Fire is hot because when it burns, it releases energy in the form of heat. This combustion process generates heat that spreads into the environment, giving off the sensation of warmth when approaching it.
Fire appears when combustion occurs: it is a chemical reaction that rapidly releases a lot of energy. Generally, this happens when a carbon-rich fuel, such as wood or coal, reacts with the oxygen present in the air. Important atoms, particularly carbon and hydrogen, then combine with oxygen, primarily forming carbon dioxide (CO₂) and water vapor (H₂O). During this process, the energy stored in the molecules is released in the form of heat and light. The faster and more intense this reaction is, the more impressive the heat released will be, creating the hot and bright phenomenon known as fire.
When a fuel (like wood or gas) burns, it reacts with oxygen. This chemical reaction breaks the bonds of the fuel molecules and releases a lot of stored energy, mainly in the form of heat. The atoms then recombine to form new molecules (carbon dioxide, water vapor), which are more stable but contain significantly less energy. The difference is released as intense heat into the environment, which we feel as the warmth of the fire.
When a material burns, it produces hot gases resulting from combustion. These gases are primarily made up of carbon dioxide, water vapor, and sometimes other substances depending on what is burning. Since they are formed at very high temperatures, they occupy much more space and become extremely light. It is because these hot gases rise quickly that we see the flames stretching upward. As they rise, they carry a large amount of heat that spreads all around. This phenomenon is what makes it possible to feel heat on the skin immediately, even at a certain distance from a fire.
When a fire burns, it does not keep the heat to itself: it loses it to its immediate environment in several ways. First by conduction, which is when objects in contact with the flames or embers heat up directly. If you slide a spoon into your bowl of hot soup, it eventually heats up too; it’s exactly the same with a branch in a fire. Next, hot gases naturally rise; this is convection. Hot air rises, making room for cold air that is then heated in turn. This movement explains why, when you stand close to a fire, you feel a wave of heat on your face. Finally, there is radiation, which sends thermal waves in all directions, even crossing through cold air to warm you from a distance. You can feel this radiant heat even if the surrounding air remains cool. These three processes—conduction, convection, and radiation—work together to quickly and efficiently diffuse the heat of the fire all around it.
When a fire burns, a good portion of its heat is transmitted by thermal radiation. These are electromagnetic waves, primarily infrared, that we may not necessarily see but clearly feel as heat as we get closer. These radiations travel in all directions at the speed of light. So, even without directly touching the flames or the hot air, you immediately feel their effect on your skin. The hotter an object is — like the flames of a fire — the more intensely it emits this type of radiation. That’s why, in front of a fireplace or a campfire, you feel this characteristic warmth even from a distance.
The color of the fire depends on the temperature: a blue flame indicates a higher temperature than yellow or orange. For example, a blue flame can reach up to 1,500°C!
Some chemical reactions can produce cold flames, which have temperatures that are very low compared to regular flames: they can even be touched with bare hands without an immediate burn!
Astronauts living in microgravity observe that flames take on a nearly spherical shape rather than their traditional elongated form. Gravity strongly influences the convection of hot gases and the appearance of flames.
Humans have mastered the use of fire for about 400,000 years. This mastery has been a crucial step in the evolution and development of our species.
Organic substances are primarily composed of carbon and hydrogen, which readily react with oxygen to form water, carbon dioxide, and other gaseous compounds. This chemical reaction, known as combustion, releases a significant amount of energy in the form of heat and light.
Yes, combustion can occur without a visible flame. This phenomenon is often referred to as slow combustion or smoldering, as when coal or certain organic materials burn, producing heat and gas, but without emitting visible flames.
We feel the warmth before directly touching the fire thanks to thermal radiation, or infrared radiation. These radiations carry the thermal energy from the fire to the skin directly through the air, creating that sensation of heat from a distance.
The best way to extinguish an electrical fire is to use a fire extinguisher specifically designed for this type of fire, often a CO₂ (carbon dioxide) or dry powder extinguisher. It is crucial to never use water, as it is conductive and could worsen the situation or cause electrocution.
The color of fire primarily depends on the temperature and the chemical substances present. The higher the temperature, the closer the color is to white-blue. Certain chemical substances produced during combustion can color the flames yellow, orange, or even green and blue depending on their specific properties.
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