Physics

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Video

Materials

• Ethanol
• 1 small towel
• 1 pair of grill tongs
• A 1 dl measuring cup (or a 1 cup measuring cup)
• 1 teaspoon measuring spoon (5 mL)
• 1 bowl
• 1 spoon
• 1 lighter
• Salt
• Water
• Safety equipment: 1 fire extinguisher, 1 bucket of water, 1 pair of safety goggles

Short explanation

Most of the heat released when the ethanol burns is absorbed by the water. This prevents the towel from igniting.

Long explanation

When you hold a flame close to liquid ethanol, the liquid heats up so much that it first evaporates and then starts to burn. What happens then is that the ethanol react rapidly with oxygen and form water and carbon dioxide (and small amounts of other by-products). This chemical reaction is exothermic, which means that energy is released. In this specific chemical reaction, energy is released in the form of radiant energy (including light) and thermal energy (heat).

The chemical equation of when ethanol burns is as follows:

C₂H₅OH + 4 O₂ → 2 CO₂ + 3 H₂O + energy

Since energy can neither be created nor destroyed, only transformed, one may ask where this energy came from. Well, when the ethanol and oxygen molecules were formed (whenever that was), energy was required to move the electrons in these molecules so that the constituent atoms would sit together. This energy was then stored as potential energy in these molecules, which is called chemical energy. That is, until now, when the electrons in this chemical reaction were moved to less energy-demanding positions in newly formed molecules.

The flame in this demonstration consists of ethanol and oxygen that are being converted to water and carbon dioxide, as well as the radiant energy (light) and thermal energy (heat) that is formed as a by-product in that process. It's possible that part of the flame contains ethanol and/or oxygen that has become so hot that it has been ionized. This means that some of the electrons of these molecules are completely detached. This state is called plasma and is the state of matter that the chemical substances in the Sun have. But it's unclear whether the chemical reaction in this demonstration really releases so much heat that ionization occurs - and to our eyes, burning plasma and burning gas look pretty much the same.

The big question, though, is: why doesn't the towel burn up in this demonstration? The answer is water. The water in which the towel is also soaked absorbs most of the energy released in the exothermic chemical reaction.

Temperature is a measure of how much the particles in a substance move. In cold water the water molecules move a little and in hot water they move a lot. But it takes a lot of energy to heat water and get the molecules moving. This is because much of the energy added to water is used to bend and break the hydrogen bonds that hold the individual water molecules together. A lot of energy is absorbed by this process, instead of causing the water molecules to start moving. So water can be heated and heated, but still remain quite cool. Another way to put it is that water has a high heat capacity. However, when enough energy has been added to water, the bonds between the water molecules break completely. The water molecules are "dispersed by the wind" - which means that the water changes from a liquid state to gaseous state (water vapor).

Why add the salt? Well, this is to see the fire better. Ethanol burns with a rather invisible flame, while salt burns with a distinct yellow flame.

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 leave a corner of the towel dry?
• What happens if you halve the amount of ethanol?
• What happens if you don't add the salt?
• What happens if you replace the salt with sugar?

Variation

A smaller version of this demonstration is to use a bill instead of a towel. You can find this experiment here: Burning money.

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