When discussing energy, safety is an unavoidable topic. As sources of energy, materials themselves are relatively stable under normal conditions—otherwise, we likely wouldn’t encounter them at all.
Take a simple example: you’ve definitely seen salt (sodium chloride), but many people will never see metallic sodium in their lifetime. That’s because sodium is highly unstable in its pure form and rarely exists as such. The most common fuels in civilization—wax, oil, and gas—are similar. They remain calm and stable until actively ignited.
What Is Combustion?
From everyday experience, we know that fire requires ignition. Without that initial spark, fuel will not burn. In essence, combustion is a chemical reaction between fuel and oxygen. This reaction requires activation energy (ignition) to begin. Once triggered, as long as fuel and oxygen are continuously supplied, combustion can sustain itself.
Light and heat are both byproducts of combustion. Traditionally, humans utilize heat energy, but light is also a form of energy.
By the way: which has higher energy, a blue flame or an orange flame? The answer is blue. Blue flames burn at higher temperatures and more completely, while orange flames typically indicate incomplete combustion with more carbon particles being emitted.
What About Explosions?
Think of a balloon. As you keep blowing air into it, the tension of the balloon’s surface increases the internal pressure. When that pressure exceeds what the balloon can withstand, the high-pressure gas inside is suddenly released—resulting in what we see as a “burst” and hear as a “bang.”
The essence of an explosion is the instantaneous release of high pressure. A large pressure difference, an extremely short time frame, and a large volume—these three conditions are all required.
Let’s analyze a common example: a gas explosion. The pressure inside a gas cylinder increases beyond what the container can handle, leading to a sudden and massive release. But why does the pressure inside rise?
Under normal conditions, the cylinder contains liquefied gas along with gaseous gas in the upper space—similar to the relationship between water and steam. When the valve is opened, gas escapes, and the liquid continues to evaporate to replenish it. However, when the temperature rises dramatically—often due to ignition inside the cylinder—the rupture of the container becomes only a matter of time.
Can Fuel Explode in Open Spaces?
Consider the scene of a car catching fire and the fuel tank exploding. Fire travels back to the tank, and due to the enclosure, internal pressure begins to rise. In combustion, liquids must first vaporize into gas before burning. As temperature increases rapidly, pressure spikes, eventually reaching explosive conditions.
What about a candle? No one has seen a candle explode. Wax must first melt from solid to liquid, then vaporize through the wick to sustain combustion. This process limits how quickly pressure can build. Even if you burn a candle in a sealed jar, it will quickly extinguish—not explode—because the oxygen is depleted.
In a completely open space, spilled gasoline may burn, but it cannot accumulate pressure fast enough to explode. The same applies to leaked gas. Everyday experience with gas stoves supports this: they burn, but they don’t explode.
However, in a partially enclosed space—such as a closed kitchen—if gas accumulates to a certain level and is then ignited, the outcome is something you’ve likely seen depicted in movies.
That said, there’s no need to worry too much. Many people have experienced a gas stove failing to ignite a few times, only to suddenly flare up when it finally does. This shows that reaching explosive conditions is not as easy as it might seem.
This article is published by the Tellus Materials editorial team. Tellus Materials focuses on hydrogen energy and energy storage solutions, continuously advancing Taiwan’s energy transition.
