Fire and energy

Every fire needs three things at the same time. It needs fuel, something to burn. It needs oxygen, usually from the air. And it needs heat, enough to set the fuel burning and to keep it burning. Firefighters draw the three as the sides of a triangle and call it the fire triangle.

The picture is useful for what follows from it. Take away any one side and the fire goes out, however much of the other two is left. A pile of dry wood in the open air has fuel and oxygen and does not burn, because the heat is missing. A gas cooker goes out the instant its fuel is shut off. A candle under a glass dies for want of oxygen. Every way of putting out a fire, as the last passage shows, is a way of removing one of the three.

A candle looks simple, and it is a small chemical works. Solid wax cannot burn as it is. The heat of the flame melts a pool of wax at the top, and the job of the wick is to carry melted wax upward, soaking it up as a paper towel soaks up a spill. Near the flame the heat turns the liquid wax to vapour, and it is this vapour, mixing with the air around it, that burns. The wick itself hardly burns at all.

Why is the flame yellow? Air reaches the vapour only from outside, so in the middle of the flame there is too little oxygen to burn all the carbon at once. Some of it gathers into tiny solid specks called soot. The specks glow yellow-white in the heat, like the wire of an old light bulb, and that glow is the candlelight.

A gas cooker burns the same two elements as a candle, carbon and hydrogen, yet its flame is blue. The difference is air. In the cooker, air is mixed into the gas before it reaches the flame, so every part of the fuel meets enough oxygen. All the carbon burns to carbon dioxide, no soot is left to glow, and the flame is blue and hotter. This is complete combustion.

Starve a flame of air and the burning is incomplete. Some carbon leaves as soot, and some as carbon monoxide, a gas with one oxygen atom to each carbon atom where carbon dioxide has two. It has no colour or smell, and it is poisonous. It clings to haemoglobin, the protein that carries oxygen in the blood, over two hundred times as tightly as oxygen does. A faulty heater can produce it, which is why homes have alarms for it.

Every chemical reaction either gives out energy or takes it in. A reaction that gives out heat to its surroundings is called exothermic. Burning is the obvious example. A disposable hand warmer is a quieter one. It holds iron powder, and when the packet is opened to the air the iron rusts quickly and stays warm for hours.

A change that takes heat in from its surroundings is called endothermic. Squeeze an instant cold pack and a salt inside it dissolves in water, drawing in heat as it does so. The pack does not make cold. It takes warmth out of whatever it touches, and a sprained ankle feels that as cold.

The two words are built from Greek. Exo means outside, endo means within and therm means heat. In one the heat goes out, and in the other it goes in.

Where does the heat of a fire come from? It was there all along, but not as heat. The fuel and the oxygen held it as chemical energy, in their chemical bonds, the links between atoms.

Follow methane, the gas in a cooker, as it burns. The bonds inside the methane and the oxygen must first be broken, and breaking a bond always costs energy. Then the atoms join again as carbon dioxide and water, and forming a bond always gives energy out. The new bonds are stronger than the old ones, so they give out more than the breaking cost. For every 100 units of energy spent, about 130 come back. The surplus is the flame’s heat and light.

So the heat does not come from breaking the fuel apart. A reaction is exothermic when its new bonds are stronger than the old, and endothermic when they are weaker.

If burning pays out energy, why does a match not burst into flame in its box? Because old bonds must be broken first, and that takes a push. Molecules of fuel and oxygen must collide hard enough to break apart before new, stronger bonds can form. The energy needed to get a reaction started is called its activation energy.

Picture a ball resting in a hollow near the top of a hill. A long slope runs down the far side, but first the ball must be pushed over a low rise. The push is the activation energy. The long slope is the energy the reaction gives out.

Striking a match supplies the push. Dragging the head along the rough strip heats it by friction, and that small burst of heat is enough to set the chemicals of head and strip reacting. From then on the flame makes its own heat.

