Physics

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Video

Materials

• 1 drinking glass
• 1 plate
• 1 tealight
• 1 lighter or matchbox
• Food coloring (optional)
• Water
• Safety equipment: 1 fire extinguisher

Short explanation

When the candle goes out - due to lack of oxygen - the air in the glass cools and its pressure on the water surface decreases. The air outside the glass then pushes comparatively more on the water surface, and pushes water into the glass.

Long explanation

When a candle burns the wax (or whatever fuel that the candle is made of) rapidly reacts with oxygen and forms water, carbon dioxide and some residual products. This chemical reaction is exothermic, which means that energy is released into the surroundings. In this chemical reaction, energy is released in the form of radiant energy (including light) and thermal energy (heat).

When the candle has been trapped in the glass for a while, it goes out. This is because the oxygen is depleted. Or maybe not depleted, but the concentration of oxygen in the air is too low to sustain the fire.

Since oxygen makes up 21 % of the gas mixture called air, and oxygen is consumed when the candle burns, one can easily believe that the result of this demonstration is due to the volume of air decreasing during combustion, and that water is sucked in to replace this volume. However, this is not the case. After all, carbon dioxide and water are formed during combustion, and these gases replace the lost oxygen gas. The amount of matter in gaseous form in the glass actually increases, because all oxygen atoms remain in the air, but now in carbon dioxide and water molecules - together with atoms from the candle. But whether the volume of gas in the glass increases or decreases during combustion is still a bit difficult to know, partly because part of the water vapor condenses to liquid water and that carbon dioxide tends to dissolve in the water.

The explanation is instead something else. Surely you noticed that the water was sucked into the glass only after the candle went out?

When a gas (such as air) is heated, the particles (atoms or molecules) that make up the gas begin to move faster. Temperature is in fact a measure of this kinetic energy of a substance's particles. Consequently, if the gas is kept within an enclosed space, the frequency and speed of the particles' collisions with each other and their surrounding also increase. In each collision, the gas particle pushes on what it collides with. So, when a gas is heated, its pressure also increases.

Because the particles in a gas repel each other all the time, gas escapes from its confined space if it can. If you quickly put the glass over the candle, you will see hot air bubble out of the glass. But this "escape" ends as quickly as it began, and this is because the air pressure inside the glass has now become equal to the air pressure outside. At this moment, the air inside the glass has a higher temperature and lower density (the air particles are far from each other) than the air outside, which has a lower temperature and higher density (the air particles are close to each other). But their air pressure will be equal. The air outside the glass pushes - via the water - as much on the air inside the glass as vice versa. Imagine how the air pushes down on the water surface, both from inside the glass and outside.

When the candle goes out due to lack of oxygen, the air inside the glass starts to cool down again. This causes the air pressure inside the glass to decrease, and it is now lower than the air pressure outside. Therefore, the air outside the glass now pushes water into the glass (this is alternative A). This happens until the air pressure inside and outside the glass is the same again. Nature hates pressure differences.

This demonstration has an alternative result (alternative B), which happens if the glass is tight against the bottom. Then no water can be forced into the glass and the air pressure in the glass remains low. This means that the air outside will continue to push more on the glass than the air in the glass does, thus pushing the glass against the plate.

It is easy to talk about "suction" in this demonstration, but there is no kind of "suction force" or something like that - just pressure.

Experiment

You can turn this demonstration into an experiment. This will make it a better science project. To do that, try answering one of the following questions. The answer to the question will be your hypothesis. Then test the hypothesis by doing the experiment.

• What happens if you put the glass over the candle very quickly?
• What happens if you use a larger glass?
• What happens if you use a taller, narrower, glass?
• What happens if you use a different type of candle (for example, a small cake candle?
• What happens if you use multiple candles (you may need a larger glass or a jar)?
• What happens if you use a bowl of water instead (see below)?

Variation

Instead of a plate of shallow water, you can use a bowl of deep water. Place the tealight on the surface (it floats) and light it. Then take the glass, place it directly above the tealight, and push the glass straight down to the bottom. Since there is air in the glass, the tealight and water surface will be pushed down to the bottom. For a while, the candle will burn well below the water surface, trapped inside the glass. This transparent "diving bell" with a burning candle in it is a pretty cool sight. Then the candle goes out as usual and water will either be sucked up into the glass, or the glass will stick to the bottom (again depending on how tight the glass is to the bottom).

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