How to make a fireproof balloon

This activity is part of our Science Lab series. Check out the ANU Science Lab for more experiments! In this activity, you will conduct a classic thermal physics demonstration to discover how water's unique heat capacity can create a "fireproof" balloon. Watch the video above to see how it's done.

Fireproof balloon materials & ingredients

For this experiment:

• 2 standard rubber balloons
• Tap water
• A small candle or tealight
• Matches or a lighter
• A sturdy glass or candlestick holder (to elevate the candle safely)

Safety Note: This activity involves open flames, hot wax, and popping balloons! Always ensure adult supervision, tie back long hair, roll up loose sleeves, and keep flammable materials clear of your workspace.

How to make a fireproof balloon: step-by-step instructions

1. Inflate one balloon with air to a standard size and tie it off securely. This will serve as your control test.
2. Take your second balloon and stretch the neck over a tap. Fill it with water until the uninflated balloon is mostly full.
3. Carefully blow air into the balloon until it reaches about the same size as your control balloon, then tie it off. You should have a balloon with a small pool of water resting at the bottom.
4. Place your candle securely on top of a glass and light it using a match.
5. Standing back at a safe distance, slowly lower the air-filled control balloon directly over the flame. Watch how quickly the balloon pops!
6. Relight your candle if the popping balloon blew out the flame.
7. Carefully lower the water-filled balloon over the flame, ensuring the bottom area where the water sits makes direct contact with the top of the flame. Observe what happens.

How does a fireproof balloon work? The science explained

Your fireproof balloon demonstrates a fundamental principle of thermodynamics known as specific heat capacity.

When you place the air-filled balloon over the flame, the heat quickly transfers into the thin rubber wall. Because air is a poor conductor of heat, the rubber absorbs the thermal energy directly, rapidly weakening and stretching until the pressurized air inside bursts the balloon.

When you place the water-filled balloon over the flame, the outcome changes completely due to water's special properties:

• High Specific Heat Capacity: Water requires a massive amount of heat energy to increase in temperature compared to rubber or air.
• Thermal Conduction: As the flame heats the bottom of the balloon, the water resting inside immediately conducts and absorbs that thermal energy away from the thin rubber skin.
• Heat Convection: Warm water rises away from the bottom while cooler water sinks down to replace it, creating a continuous cooling current inside the balloon.
• Because the water continually draws heat away, the rubber remains cool and strong below its melting point, preventing the balloon from popping.

Science fair projects & taking it further: experimenting with variables

Once you have tested a basic water balloon, try tweaking your setup to test the limits of thermal absorption:

• Water Volume: What happens if you only use a tiny splash of water versus filling the balloon halfway? Does more water allow the balloon to withstand the flame longer?
• Initial Water Temperature: Test cold ice water versus warm tap water. How does starting temperature affect how long the balloon lasts over the candle?
• Alternative Liquids: Try using other household liquids like cooking oil or saltwater to compare their heat absorption abilities against plain water.

Enjoyed this experiment? Explore more hands-on activities on the Science Lab ANU YouTube channel.