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Introduction

Combustion involves the reaction of a substance - the fuel - with oxygen in such a way that heat and light are produced. We can think of a fuel as a substance that can be safely burned to produce heat and light. For combustion to take place, there must be a fuel, a supply of oxygen (usually air) and a source of heat to start and to maintain the process. This is often known as the 'fire triangle'. For combustion to be possible, the triangle must be complete. If any one of the three factors is missing, combustion is not possible.

Task 1: Combustion


Fire triangle

  1. Open Yenka file Model 1. Transfer the carbon to the gas tube above the Bunsen burner and open the oxygen cylinder using the control to the left of the cylinder.
    (a) Does the carbon catch fire?
    (b) Explain the observation.
    Answer
    (a) The carbon does not catch fire.
    (b) There is no source of heat.
  2. Now turn on the Bunsen burner.
    (a) What happens?
    (b) Explain the observation.
    (c) What is the word equation for the reaction that takes place?
    Answer
    (a) The carbon burns.
    (b) There is now a supply of heat. We have a fuel, oxygen, and a supply of heat, so combustion can take place.
    (c) Carbon + oxygen → carbon dioxide
  3. Now reload the model by using the F5 key. Open the oxygen cylinder and turn on the Bunsen burner. When the temperature is high enough, the carbon will start to burn. When this happens, turn off the oxygen.
    (a) What happens?
    (b) Explain the observation.
    Answer
    (a) The carbon stops burning.
    (b) The oxygen supply has been removed. We still have a fuel and a heat supply, but we do not have an oxygen supply.
  4. Reload the model. Add the sulfur to the tube. Turn on the supply of oxygen and the Bunsen burner. The sulfur will catch fire. Now turn off the Bunsen burner.
    (a) What happens?
    (b) Write the word equation for the reaction.
    (c) Can you think of any reason why sulfur is not used as a fuel, despite the fact that it burns easily?
    Answer
    (a) The sulfur continues to burn.
    (b) Sulfur + oxygen → sulfur dioxide
    (c) When we burn sulfur it forms sulfur dioxide. Sulfur dioxide is a poisonous gas, and pollutes the atmosphere.
  5. Suggest why the sulfur continued to burn even when the Bunsen burner was turned off.
    Answer
    When sulfur burns it gives out enough heat to keep the burning going.
  6. Open Scene 2 by clicking on the button in the main toolbar which says 'Scenes' when hovered over. Add the sulfur to the beaker and heat it using the Bunsen burner. When it catches fire, move the beaker so that its base enters the water in the water bath.
    (a) What happens?
    (b) Explain the observation.
    Answer
    (a) The sulfur stops burning.
    (b) The water in the bath is much colder than the burning sulfur. It removes the heat from the burning sulfur, so that it can no longer burn.
  7. Where does the heat from the burning sulfur go?
    Answer
    It goes into the water in the water bath. If you reload the model and repeat what you have just done, you should see that the water gets hotter. This is because the heat from the sulfur has been transferred to it.
  8. Open Scene 3. Add the gunpowder to the beaker and turn on the Bunsen burner. Note what happens when the gunpowder becomes hot. Why would we not think of gunpowder as a fuel?
    Answer
    Although it burns, it burns explosively - it gives out all its energy at once.

Summary

In order for burning to take place, three things are required - a fuel, a supply of oxygen, and heat. If any of these are removed, burning stops. Often the heat required comes from the actual burning itself, but if the fuel is cooled down enough it will stop burning. Generally, when we talk about a fuel, we mean something that can be safely burned. A good fuel does not pollute the atmosphere and it does not burn in a dangerous way - for example, explosively, as gunpowder does.

Teacher Summary

  • This activity could be used to support a topic on fuels or combustion. It could also be used to support a topic on safety or fire fighting. It could be used to explain why water is used to put out fires - it removes the heat required for continued combustion. Enough water will also exclude air. It might be worth commenting that most explosives contain their own supply of oxygen (e.g. as potassium nitrate in gunpowder) and this accounts for the fact that some explosives work by extremely rapid combustion.