Science questions in short. [3 marks ]

It is difficult to burn a heap of green leaves but dry leaves catch fire easily. The green leaves contain a lot of water. This water does not allow the green leaves to get heated to their ignition temperature and makes burning of green leaves difficult. Dry leaves do not contain water, they get heated to their ignition temperature easily and hence catches fire easily.

Fuel: There must be something to burn.
Air: There must be a continuous supply of oxygen to support the process of combustion.
Heat: The temperature of the fuel must be raised above the ignition temperature to start the process of combustion.

Carbon dioxide is the best fire extinguisher to put out fire caused by inflammable materials like oil and petrol and electrical equipment’s. Carbon dioxide is heavier than air and it covers the fire and cuts off the supply of oxygen and puts out the fire. Carbon dioxide is stored at high pressure as liquid in cylinders. When released from the cylinder, carbon dioxide expands enormously in volume and cools down. Carbon dioxide not only forms a blanket around the burning substance, it also cools down the burning substance.

Solid Fuels: Wood, charcoal, coal, coke, cow-dung cakes.
Liquid Fuels: Kerosene, petrol, diesel, alcohol.
Gaseous fuels: Coal gas, natural gas.

For the combustion reaction to be successful a continuous supply of oxygen is required. Normally this demand of oxygen is met with normal air supply. Hence, if we cut off the supply of air for combustion of a substance then the combustion reaction will stop and the substance will stop burning.

The yellow part of the flame is the middlemost part where Abida has kept the beaker. The fuel vapours burn partially in the middle zone because there is not enough air of burning in this zone. The partial burning of fuel in the middle zone produces carbon particles. It has moderate temperature. Hence, Abida’s beaker will take time to heat up. Ramesh’s beaker is kept in the outermost part of the flame. In this zone, complete combustion of the fuel takes place because there is plenty of air around it. The outermost zone has the highest temperature in the flame. It is the hottest part of the flame. Hence Ramesh’s beaker gets heated in a shorter time.

Water extinguishes heat by cooling down the burning substance such as wood and paper. When water is thrown on these substances, it gets cooled below its ignition temperature and also stops burning. The water vapour produced by the action of heat of fire on water surround the burning material and help in cutting off the supply of air.

Burning of coal, petrol and diesel produces Sulphur dioxide which goes into the air. It is an extremely suffocating and corrosive gas. It may damage our lungs. The burning of petrol and diesel in the engines of vehicles also releases nitrogen oxides into the air. These oxides produced by the burning of fuels dissolve in rain water and form acids. The rain water containing acid is called acid rain. It is harmful for crops, soil and damages buildings.

Luminous flame	Non-luminous flame
It consists of unburnt carbon particles.	It does not consist of unburnt carbon particles and is the zone of complete combution.
It is the least hot part of the flame.	It is the hottest part of the flame.

Carbon dioxide is the best fire extinguisher to put out fire caused by inflammable materials like oil and petrol and electrical equipment’s. Carbon dioxide is heavier than air and it covers the fire and cuts off the supply of oxygen and puts out the fire. Carbon dioxide is stored at high pressure as liquid in cylinders.

A piece of paper wrapped around aluminium pipe does not catch fire easily. This is because aluminium, being a metal, is a good conductor of heat and hence heat is conducted away. Also, the ignition temperature of paper is lower compared to that of aluminium. When paper is wrapped around an aluminium pipe; the ignition temperature increases. That is why paper itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not.

Burning of woods produces smokes and a lot of particulate pollutants. It also produces unburnt carbon particles. These fine particles are dangerous pollutants causing respiratory diseases.
Incomplete combustion of theses fuels gives carbon monoxide gas which is a poisonous gas. Moreover they produce carbon dioxide upon combustion which causes global warming.

When coal burns in air, it produces carboh dioxide which is non-poisonous. But, when angithi or coal burns in a closed room, the air or oxygen cannot enter the room. So, in the incomplete supply of oxygen, coal burns to produce carbon monoxide which is highly poisonous gas. It causes suffocation in a closed room. So, door and windows should be open.

Anu should keep her test tube in the outermost zone or non-luminous zone of the flame because it is the hottest zone of a flame and has more temperature.In the outer zone of a flame, complete combustion of the fuel takes place because there is plenty of air around it so, water in a test tube will boil quickly in this zone.

When a cracker is ignited, a sudden reaction (very rapid) takes place with the evolution of heat, light and sound and a large amount of gas is liberated. Such a reaction is called explosion. The gases produced are heated by the heat evolved in the reaction. The hot gases expand rapidly and cause an explosion (producing a loud sound).

The burning of fuels such as coal, petrol and diesel releases sulphur dioxide and gaseous oxides of nitrogen into the air. These gases are extremely suffocating and corrosive. When these oxides of sulphur and nitrogen dissolve in rainwater they form acids. Such a rain is called an acid rain. The harmful effects of acid rain are that it is very harmful to crops, buildings and soil and leads to extensive loss of property.

When fire brigade arrives to the rescue, the fire man throws a strong stream of water on the building on fire, the burning material get cooled down to below their ignition temperature and fire is extinguished. The water vapour produced by the action of heat of fire on water surround the burning material and help in cutting off the supply of air. Fire-men extinguishes the fire by throwing water under pressure on the burning things.