Why Is Fire Hot? How Hot Is It?

Fire is hot because thermal energy (heat) is released when chemical bonds are broken and formed during a combustion reaction. Combustion turns fuel and oxygen into carbon dioxide and water. Energy is required to start the reaction, breaking bonds in the fuel and between oxygen atoms, but much more energy is released when atoms bonds together into carbon dioxide and water.

Fuel + Oxygen + Energy → Carbon Dioxide + Water + More Energy

Both light and heat are released as energy. Flames are visible evidence of this energy. Flames consist mostly of hot gases. Embers glow because the matter is hot enough to emit incandescent light (much like a stove burner), while flames emit light from ionized gases (like a fluorescent bulb). Firelight is a visible indication of the combustion reaction, but thermal energy (heat) may be invisible, too.

Why Fire Is Hot

In a nutshell: Fire is hot because the energy stored in fuel is released suddenly. The energy required to start the chemical reaction is much less than the energy released.

How Hot Is Fire?

There is no single temperature for fire because the amount of thermal energy that is released depends on several factors, including the chemical composition of the fuel, the availability of oxygen, and the portion of the flame being measured. A wood fire may exceed 1100° Celsius (2012° Fahrenheit), but different types of wood burn at different temperatures. For example, pine produces more than twice as much heat as fir or willow and dry wood burns hotter than green wood. Propane in air burns at a comparable temperature (1980° Celsius), yet much hotter in oxygen (2820° Celsius). Other fuels such as acetylene in oxygen (3100° Celsius) burn hotter than any wood.

The color of a fire is a rough gauge of how hot it is. Deep red fire is about 600-800° Celsius (1112-1800° Fahrenheit), orange-yellow is around 1100° Celsius (2012° Fahrenheit), and a white flame is hotter still, ranging from 1300-1500 Celsius (2400-2700° Fahrenheit). A blue flame is the hottest one of all, ranging from 1400-1650° Celsius (2600-3000° Fahrenheit). The blue gas flame of a Bunsen burner is much hotter than the yellow flame from a wax candle!

Hottest Part of a Flame

The hottest part of a flame is the point of maximum combustion, which is the blue portion of a flame (if the flame burns that hot). However, most students performing science experiments are told to use the top of the flame. Why? Because heat rises, so the top of the flame's cone is a good collection point for the energy. Also, the cone of the flame has a fairly consistent temperature. Another way to gauge the region of most heat is to look for the brightest portion of a flame.

Fun Fact: Hottest and Coolest Flames

The hottest flame ever produced was at 4990° Celsius. This fire was formed using dicyanoacetylene as fuel and ozone as the oxidizer. Cool fire may also be made. For example, a flame around 120° Celsius may be formed using a regulated air-fuel mixture. However, since a cool flame is barely over the boiling point of water, this type of fire is difficult to maintain and goes out readily.

Fun Fire Projects

Learn more about fire and flames by performing interesting science projects. For example, learn how metal salts affect flame color by making green fire. Up for a truly exciting project? Give firebreathing a try.

Source

• Schmidt-Rohr, K (2015). "Why Combustions Are Always Exothermic, Yielding About 418 kJ per Mole of O₂". J. Chem. Educ. 92 (12): 2094–99. doi:10.1021/acs.jchemed.5b00333