Burning
It had been a hot dry week. The people working at the wood yard were glad it was Friday afternoon and their work would soon be over. All week long lorries had been delivering huge logs. They had been sawn up into planks and stacked in huge open sheds to dry. In other sheds nearby were planks that had dried out. Builders had been calling all week for them, as they were needed to make roofs at a housing estate on the other side of town. By Friday evening the shed of dry planks was only half full.

"That's it", cried the site manager as she locked the wood yard gates. "I won't be seeing you till Monday." But she was wrong.

On Friday night someone passing the wood yard threw down a glass bottle after they had finished a drink. The bottle did not break but rolled into a tuft of dry grass by the wood yard fence. In the morning the Sun rose in a clear sky and it shone on the bottle. The curved glass of the bottle focussed the Sun's rays onto the grass. The dried leaves of the grass became hotter and hotter. Eventually they became so hot that they caught fire and small flames licked against the fence.

When something catches fire, it forms a vapour, which mixes with oxygen. Heat and light are given out and the oxygen joins with a substance in the burning object to form carbon dioxide. The hot air produced by the flames rises and cooler air with more oxygen rushes in to take its place. All this was happening quickly in the tuft of dry grass and in a short time it was a roaring blaze and the bottom of the fence was beginning to get hot. The fence was made of wood and it was not long before it was ablaze, too.

By now someone had seen the smoke and flames and had called the fire brigade. As the sirens wailed through the town the fire crept along the fence and entered the shed of dry planks. In the short time it took the fire brigade to arrive, the shed of dry planks was beginning to burn strongly. Four metre long flames danced above the roof and dipped towards the other sheds and the sawmill.

The site manager had been called and she had returned to open the gates. As the gates were thrown apart firefighters rushed in. Some carried a hose and began to spray the wood with water. Others rushed to a pile of wood in the middle of the yard. If they could reach it before the flames did there was a chance the other parts of the wood yard could be saved. The firefighters used long poles to push the wood further from the flames. Their action meant that the fire had been robbed of fuel and could no longer grow.

Meanwhile the water from the hose was hitting the hot wood, picking up heat and turning into steam. As the water took heat from the fire the wood cooled down and the flames died away.

At the height of the blaze the rising hot air above the fire was moving so strongly that it lifted up burning splinters. One had fallen through an office window, which had been carelessly left open. It fell onto a carpet by a wastebasket and set them ablaze.

Fortunately the wood yard site manager had entered the office to check that everything was safe. When she saw the fire she pulled a fire extinguisher off the wall and squirted foam over the blaze. The foam forms a blanket over the fire and stops oxygen reaching it. In moments the fire was out.

A major disaster had been avoided because the fire was robbed of the three things it needed - fuel, heat and oxygen.

Why did the fire brigade use water?
To cool down the wood. When the cold water landed on the hot wood, heat moved from the wood to the water. As the water warmed up, it evaporated and even boiled. The hot air lifted the water vapour into the air. As the water vapour rose, it cooled down and condensed to form a cloud most people call steam. When firefighters are using a hose they look at the cloud of steam. If it is large they know they are hitting the heart of the fire and removing large amounts of heat to bring the fire under control.

Some fire extinguishers say they produce carbon dioxide. How does that help to put a fire out?
Carbon dioxide is a gas that is heavier than air. If it is sprayed over a fire it pushes air out of the way. There is oxygen in the air so this is pushed out of the way too. As the fire is then starved of oxygen it can no longer burn and it goes out.

Is burning an irreversible change?
Yes, it is. When a substance burns new substances are made. For example, a candle is made of wax. This material contains substances called hydrocarbons. There are two kinds of particles in hydrocarbons: hydrogen atoms and carbon atoms. When the wax burns, the carbon atoms join with oxygen atoms in the air and form carbon dioxide. At the same time, hydrogen atoms join with other oxygen atoms in the air and form hydrogen oxide. We know this substance as water. As the flame is so hot, the water can only exist as water vapour. It rises unseen in the hot air above the candle flame.

Why are flames yellow?
The colour is made by glowing particles of carbon. If you look at the bottom of a candle flame you will see a blue coloured region. This is made where the vapour from the wick mixes with oxygen in the air and burning is complete. Just above the blue region there is a clear region in the flame. In this region the vapour from the candle is starved of oxygen and cannot burn. Above this you see the yellow part of the flame. Here oxygen has mixed with the unburnt vapour and burning occurs again. Heat from the burning causes the carbon particles to glow and make the yellow colour. At the top of the flame, most of the carbon atoms in the particles join with oxygen atoms to form carbon dioxide. Some carbon particles may escape, however, and form smoke.