Burning looks like the most natural thing in the world, but fire is actually quite picky: three conditions have to be met at the same time, and missing any one of them means no flame. In this experiment a few candles and two glasses form a controlled comparison — you remove the three conditions one at a time and watch the flame die each time. Read the process backwards, and you have the three firefighting methods from the textbook.

Safety first (read this section before you start)

  • This experiment uses an open flame. An adult must supervise the entire time — never do it alone.
  • Clear the work surface completely: move away tissues, curtains, books, alcohol and anything flammable. Tie back long hair and roll up loose sleeves.
  • Keep a cup of water or a damp towel within reach for emergencies. When you finish, make absolutely sure every candle is fully out and no wick is still smoldering.
  • After a glass has covered a candle for a while, its walls and rim get hot. Pick it up with a dry cloth, or wait until it has cooled completely. Mist forming on the inside of the glass is normal.
  • Never grab a freshly heated glass with wet hands — sudden temperature changes can crack the glass. Don't use thin-walled disposable glassware either.
  • Never suck on or blow into the glass to put out the candle, and never lean your face over the mouth of the glass to watch.

Materials and equipment

  • Three short candles (birthday candles or tea lights both work), fixed to a ceramic plate;
  • Two glasses, one large and one small (heat-resistant glass is best);
  • A lighter or long-reach igniter, and a pair of tweezers;
  • A small pebble, a glass bead or a short piece of wire, as the "non-flammable" comparison;
  • A stopwatch (a phone is fine), a damp towel, and a ceramic plate or heat-proof mat.

Step 1: three candles, three ways to kill a flame

Fix the three candles to the plate at least ten centimeters apart, light them all, and call them A, B and C.

  1. A: take away the fuel. Grip the base of candle A's wick with tweezers and carefully lift out the whole wick together with the pool of liquid wax at the top (or simply move candle A away from the flame source).
  2. B: take away the oxygen. Turn the small glass upside down over candle B, rim pressed tight against the plate, and start the stopwatch at the same moment.
  3. C: take away the heat. Blow gently on candle C to put the flame out — then immediately watch the wisp of white smoke rising above the wick.
One extra step (worth doing): hold the pebble and the glass bead above the flame for a while, then take them away. They never catch fire, because they simply are not fuel — no amount of heat or oxygen will change that.

Step 2: the big-glass, small-glass comparison

Relight two candles and cover them at the same moment, one with the large glass and one with the small, timing how long each takes to go out. Repeat three times and average the results.

You'll find that the candle under the big glass burns noticeably longer. The two candles are identical; the only difference is how much air each glass holds. This step shows, in a measurable way, that burning really does consume the oxygen in the glass.

Interactive simulation: this experiment has a drag-and-play simulator in our Interactive Lab. The lab interface is currently Chinese-only — the controls are simple sliders and switches, so it is still easy to explore.

What to record

Check your observations against the list below and write them into your lab notes:

  • A: the flame vanishes the instant the wick is removed, leaving only solid wax on the plate — no fuel, no fire.
  • B: under the glass, the flame first shrinks and dims, then goes out after a few seconds to a dozen or so; mist appears on the inside of the glass.
  • C: after blowing the candle out, a wisp of white smoke rises from the wick — and if you quickly bring a flame close to that smoke, the fire "jumps" back down the smoke to relight the wick.
  • The candle under the big glass takes clearly longer to go out than the one under the small glass — often more than twice as long.
  • The pebble and the glass bead get very hot, but never burn.
  • The three candles die for three different reasons, but the result is identical: take away one condition, and burning stops.

How it works

Combustion is a rapid oxidation reaction between a fuel and oxygen that gives off light and heat. For it to happen, three conditions must be met at the same time: there is a fuel; the fuel is in contact with oxygen (air); and the temperature reaches the ignition temperature. The ignition temperature is the lowest temperature at which a fuel starts to burn, and it is a property of the substance itself — about 40 °C for white phosphorus, and roughly 250–300 °C for wood.

These three conditions are often drawn as a fire triangle: only a closed triangle burns. Flip that around and you get the rule that breaking any one side puts the fire out — which is exactly the three firefighting methods: remove the fuel (cutting firebreaks in a forest fire, shutting off a gas valve), cut off the oxygen (smothering a burning pan with its lid, a CO₂ fire extinguisher), and cool below the ignition temperature (putting fire out with water, which absorbs a huge amount of heat as it evaporates). The three candles lost precisely these three things.

Now look back at the details. The mist inside glass B shows that the burning candle produced water. The oxygen in the glass was not used up completely — its concentration simply dropped below what a flame can live on, and the fire died. Candle C's white smoke is wax vapor condensing into tiny solid particles, which are themselves fuel — that is why a flame can travel down the smoke and relight the wick. It also proves that a candle burns not solid wax, but wax vapor produced by heating.

Blowing out a candle works by the same logic: one puff scatters the hot wax vapor and carries the heat away, dropping the temperature near the wick below the ignition point. Yet fanning a stove makes the fire fiercer — the airflow delivers more oxygen while the heat stays put. Same "blowing," different condition broken, opposite result.

Tips for teachers and parents

  • Timing and grouping: 25 minutes, mainly as a teacher demonstration; students time the candles in groups.
  • Before hands go on: an adult must watch every open flame, and a damp cloth must be ready.

The step most often skipped. Have students predict "big glass or small glass — which candle burns longer?" and vote by show of hands before revealing the timings. Each time you "switch off" one of the three conditions, follow up with the question: "which real-world firefighting method is this?"

How to know it has really landed. A student meets the bar when, for any flame that goes out, they can name the condition that was broken and the matching firefighting method.

Going further, and common misconceptions

  • Misconception 1: "Water puts out fire by lowering the fuel's ignition temperature" — wrong. The ignition temperature is a property of the substance and never changes; what water lowers is the temperature. This is the single most commonly lost mark on this topic.
  • Misconception 2: "Burning always makes a flame" — not necessarily. Iron wire burning in oxygen only throws off sparks; generally, only substances burning as gases produce flames.
  • Misconception 3: "Any fire can be doused with water" — no. Water on burning oil makes the oil float and spread, feeding the fire; water on live electrical equipment risks electric shock. For a kitchen oil-pan fire, the right move is to turn off the heat and cover the pan with its lid.
  • Misconception 4: "Slow oxidation doesn't count as oxidation" — it does. Rusting and food spoilage are essentially the same reaction as burning, just releasing heat far more slowly. If that heat builds up with nowhere to go, the temperature can climb to the ignition point and cause spontaneous combustion — a real risk with oily rags left piled up too long.
  • A hands-on extension: to see the effect of oxygen concentration, pour home-made carbon dioxide slowly down the inside of a glass toward burning candles at different heights — the flames go out from lowest to highest. That is exactly how a CO₂ fire extinguisher works.
  • Knowing where your home fire extinguisher is and how to use it matters more than any conclusion in this article. If a fire grows at all, evacuate immediately and call the fire department — never try to fight it yourself.

When a student can look at any extinguished flame and explain exactly which condition was broken, this experiment has done its job.