Grade 8, Science Chemistry
Define Combustion??
- A chemical process in which a substance burn in oxygen to give off heat and light (sometime may produce sometimes may not produce) is called combustion
- Ex: Burning of magnesium and charcoal
- It is not a must that light must be produced during combustion
Difference between Combustible and Non combustible substances??
Combustible substances | Non combustible substances |
Substances that burn easily in air to produce heat and light are called combustible substances | Substances that do not burn in air are called Non combustible substances |
Ex: Cloth, Wooden stick, dry leaves, Coal, Petrol, Kerosene etc. | Ex: Iron nail, Glass, Stone, Steel spoon etc |
What are the three types of combustion??
- Rapid combustion
- Spontaneous combustion
- Explosive combustion
Rapid combustion
When a combustible substance burns at rapid rate by producing large amount of heat and light in a short time it is called as rapid combustion
Ex: Burning of candle, Matchstick, LPG
Spontaneous combustion
When combustible substances burns on its own without the help of any external heat it is called as Spontaneous combustion
Ex: Forest fire, Fire in coal mine, Burning white phosphorus etc.
Explosive combustion
Combustion reaction that occurs suddenly with a large amount of heat light and sound is called Explosive combustion
Ex: Firecrackers
What are the conditions necessary for combustion??
- Presence of combustible substance
- Presence of supporter of combustion
- Attainment of ignition temperature of combustible substance
Explain in detail the conditions necessary for combustion??
Combustible substance
- The presence of combustible substance is necessary for combustion
- Paper cloth LPG petrol diesel like this any combustible substance is necessary
Supporter of combustion
- Oxygen is necessary for combustion
- No oxygen no combustion
- Combustion is a chemical reaction that takes place between combustible substance and oxygen to produce heat
Attainment of Ignition temperature:
The lowest temperature to which substance must be heated to catch fire and start burning is called ignition temperature
Inflammable substance
The substances that have very low ignition temperature and can easily catch fire with flame are called inflammable substances.
Ex: LPG, Kerosene, Paper, Fabrics, Straw etc.
Fire fighting : It means to control and extinguish the unwanted fire
Principal behind fire fighting
- Bringing down the temperature of the burning substances below the ignition temperature
- Cutting off the oxygen supply
What are the various types of fire extinguishers???
- Water
- Carbon dioxide
- Foam type
How Water act as a fire extinguisher??
- Fire service = 101
- When water is thrown on fire it cools the combustible substance by bringing low the ignition temperature and prevent the fire from spreading
- Water when heated by fire forms water vapour that surrounds the combustible substance and helps to cut off the oxygen
What are the limitations in using water as a fire extinguisher??
- Water cannot be used to extinguish the fire in electrical wiring caused by short circuit
- Water is a conductor of electricity so it can cause electric shock
- Water cannot be used to put off the fire caused by burning oil or petrol.
- Since water is heavier than oil, oil floats on water and continues to burn
Carbon dioxide as a fire extinguisher
- It is used as a fire extinguisher for fire involving electrical equipment’s, Petrol Kerosene etc.
- Carbon dioxide is heavier than oxygen
- Carbon di oxide surrounds the combustible substance as a blanket and cuts of oxygen supply and brings down the temperature
- It does not harm the electrical equipments
- Carbon di oxide fire extinguishers are found in schools, offices, godowns hospitals etc.
Foam type fire extinguisher
- Foam type can be safely used to extinguish the fire by electrical appliances and inflammable substance such as Petrol, Kerosene etc.
- It consist of a metallic cylinder with knob and nozzle tube
- Metallic cylinder filled with saturated solution of sodium bicarbonate
- Glass bottle containing Aluminium sulphate is placed inside the metallic cylinder
- When knob is strike on hard surface glass bottle brakes and Aluminium sulphate reacts with sodium bicarbonate to form carbon dioxide
- The substance called a saponin is added to sodium bicarbonate to produce a foam of carbon dioxide
List few steps to take care of fire victims??
- Bring out the fire victim from the fire place this is because breathing of carbon dioxide or smoke can cause suffocation and breathlessness
- Wrap up blanket around the person whose clothes get fire this will cut off the supply of oxygen
- Pour water over the fire victim. This will cool the body and bring down the ignition temperature and prevent blister formation.
- Immediately call the ambulance or take the fire victim to the nearby hospital.
Flame
Define Flame??
A flame is a region where combustion of the fuel takes place to produce heat and light
Why flame from cooking stove is blue in colour and candle is yellow in colour??
- In cooking stove the flame of the LPG is blue in colour.
- This is because there is plenty of oxygen available and hence complete combustion of the gas takes place.
- It is also smokeless
- Where as in candle, incomplete combustion takes place.
- Candle is made up of paraffin wax.
- This wax does not burn completely and gives out light and smoke and hence the flame is yellow in colour
How candle catches the fire to produce the flame??
