The Science Behind the Floating Candle Trick

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Today’s experiment will have your kids thinking you are a magician, but it’s not magic, it’s science!

The Floating Candle experiment demonstrates some fundamental weather topics like how atmospheric pressure works, how temperature and pressure are related, and heat transfer.

This experiment is very easy to set up and is one you can easily repeat.

Let’s get started!

How to make the Floating Candle experiment

Supplies you will need

For this experiment, you’ll need:

Thin, shallow dish (like a salad plate)
Water
Food coloring
Tealight candle
Lighter
Long, skinny glass (clear, so you can see through it)

Supplies needed for the Floating Candle experiment

Before you start

Please watch your children around the flame.

Instructions

Here is how to do this experiment with your child:

Step 1: Add a thin layer of water and food coloring to the dish

Add a few drops of food coloring to your container of water and pour a thin layer of the colored water into your shallow dish.

Start with a very small amount and work your way up, depending on the height of your candle.

You can add as much water as you like, as long as it does not cover the top of the candle. In fact, the more water, the better!

Add a few drops of food coloring to the water

Pour a thin layer of water into the shallow bowl

Step 2: Place the candle in the dish and light it

Place your small candle in the center of your shallow dish. If it’s a new candle, be sure to pull the wick up so it is perpendicular to your candle.

Light your candle with a lighter, making sure to keep everyone’s fingers away from the flame!

Place the tealight candle in the dish with water in it

Step 3: Place glass over the candle

Next up, we’re going to place the tall, thin glass over the candle.

If you start by tipping the glass and gradually placing the glass opening down flat, it will help alleviate any air bubbles.

Step 4: Watch the candle rise!

This part happens fast, so be sure to watch carefully!

As the candle heats the air inside of the glass, you will see the water level slowly starts to rise.

Once the candle uses up all of the oxygen in the glass, the flame will extinguish, causing the temperature in the glass to decrease. The water (with the candle) will then quickly rise in the glass! So cool!

You will also notice some condensation forming at the top of the glass. The air inside of the glass begins to reach saturation, otherwise known as 100% relative humidity.

Warmer air can hold more water than cooler air, so when the warmth from the flame is extinguished and the air cools inside the cup, the air inside of the cup becomes supersaturated and condensation forms.

The candle and water level rises after the candle flame is extinguished

There should be a small amount of condensation on the inside of the glass

The STEM behind the Floating Candle experiment

This experiment teaches:

Atmospheric pressure
Heat transfer
Close (and careful) observation

How it works

The Floating Candle experiment demonstrates fundamental weather topics like how atmospheric pressure works, how temperature and pressure are related, and heat transfer.

When we place a cup over a lit candle, we heat the inside of the cup with the candle flame.

When the candle flame eventually goes out because of the lack of oxygen in the closed system, the temperature quickly cools down, and subsequently, the pressure also reduces inside the cup.

To keep the pressure equalized, the water level rises in the bottom of the cup and our candle floats up!

Atmospheric pressure

Atmospheric pressure, in basic terms, is the measurement of how much air is right above a point on Earth. If there is low pressure, there is less air. If there is high pressure, there is more air.

As the temperature goes down in that column of air above the point, the pressure goes down as well.

The same thing is happening in our experiment!

The candle is heating up the air inside of the glass. When the candle goes out, the temperature begins to drop, and the pressure drops. Water is pulled into the bottom of the glass to help equalize the pressure, resulting in the candle rising with the rising water.

Heat transfer

As the candle is burning inside the glass, a lot is going on!

The candle burns inside the glass and the temperature inside of the glass wants to reach an equilibrium. Heat is transferred from the hot candle to the air around it as the air tries to reach an equilibrium, heating up the air inside.

Close (and careful) observation

This experiment exercises our ability to watch carefully:

What is happening in the glass when the candle is still burning? What do we see?
How long does it take for the water to start rising when the candle burns out?
What is happening at the top of the glass while we run the experiment?