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March 23, 2026

Chemistry of Fire – Copy

Let's Continue with our previous part of Chemistry of Fire

Chemistry of Fire-2

Following the first part, we discussed heat and temperature. Now, let’s continue on the same topic.
  • Melting; Boiling & vaporisation
  1. The melting point is the temperature at which a solid starts to change into a liquid.
  2. The freezing point is the temperature at which a liquid changes state from liquid to solid.
  3. The boiling point is the temperature at which a liquid starts to form vapour and boil.
 
Evaporation is the process in which, below the boiling point, some molecules on the surface of a liquid gain enough energy to turn into a vapour and escape.
Measuring temperature shows how heat affects substances. Thermometers work by using thermal expansion.
A thermometer is a thin glass tube, closed at one end and equipped with a small bulb at the other. This bulb is filled with mercury. Mercury’s boiling point is 100 degrees Celsius, and its freezing point is 0 degrees Celsius.
 
 
Thermometer Scale: The scale is based on the midpoint between two fixed points. Temperature is measured in Celsius. Atmospheric pressure is based on the melting point of ice to the boiling point of water. Generally, two thermometer scales are used.
  1. Celsius or centigrade scale
    1. In this type of scale, the lower fixed point is ‘zero’ & the upper fixed point is 100. The space between these two points is divided into 100 equal points. These divisions are called degrees Celsius.
  3. Fahrenheit Scale
    1. This scale sets the freezing point at 32°F and the boiling point of water at 112°F, divided into 180 equal Fahrenheit degrees.
  5. Kelvin Scale
    1. Another scale is the thermodynamic, or Kelvin, scale. Its unit is K, and it measures thermodynamic temperature, down to -273°C. Named for British scientist Baron Kelvin, it is widely used in science. Zero on this absolute scale is -273°C, with other degrees expressed as Celsius. Symbol: K.
 
 
Unit of Heat: The joule is the unit that measures the amount of heat energy in a substance, similar to how length is measured in meters and temperature in degrees. Joules measure heat energy.
 
Joules
Scientists and engineers use the same unit of heat as for other forms of energy, the JouScientists use the joule as the unit of heat. Sir James P. Joule showed that raising the temperature of water requires work, which transfers heat energy. Both work and energy transfer change internal mechanical energy into other forms. Joule’s experiments proved the heat-work relationship, which was later verified by others. For example, heating water in a test tube creates enough steam energy to pop a cork. Internal energy converts to mechanical energy. With one Joule of work, by applying a 1 Newton (N) force, a distance of one meter is covered in the direction of the force. A joule is used to measure heat energy and work done by it. The units of joules are measured in kilojoules and megajoules.
1 Kilo Joule = 1000 Joules
Calories: A calorie is the amount of heat needed to raise the temperature of one gram of water by one degree Celsius.
We know that heat energy always flows from higher temperature objects to lower temperature objects, and this flow of heat energy continues until the temperature of both objects becomes equal, that is, the colder object absorbs the same amount of heat as the hotter object. When heat is measured in Joules, applying heat to an object increases its temperature.
The temperature of an object depends on three major factors.
  1. The amount of heat energy supplied to the objects
  2. The mass of the object
  3. On the specific health capacity of the object
The amount of heat required to raise the temperature of 1 kg of substance by 1 °C is called its specific heat capacity. This is specific to each substance. Therefore, it is expressed in Joules per Kilogram per degree Celsius. The higher a substance’s specific heat capacity, the more energy is required to heat it or raise its temperature. On the other hand, if a substance has a low specific heat capacity, it will ignite more quickly than a substance with a higher specific heat capacity.
Water has an extraordinary specific heat capacity, approximately 4200 Joules/kg per degree Celsius; very few substances have a higher specific heat capacity. This is one of the main reasons water is considered an excellent fire extinguisher: it can absorb a large amount of heat.
Some substances, such as petrol, alcohol, diesel and many others, have very low specific heat capacities and can quickly vaporise, becoming dangerously flammable. A combustible substance with a very low specific heat capacity has a greater potential to fuel a fire.

Keep an image of the solid-to-liquid-to-gas-phase triangle.

 
Change of State: –
Here, a change of state means changing from a solid to a liquid, from a liquid to a gas, etc. Freezing, melting, boiling, liquefaction, etc., all the state changes are with each other.
Latent Heat of Vaporisation: –
When a kettle full of water is placed on a fire to boil, heat from the fire passes through the kettle’s metal into the water, and the water reaches 100 C. At this temperature, the water begins to boil, forming vapour bubbles, some at the bottom and some at the top. They burst out from the surface, and some of them fly away. Once the water starts boiling, the temperature stabilises at 100 C. This temperature will not exceed 100 C, but the water’s heat energy continues to pass through the kettle’s metal. This heat energy will no longer increase the water’s temperature; instead, it will continuously be used to convert water molecules into vapour. This heat energy is used to convert liquid water to water vapour.
Latent heat of vaporisation is the amount of heat required to turn 1 kilogram of water into vapour at its boiling point, without changing temperature. For water, this is 226,000 Joules per kilogram.
When measuring liquid water, heat is transferred to the measuring device as latent heat, which is why measuring devices can cause serious burns.
Latent heat is measured in joules per kilogram or kilojoules per kilogram. Effect of varying pressure on boiling point and latent heat. Water normally boils at 100 C, while atmospheric pressure is equal to 1.013 Bar.
If atmospheric pressure increases, the boiling point of a substance increases; if the pressure decreases, the boiling point decreases. This effect is exploited in pressure workers and pressurised cooling. Systems of car engines, where increased pressure increases the boiling point of the liquid in the system.
This effect is also used to store gases like propane and butane. When pressure is increased, these gases liquefy, allowing larger quantities to be stored in smaller spaces.

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