Science questions in short. [2 marks ]

(i) Combustible - charcoal, straw, cardboard, paper, candle, wood.
(ii) Non combustible - chalk, stone, iron rod, copper coin, glass

Fuel efficiency is determined by its calorific value which is the amount of heat energy produced on complete combustion of 1 kg of a fuel. The calorific value of a fuel is expressed in kJ/kg.

We will prefer CNG (Compressed Natural Gas) because the calorific value of CNG is higher than that of petrol. Therefore, CNG will be more economical. CNG will produce large amount of heat energy than petrol. At the same time, it produces the least air pollutants.

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When a cracker is ignited, a sudden reaction takes place with evolution of heat. Light and sound and a large amount of gas liberated. Such a reaction is called explosion.
So, cracker on ignition produces because of sudden formation of large amount of gas due to chemical reaction.

A matchstick does not catch fire on its own at room temperature because the ignition temperature of matchstick is much higher than the room temperature. The matchstick starts burning on rubbing it on the rough side provided on the matchbox. It is because the heat produced by friction heats the chemical at the head of the matchstick to its ignition temperature and makes it burn.

A matchstick can burn a splinter of wood directly because it has low ignition temperature, whereas a matchstick cannot burn a log of wood directly as it has high ignition temperature.
In order to burn a log of wood, a small fire is first started by burning straw (or dry grass), or paper by a matchstick, then the log of wood is placed over this fire. This fire heats the log of wood to its ignition temperature due to which it starts burning.

The head of the safety match contains antimony trisulphide and potassium chlorate. The rubbing surface has powdered glass and a little red phosphorus. When the match is struck against the rubbing surface, some red phosphorus gets converted into white phosphorus This immediately reacts with potassium chlorate in the matchstick head to produce enough heat to ignite antimony trisulphide and start combustion.

When the clothes of a person catch fire, the person is covered with a blanket to extinguish the fire This is because the supply of air to the burning clothes is cut off by blanket, due to which the clothes stop burning and the fire gets extinguished.

Necessary Conditions for Combustion
There are three necessary conditions for combustion. These are as follows
1. Presence of a Combustible Substance
Presence of substance like petrol, LPG, wood, coal, etc. is necessary for combustion process.
2. Presence of a Supporter of Combustion
A substance which helps in combustion is called a supporter of combustion, e.g. oxygen (or air).
For example, when the clothes of a person catch fire, the person is covered with a blanket to extinguish the fire. The blanket cuts off the supply of air which is needed for the burning of clothes. This shows that air is necessary for combustion.
3. Ignition Temperature
The lowest temperature at which a substance catches fire and starts burning is called its ignition temperature.
Before a substance catches fire and starts burn, it must be heated to a certain minimum temperature by supplying heat. A combustible substance cannot catch fire if its temperature is lower than its ignition temperature. 
For example, a matchstick does not catch fire on its own at room temperature. When matchstick rubbed on the rough side of matchbox the heat produced increases its temperature to ignition temperature and its start burning.

Combustible and Non-Combustible Substances
All substances around us do not burn. Based on this, substances are divided into two types
(i) Combustible Substances The substances which burn in the presence of air or oxygen are called combustible substances, e.g. petrol, LPG, kerosene, etc.
(ii) Non-combustible Substances The substances which do not burn in the presence of air or oxygen are called non-combustible substances, e.g. water, glass, sand, etc.

It is because the ignition temperature of paper is less, so it catches fire easily. It does not catch fire, when wrapped around aluminium pipe because aluminium absorbs the heat by conduction, so paper does not attain its ignition temperature.

LPG is a better domestic fuel than wood because LPG has more calorfic value and produces no pollution while wood has less calorific value and produces smoke and causes more pollution.

Water is not used to control fires involving electrical equipments because water conducts electricity. So, it can causes danger electric shocks.

The process of rusting cannot be called combustion because in this process, no heat and light is produced. Due to this reason, iron is not considered as combustible substance.

A goldsmith uses the outer zone or non-luminous zone of a flame for melting gold and silver because it is the hottest zone of a flame and has more temperature.

It is difficult to burn a heap of green leaves because the green leaves contain some water, due to which the ignition temperature of leaves increases and they do not catch fire easily, while dry leaves have no water, so they catch fire easily.

Carbon dioxide (CO₂) is the best fire extinguisher. Being heavier than oxygen, it covers the fire like a blanket. Since, the contact between the fuel and oxygen is cut-off and hence, the fire is controlled. It also brings down the temperature of the fuel.

CNG has replaced diesel and petrol as fuel in automobiles because petrol and diesel produced a lot of unburnt carbon particles and emitted carbon dioxide, nitrogen oxide and sulphur dioxide. These all gases are poisonous gases and cause various environmental hazards. The main causes of acid rain are nitrogen oxides and sulphur dioxide. But CNG is safe because it produces these harmful substances in very small amounts. Thus, the use of CNG is reducing pollution in our cities.

Necessary conditions for combustion are as follows
(i) Presence of combustible substance.
(ii) Proper supply of air to provide oxygen.
(iii) Attainment of ignition temperature.