Propane torch
A propane torch is a tool
A tool is a device that can be used to produce an item or achieve a task, but that is not consumed in the process. Informally the word is also used to describe a procedure or process with a specific purpose. Tools that are used in particular fields or activities may have different designations such...

 for burning the flammable gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...

Propane is a three-carbon alkane with the molecular formula , normally a gas, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used as a fuel for engines, oxy-gas torches, barbecues, portable stoves, and residential central...

 (C3H8). The maximum adiabatic flame temperature a propane torch can achieve with air is 2,268 kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

s (1,995 °C
Celsius is a scale and unit of measurement for temperature. It is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...

/3,623 °F
Fahrenheit is the temperature scale proposed in 1724 by, and named after, the German physicist Daniel Gabriel Fahrenheit . Within this scale, the freezing of water into ice is defined at 32 degrees, while the boiling point of water is defined to be 212 degrees...

). Some propane torches are also used with a tank of pure oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 to achieve a flame temperature nearing 3,095 kelvins (2,820 °C/5,110 °F).

The temperature in these flames is lower because of incomplete combustion, heat loss in combustion, oxygen quantity etc.
Propane torches are frequently employed to solder
Solder is a fusible metal alloy used to join together metal workpieces and having a melting point below that of the workpiece.Soft solder is what is most often thought of when solder or soldering are mentioned and it typically has a melting range of . It is commonly used in electronics and...

Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

 water pipes. It can also be used for some low temperature welding
Welding is a fabrication or sculptural process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material that cools to become a strong joint, with pressure sometimes...

 applications, as well as for brazing
Brazing is a metal-joining process whereby a filler metal is heated above and distributed between two or more close-fitting parts by capillary action. The filler metal is brought slightly above its melting temperature while protected by a suitable atmosphere, usually a flux...

dissimilar metals together.

In addition to above, a large factor in the temperature of the flame is the percent of oxygen mixing with the propane. With air/fuel torches, since air contains about 21% oxygen, to obtain the maximum flame temperature with air, you must use a very large ratio of air/fuel. Even these glass beadmaking torches, which are essentially bunsen burners with an added air pump can only achieve temperatures of 2012 °F (1,100 °C).

With oxygen/propane torches, the air/fuel ratio is much lower. The stoichiometric equation for full combustion of propane with 100% oxygen is:

C3H8 + O2 → H2O + CO2

C3H8 + 5 (O2) → 4 (H2O) + 3 (CO2)

As you can see the only products are CO2 and water. This is only true with complete combustion.

To get the ratio, first find the atomic mass of C, H, and O.
  • O = 16 g
  • C = 12.01 g
  • H = 1.008 g

  • Propane weighs ((3×12.01) + (8×1.008)) = 44.094 g/mol.
  • Oxygen gas weighs (2×16) = 32 g/mol.

The balanced equation shows to use 1 mol of propane for every 5 mol of oxygen. / (44.094 g) = 160/44.094 = 3.63 grams oxygen gas for each gram of propane,

If using an oxygen/propane torch, the oxygen/fuel ratio is 3.63 by mass, 5:1 molar ratio.

For an air/propane torch: 3.63 g O2 × (100 O2)/(21% O2) = 17.28 g air/propane by mass,
23.81 mol O2 / 1 mol propane = 23.81:1 molar air/fuel ratio.

This means that it is much harder to achieve complete combustion with air than with oxygen.

If the propane does not receive enough oxygen, some of the carbon from the propane are left unburned:
Complete combustion:
C3H8 + 5 (O2) → 4 (H2O) + 3 (CO2)

Examples of incomplete combustion:
C3H8 + 4 (O2) → 4 (H2O) + 2 (CO2) + 1 C

The extra carbon mole will cause soot and the less oxygen used the more soot you will get. There are other unbalanced ratios where incomplete combustion products such as Carbon Monoxide (CO) are formed, such as:
6 (C3H8) + 29 (O2) → 24 (H2O) + 16 (CO2) + 2 CO
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