Combustible vs Flammable – What is the Difference?

Combustible vs Flammable

Let us understand the meaning of combustible and flammable in simple language.

Combustible and flammable substances are the materials that can burn in the air. They can be solid as well as liquid.

Then, what is the basic difference between them?

The combustible and flammable materials can be distinguished on the basis of their flashpoint. Now, what is a flashpoint?


Flashpoint and Ignition Temperature

flash point

Flashpoint is the minimum temperature, which is required for burning/igniting vapors of the material by supplying an ignition source.

Examples of ignition sources are sparks, open flames, static electricity, and hot surfaces. Here, volatile materials are considered for defining flashpoint.

The volatility of the material depends upon its vapor pressure. The higher the vapor pressure of the materials, the more it will be volatile.

Flashpoint can be related to the volatility of the materials. The higher the volatility of the substance, it will have lower the flashpoint.

For example, The flashpoint temperatures of the petrol, diesel, kerosene, and vegetable oil are 21 °C, 52 °C, 38-72 °C, and 327 °C, respectively. It means that if we provide an ignition source to these materials, they will catch fire at their respective flashpoint, not before that.

But we should not get confused the flashpoint with (auto)ignition temperature.

(Auto)Ignition temperature is the lowest temperature at which material will catch fire automatically without an ignition source.

For example, The (auto)ignition temperature of the petrol, diesel, kerosene, and vegetable oil are 280 °C, 210 °C, 220 °C, and 424 °C, respectively. It means that these materials will catch fire automatically without an ignition source at their respective autoignition temperature.

Now, let us understand the real definition of Combustible and Flammable material.


Definition of Combustible and Flammable

Combustible materials are materials that can combust in the air with an ignition source with some effort. The combustible liquids have flashpoints in the range of 60-93 °C. It means they are less volatile and hence, they will not burn easily at working temperature i.e., 25- 35 °C.

Flammable materials are those combustible materials, which can burn in the air easily by providing an ignition source. It means that they have lower flashpoint temperature as compared to the Combustible materials.

The flashpoint temperature of the flammable liquid varies between 0 to 60 °C. They are more volatile than combustible substances. They are also known as inflammable substances.

Flammable liquid can catch fire easily at working/room temperature and hence, they are handled carefully while working.

This particular flashpoint range has been defined by the Globally Harmonized System of Classification and Labeling of Chemicals, which is created by the United Nations to provide the same range of flashpoints for the definition of combustible and flammable across different countries.


Examples of Combustible Materials

Cyclohexane, kerosene, formic acid, naphthalene, dichlorobenzene, oils, and hydrazine are some of the examples of combustible liquids.

The flashpoint and fire points of combustible liquids are smaller than their boiling points. Examples of combustible materials include paper and wood.



Examples of Flammable Materials

Flammable liquids are pentane, hexane, ethanol, petrol (gasoline), ethyl acetate, nitromethane, acetone, methanol, and isopropyl alcohol.

Some examples of flammable solids are metallic sodium, oily fabrics, matches, and nitrocellulose products.



Nonflammable Materials

These materials will not catch fire or burn in the air with the supply of an ignition source. Water, diesel, and carbon tetrachloride are some examples of nonflammable liquids.


Classification of flammable Materials

The flammable materials are classified and rated from 0 to 4 depending on their degree of flammability, which can be determined by conducting a fire test. Some of the well-known illustrations of fire tests are Underwriters Laboratories UL 94 Flammability Testing, National Fire Protection Association (NFPA) i.e., NFPA 287, NFPA 701, NFPA 850.

Classification of flammable materials is given below in the table with examples.

Rating  Degree of Flammability  Examples
Materials do not burn  Water
Materials must be preheated before ignition  Cooking oils,  lubricating oils
Materials must be preheated either at moderate  temperature or at high temperature before ignition Diesel Fuel
Materials ignite at all temperatures  Petrol (gasoline),  ethanol
Materials vaporize at standard room temperature  and atmospheric pressure Propane, butane,  natural gas


Factors affecting the degree of Flammability or Combustibility of a Material

The degree of flammability is affected by the volatility of the material, which in turn depends upon the vapor pressure.

Vapour pressure is further dependent on the temperature. The degree of flammability also depends upon the surface area of the material.

Let us understand the effect of these interdependent factors on the degree of flammability in detail.

1. Volatility: More volatile the substance, the larger the degree of flammability of material i.e., the more volatile substance will burn easily as compared to the less volatile compound.

2 Vapour Pressure: Vapour pressure is the pressure exerted by the vapor of liquid on the surface of the liquid at equilibrium i.e, the rate of condensation of vapors is equal to the rate of evaporation of liquid.

For example: If the substance has high vapor pressure, it will be more volatile and hence, it will combust in the air easily.

3 Temperature: Vapour pressure of the liquid increases with an increase in temperature owing to an increase in the kinetic energy of the liquid molecules, which increases the number of liquid molecules going to the vapor phase.

4 Surface area: The degree of flammability can be increased by increasing the surface area of material i.e., the use of finely divided material.

The increased surface area of the material will enhance the number of vapors of the material and hence, the substance can burn easily in the air.



The Dangers related to Combustible and Flammable Materials

As we get to know that the flashpoint temperature of combustible materials is greater than flammable materials. But it does not mean that combustible materials are less dangerous than flammable materials.

Both combustible and flammable materials are used, transported, and stored at a temperature greater than their respective flashpoints. Extreme precautions are needed for handling these materials to prevent ignition.

There should be proper ventilation in the room otherwise vapors of flammable materials can catch fire easily at working temperature.

If the temperature of the environment is above the flashpoints of combustible materials, then they can release flammable vapors and hence, catch fire.

In brief, combustible and flammable materials can cause danger to the life of people and damage the materials used for transportation and storage. It also leads to pollution in the environment.

Therefore, it is very necessary to store, transport, and handle these chemicals very carefully with extreme precautions.

What are the precautions for storing these materials?

Let us have a look at these precautions.


Precautions while storing Combustible and Flammable Materials

The handling and storage of combustible and flammable materials require a proper procedure, which has been outlined by Australian standard i.e., AS1940-2017.

We, as common man, need to remember some points while storing combustible and flammable liquids. These points are

1 There should be proper ventilation in the room, where these chemicals are stored.

2 These materials should be completely separated from any ignition source.

3 The flammable and combustible chemicals should be segregated from incompatible chemicals so that they don’t react with them.

If flammable or combustible liquids mix with incompatible chemicals, there are chances of dangerous chemical reactions between them such as combustion and oxidation, etc.

Hence, it is necessary to segregate the incompatible class of dangerous chemicals.

For that, there is a particular separation distance between incompatible chemicals, which is determined by the dangerous goods separation chart.


Dangerous Goods Separation Chart

To avoid violent chemical reactions of incompatible chemicals with flammable or combustible chemicals, it is important to separate them

As violent chemical reactions can cause fire or maybe explosion, which in turn will release harmful gases into the environment. These harmful gases will affect the health of people as well pollute the environment.

The dangerous goods separation chart identifies the chemicals, which must be isolated and stored in separate cabinets.

It also identifies the chemicals, which must be segregated either by at least 3 m or 5 m depending on their reactivity.



In a nutshell, we have explained the difference between flammable and combustible materials along with their examples. The flammable materials can be classified into five categories depending on their degree of flammability.

The degree of flammability depends on the volatility and surface area of the material, temperature, and vapor pressure of the liquid.

Thank you for reading.

Happy learning.

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