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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.

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.
(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?
(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.
(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?
(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.
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.
(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?