Melting is an example of a physical change . A physical change is a change to a sample of matter in which some properties of the material change, but the identity of the matter does not. Physical changes can further be classified as reversible or irreversible.

Breaking and making bonds Bond-breaking is an endothermic process. Energy is released when new bonds form. Bond-making is an exothermic process. Whether a reaction is endothermic or exothermic depends on the difference between the energy needed to break bonds and the energy released when new bonds form.

Bond formation releases energy (and similarly bond breakage is endothermic), and thus neutralisation reactions are exothermic.

Burning is an example of a chemical change. Signs of chemical change include the release of bubbles, a change of color, production of an odor, release of heat and light, and production of loud sounds. Because chemical changes result in different substances, they often cannot be undone.

In principle, all chemical reactions are reversible reactions . This means that the products can be changed back into the original reactants .

A burning candle or other fire is an example of a chemical chain reaction. When a flammable material is surrounded by enough oxygen and is exposed to a source of heat or temperature above the flash point, a fire starts.

Classes of fire

• Class A – fires involving solid materials such as wood, paper or textiles.
• Class B – fires involving flammable liquids such as petrol, diesel or oils.
• Class C – fires involving gases.
• Class D – fires involving metals.
• Class E – fires involving live electrical apparatus. (

Fire emits heat and light because the chemical reaction that produces flames is exothermic. In other words, combustion releases more energy than is needed to ignite or sustain it. In order for combustion to occur and flames to form, three things must be present: fuel, oxygen, and energy (usually in the form of heat).

Oxygen, heat, and fuel are frequently referred to as the “fire triangle.” Add in the fourth element, the chemical reaction, and you actually have a fire “tetrahedron.” The important thing to remember is: take any of these four things away, and you will not have a fire or the fire will be extinguished.

When it comes to fire safety, the number one rule is that everyone goes home. Ensuring this starts the moment you drive up on scene with a good scene size up. Assessing the structure, looking for hazards, and calculating the best means of fire attack are some of the first steps to ensuring scene and firefighter safety.

A tetrahedron can be described as a pyramid which is a solid having four plane faces. Essentially all four elements must be present for fire to occur, fuel, heat, oxygen, and a chemical chain reaction. Each of the four sides of the fire tetrahedron symbolise the Fuel, Heat, Oxygen and Chemical Chain Reaction.

A class C fire is one in which an energized electrical element is the cause of the fire. “Energized” means that the electrical component (whether electrical appliance, wiring, device, etc.) is connected to a power source.

There are 6 different classes of fire, and each should be attacked in a different way.

• Class A (Solids) Class A fires are fires involving solids.
• Class B (Liquids) Class B fires are fires involving liquids.
• Class C (Gases)
• Class D (Metals)
• Electrical Fires.
• Class F (Cooking Fats & Oils)

Dry Chemical Extinguishers

Two commonly used chemicals are effective in fighting these types of fires. Monoammonium phosphate effectively smothers the fire, while sodium bicarbonate induces a chemical reaction which extinguishes the fire. Fire extinguishers with a Class C rating are suitable for fires in “live” electrical equipment.

What is a Class D fire? A Class D fire is characterised by the presence of burning metals. Only certain metals are flammable and examples of combustible metals include sodium, potassium, uranium, lithium, plutonium and calcium, with the most common Class D fires involve magnesium and titanium.

Fire Extinguishers: Your First Line of Defense Against Fires.

Before discharging the extinguisher, move toward or away from the fire so you’re standing 6 to 8 feet (1.8 to 2.4 m) (1.8 to 2.4 m) away. You can move closer once you start dousing the fire and the flames die down.

R.A.C.E: An acronym that hospital personnel use to remember their duties in case of fire. It stands for RESCUE, ALARM, CONFINE, EXTINGUISH/EVACUATE.

The US National Fire Prevention Association recommends that you install fire extinguishers close to an exit point, to enable you to keep your back to the exit when you use the extinguisher; this ensures that you can make an easy escape if the fire cannot be controlled.

ABC Fire Extinguishers are very versatile. They are often the ideal choice being that they are able to put out many different types of fires. They use monoammonium phosphate which is a dry chemical that is able to quickly put out the fire.

ABC Fire Extinguishers use monoammonium phosphate, a dry chemical with the ability to quickly put out many different types of fires by smothering the flames. This pale, yellow powder can put out all three classes of fire: Class A is for trash, wood, and paper. Class B is for liquids and gases.

Carbon dioxide fire extinguishers extinguish fire by taking away the oxygen element of the fire triangle, and by removing the heat with a very cold discharge. Carbon dioxide extinguishers can be used on Class B and C fires. They are usually ineffective on Class A fires.

Powder extinguishers Instead it ‘knocks out’ the fire and forms a blanket or crust over the fire to starve the fire of oxygen. They are a more powerful extinguisher than their foam counterparts and the physical size can therefore be smaller than a foam extinguisher for the same application.

All fires can be extinguished by cooling, smothering, starving or by interrupting the combustion process to extinguish the fire. One of the most common methods of extinguishing a fire is by cooling with water.