Butane Balloons

I love showing people how to do fascinating new things with objects and materials they might have in an ordinary house. Today we will play with butane, and have some very interesting experiences without ever even lighting it on fire.

We will need some time for preparation. An hour ought to do, but most of it will be just waiting. We also need a butane lighter refill can, and some balloons. The large 12 inch diameter balloons will work best.

First, we put the can of butane into the freezer. Make sure the freezer is set to its coldest setting (this will also make your frozen food last longer). We want the butane to be as cold as the freezer compartment can make it.

Butane has a very interesting property. It is a liquid at -0.5° Celsius (31.1 degrees Fahrenheit). Above this temperature, the liquid butane will boil, becoming a gas.

You may have seen transparent butane lighters, where you can easily see the level of butane liquid in them. Butane is a liquid at room temperature when it is under pressure. It does not take a lot of pressure to keep butane liquid. At 2.6 atmospheres of pressure, it will remain liquid up to 39° C (100 F). This is why it can be stored as a liquid in little clear plastic lighters.

On a cold winter's day, when the temperature is a few degrees or more below freezing, a can of butane (or a butane lighter) won't work very well, since the contents will remain a liquid, and won't have enough pressure to leave the container. Some camping stoves run on a mixture of butane and propane for this reason, since the propane will still be a gas in cold weather.

If the freezer has been set low enough, it will only take an hour or less for the butane can to get so cold that the butane will be a liquid.

Cold butane

Remove the butane can from the freezer using a towel or some oven mitts so your hands don't warm up the can. Stretch a balloon over the top of the can.

Butane with a balloon

Turn the can upside-down, and pinch the plastic nozzle through the neck of the balloon, and push it towards the can. Liquid butane will pour into the balloon, along with some gaseous butane that has warmed up a bit. As the butane hits the balloon, some of it will boil into butane gas. The balloon will start to inflate a little.

Pinch the neck of the balloon and remove it from the can, and tie it closed.

The first thing to notice is that frost forms on the outside of the balloon where the liquid butane has made the rubber very cold.

A warmer balloon

Hold the balloon in your hand. The cold liquid butane will boil from the warmth of your hand. You can feel it boiling, even though it is below freezing in temperature. It make a hissing sound, and the bubbles vibrate the balloon.

The balloon gets bigger

The balloon continues to get bigger. At some point, all of the liquid butane is gone, having turned into butane gas. As the gas warms to room temperature, the balloon will get a little bit bigger, but not as quickly as it did when the liquid was boiling.

The next thing to notice is that the balloon feels heavy. This was not surprising when it had a puddle of liquid in it, and it was small, but now it is a big balloon full of gas, but the weight has not changed perceptibly.

An empty balloon weighs about 3 grams.

An empty balloon weighs 3 grams

A balloon full of air still weighs about 3 grams (why is that?). But a balloon full of butane gas weighs 12 grams.

A balloon full of butane weighs 12 grams

Butane is denser than air, so any volume of butane will weigh more than that volume of air. And since air is neutrally buoyant in the air around us, it does not register on the scale. But the butane sinks in air, and presses down on the scale.

The third fascinating thing to do with our butane balloon is to hold it up to your ear. The dense gas in the roughly spherical balloon acts as a lens for sound waves. Turn your head (with the balloon still against your ear) until some source of sound, like a radio or a television, or someone talking, has to go through the balloon to get to your ear. When the balloon is directly between your ear and the sound, the volume suddenly gets louder. The balloon lens is acting like a telescope for sound.