How big a push does a fuel need? Each has its own ignition temperature, the temperature at which it catches fire without a spark. White phosphorus ignites at about 34 °C, the heat of a hot day, which is why it is stored under water. Paper needs about 233 °C. Methane, the gas in a cooker, needs more than 500 °C.

Once a fuel is alight, something important happens. The burning part gives out heat, and that heat raises the fuel beside it to its ignition temperature. That part burns and heats the next. Each step sets off the one after it, and a process of this kind is called a chain reaction. It is why a fire, once started, needs no more matches. It is why fire spreads, and why a small fire is so much easier to stop than a large one.

Candle wax, petrol and natural gas look nothing alike, yet all three are built from the same two elements, carbon and hydrogen. Substances of this kind are called hydrocarbons. Coal, oil and natural gas are fossil fuels. Much of the world’s coal is what remains of swamp forests buried about 300 million years ago. Oil and gas came mostly from tiny sea life that settled on ancient sea beds.

Those plants and sea creatures grew on sunlight. Plants used its energy to build their own substance out of carbon dioxide and water, and animals fed on the plants. So the energy in a lump of coal first arrived on Earth as sunlight. Burning the coal releases it, along with the carbon dioxide the plants once took in.

One fuel holds no carbon at all. When pure hydrogen burns it joins with oxygen, and the only product is water.

How much energy does a kilogram of fuel hold? The amount is called its energy density, and it is measured in megajoules per kilogram. One megajoule brings about three litres of cold water to the boil. Hydrogen holds about 142, petrol about 46 and butter about 30. TNT, the explosive, gives out only about 4 when it explodes.

That is no mistake. A kilogram of butter stores about seven times the energy that a kilogram of TNT sets free. What makes an explosive dangerous is how fast it releases its energy. Butter and petrol must wait for oxygen to reach them from the air, so they burn only at the surface. TNT carries oxygen inside each of its molecules, so the whole lump can react in a fraction of a second. Energy released that fast shatters whatever is near. The same energy released over an hour would only warm it.

The body runs on the same reaction as a flame, with the heat turned far down. The fuel is glucose, a sugar from food. The cells combine it with the oxygen you breathe in, and the products are water and the carbon dioxide you breathe out. This slow, controlled release of energy from food is called respiration.

No flame is involved. Enzymes, proteins that speed up reactions, take the glucose apart in many small steps at body temperature. Part of the energy is caught and put to work in moving and growing, and the rest keeps you warm.

The total is the same as if the glucose had been burned. That is how the energy in foods was first measured: a sample was burned in a sealed chamber and its heat was recorded. Food labels still rest on such measurements. A food calorie is simply a unit of heat.

A battery is a chemical reaction with its two halves held apart. At one end is a material that readily gives up electrons, the tiny charged particles that make an electric current. At the other is a material that takes them. Left touching, the two would simply react and grow warm. Kept apart, the electrons can cross only through a wire outside the battery, and on the way they light a lamp or run a phone.

Inside, the circuit is completed by charged atoms called ions, which move through a paste or liquid between the two ends. Electrons travel outside and ions inside, and if either path is broken the reaction stops.

When the chemicals are used up, the battery is flat. In a rechargeable one, the charger pushes electrons the other way and runs the reaction backwards, rebuilding the original chemicals so that it can all happen again.

Every way of putting out a fire removes one side of the fire triangle. Water on a wood fire takes away the heat, cooling the wood below the temperature at which it burns. A fire blanket takes away the oxygen. So does a carbon dioxide extinguisher, whose gas is heavier than air and settles over the flames. A firebreak, a strip cleared of trees ahead of a forest fire, takes away the fuel.

The method has to suit the fire. Never pour water on burning cooking oil. Water is denser than oil, so it sinks beneath it, meets oil far hotter than its own boiling point and flashes into steam. Steam takes up more than a thousand times the room of the water it came from, and it blasts burning oil everywhere. Slide a lid over the pan and turn off the heat, and the fire dies quietly.