consider what has happened reiterating that no air
can escape or enter the bottle – they should conclude that the air
inside the bottle has expanded
y Ask the class to consider what will happen when the bottle is
submerged in cold water (the air inside will cool down again)
y Submerge the bottle in the cold water – the air inside the bottle
contracts and the balloon deflates
Safety notes:
• Emphasise the safety precautions you are taking before performing this demonstration and insist
that students only attempt this demonstration under the strict supervision of an adult.
Surfing Scientist | Teacher Demonstrations
© 2004, Ruben Meerman, ABC Science
Explanation:
Why does the balloon expand?
The temperature of a gas is a measure of the speed of its molecules. Increasing the temperature of a gas
increases the average speed (and therefore the kinetic energy) of the molecules. This causes the
molecules to ‘spread out’ – a phenomenon called thermal expansion.
Initially, the gas (air) molecules inside and outside the bottle are at the same temperature and therefore,
moving at the same speed. Air molecules collide into the balloon with the same energy inside and
outside the balloon.
When the bottle is heated, the air molecules inside start moving faster. These molecules now collide into
the balloon with more energy resulting in increased pressure. The increased pressure causes the balloon
to expand.
Why does hot air rise?
A given amount (mass) of air takes up more space when it is hot. In other words, hot air is less dense
than cold air. Any substance that is less dense than the fluid (gas or liquid) of its surroundings will float.
So, just as wood floats because it is less than water, hot air floats on cold air because it is less dense. This
how hot air balloons get their lift.
How does the Sun produce wind on Earth?
Once the relationship between the thermal expansion and buoyancy of hot air is understood, it can be
directly related to the formation of wind.
Different air temperatures near the Earth’s surface and over vast areas result in large bodies of air either
rising or falling. Air from surrounding areas is then either pushed in underneath rising air, or pushed out
by descending air.
But as the Earth rotates in and out of sunlight, different parts of the planet heat up faster than others for
several reasons. Land absorbs heat more readily than water. As a result, air temperatures over the
continents increase more quickly than they do over oceans and large bodies of water. Land also loses
heat more quickly than water at night. The angle at which sunlight enters the atmosphere also affects
the amount of heat reaching the Earth’s surface. These differences and many other factors make the
Earth’s weather so variable and difficult to predict in the long term.
A clear example of how the difference in heat absorption by land and water results in wind can found
along many coastlines (in the absence of strong winds associated with high or low pressure systems).
During the day, the land up quickly and soon becomes much warmer than the ocean. As a result, the air
above the land expands and rises into the atmosphere. Cooler air from above the ocean rushes in to
replace the rising air resulting in cool onshore winds. At night, the land loses its heat faster than the
ocean. As the land temperature falls below that of the ocean, the process is reversed resulting in offshore
winds. Offshore winds give the ocean a smooth appearance while onshore winds result in white caps.
Internet sites of interest:
Bureau of Meteorology
http://bom.gov.au
How Stuff Works – Hot Air Balloons
http://travel.howstuffworks.com/hot-air-balloon.htm
Surfing Scientist | Teacher Demonstrations
© 2004, Ruben Meerman, ABC Science