Gunpowder is a mixture of potassium nitrate (KNO₃), sulfur (S), and charcoal (C). The substance that makes the gunpowder explode is the KNO₃. The nitrate (NO₃) unit is a molecule made out of nitrogen and oxygen and the atoms in this molecule are unhappy in their state. They would rather be pure nitrogen (N₂) and pure oxygen (O₂). However, there is an energetic barrier that prevents them from achieving their preferred state.

When setting the gunpowder on fire, the heat from the fire supplies the energy needed to overcome this barrier and the NO₃ molecule can release its nitrogen and oxygen. The nitrogen likes to stay alone and becomes (N₂), the oxygen further reacts with the carbon (C) and forms CO₂. The potassium from the KNO₃ reacts with the sulfur (S) to K₂S.

The chemical reactions is therefore:
2 KNO₃ + S + 3 C -> K₂S + N₂ + 3 CO₂

This means that the gunpowder is very different after being set on fire. Since N₂ and CO₂ are gases, they leave the powder with the reaction and the leftover powder will be only K₂S. The release of the gases is also what makes the bullet of the gun being shot out. The volume of the gases is way bigger than the volume of the powder. In a restricted space, like the barrel of the weapon, this gas quickly wants to go out and therefore has to push the bullet out too.

Gunpowder is commonly made of saltpeter (potassium nitrate), sulfur, and charcoal, which are all solids.

Burning these chemical substances changes them from solids into two other solids and two gases carbon dioxide and nitrogen. One of them, potassium sulfate, is commonly used in fertilizer; the other one, potassium carbonate, is used in making soap and glass, but is also used in making some foods (it should be noted here that not all things with names that sound "chemical" are bad for the human body). Of course, if the burning is not complete, then there could be leftover saltpeter, sulfur, or charcoal.

Gunpowder is a mixture of charcoal (mostly carbon), sulfur, and potassium nitrate. When ignited, the oxygen in the nitrate combines with the carbon in the charcoal to form carbon dioxide, the nitrogen is released as nitrogen gas, and the potassium combines with the sulfur to become potassium sulfide. The carbon dioxide and nitrogen are gasses, which means that they take up much more volume than the gunpowder itself did, and this expansion in volume is what drives gunpowder to explode in a confined space such as the wrapped-up paper interior of a firecracker or the firing chamber of a gun.

The gunpowder is very different. Fuel + oxygen makes fires and explosions. The gunpowder has fuel, and the air has oxygen that makes the gunpowder explode.

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