- When a matchstick is lighted the heat of the matchstick not only melt the wax but also covert the wax into wax vapour
- The wax vapour reacts with oxygen to produce flame
Classification of Fuels:
- Solid fuels (wax ,camphor)
- liquid fuels ( kerosene, mustard oil) vapourise on heating and burn with flame
- Solid fuels like cold charcoal do not vaporize on heating so they burn without flame
Zones of a candle flame
- A candle is column of wax with cotton thread/wick at the centre
- Candle is made up of paraffin wax this contain high percentage of carbon
- Paraffin wax vaporizers and burns on heating
- Candle flame consists of three main zones depending upon the amount of air it receives.
- They are
- Innermost zone Middle zone and Outermost zone
Innermost zone
- It is the least hot region of the flame
- There is no oxygen in this zone so no combustion
- It appears black and contains unburnt wax vapour.
Middle zone
- There is not enough oxygen for complete burning of wax vapour.
- Partial or incomplete burning of wax vapour produces carbon particles and carbon monoxide.
- Unburnt carbon particles become hot and yellow light. This makes the middle zone luminous.
- The carbon particles leave flame as smoke and soot.
- Middle zone is moderately hot and becomes the major part of candle flame.
Outermost zone
- There is plenty of air around it.
- So complete combustion of wax vapour takes place to produce carbon dioxide and water vapour
- It is the hottest zone of the flame.
- It is thin and non luminous zone
- At the base of wick that is Oxygen rich region where carbon monoxide burns with blue colour
- This region of the flame is known as blue zone.
How goldsmith melt gold and silver by blowing through the metallic pipe??
- Goldsmith blow the outermost zone of a flame with metallic blow pipe.
- This is done to melt gold and silver into desired shape to make jewellery
- When air is blown through blow pipe into flame it helps in the rapid combustion of unburnt fuel hence makes the flame hotter to melt gold and silver.
Fuel
- The material that are burnt to produce large amount of heat energy is known as Fuel.
- Example Wood, Coal, Kerosene, Diesel, Petrol etc.
- Fuels are used in homes vehicles and industries.
Classification of fuels
- Fuels are classified based on the physical state in which they occur
- Solid fuels liquid fuels and gaseous fuel
Solid fuels
Fuels that exist as solids at room temperature
Ex: Coke, coal, wood charcoal
Liquid fuels
Fuels that exist as liquids at room temperature.
Ex: Kerosene, Petrol, Diesel
Gaseous fuel
Fuels that exist as gas at room temperature.
Ex: LPG CNG biogas
Calorific value of fuel
- The amount of heat energy produced on complete combustion of 1 kg of fuel is called its calorific value
- It is expressed in KJ/Kg. Kilojoule per kilogram
- Ex: LPG – 55,000
- Coal – 25,000 – 33,000
- Petrol – 50,000
Characteristics of fuels:
- It should be cheap and easily available
- It should be easy to handle, safe to transport and convenient to store
- It should burn smoothly and have high calorific value
- It should not produce any harmful or toxic chemicals or fumes during burning
- It should not leave Ash after burning
- It should have proper ignition temperature. Not too low like room temperature or too high ignition temperature.
What are the harmful products released during burning of fuels??
The burning of fossil fuels like coal petroleum diesel produces various air pollutants like Carbon Monoxide, Carbon dioxide, Oxides of Sulphur and Nitrogen and Unburnt carbon particles which cause air pollution
How Carbon Monoxide is formed??
When a fuel (Hydrocarbons) burns with insufficient supply of oxygen it leads to incomplete combustion of fuel and produce carbon monoxide water and energy
2CH4 + 3O2 2CO + 4H2O + Energy
What happens when we inhale carbon monoxide??
- Carbon Monoxide is a very poisonous gas.
- When we inhale carbon monoxide it reacts with haemoglobin to form carboxy haemoglobin.
- Due to this the blood cannot carry enough oxygen to various parts of the body. It lead to suffocation and under extreme conditions it can lead to death
Why it is dangerous to sleep in a closed room with coal burning inside??
- In hilly areas people used to burn coal inside their home to keep them warm.
- When the doors are closed, fresh air cannot enter the room
- So coal burns with insufficient supply of oxygen leading to the production of carbon monoxide.
- This can lead to suffocation or may even death
How carbon dioxide is formed??
When a fuel burns with sufficient supply of oxygen it leads to complete combustion and produce carbon dioxide water and energy
CH4 + 2O2 CO2 + 2H2O + Energy
Define Greenhouse Effect??
- Carbon dioxide is not a poisonous gas
- But if excess of carbon dioxide is released into the atmosphere it traps more sun rays in the earth’s atmosphere
- Heating up of atmosphere due to trapping of sun rays by carbon dioxide in the atmosphere is called Greenhouse effect.
Global Warming:
- The increase in the concentration of carbon dioxide in the atmosphere increases the rate of greenhouse effect
- This leads to increase in average temperature of the earth called as Global Warming
What are the Effects of Global Warming??
- It adversely affect the monsoon rains
- It causes changes in seasonal pattern
- It causes excessive melting of polar ice leading to increase in the sea level and cause flooding.
- Flooding leads to submerging of coastal areas
Write a short note on Sulphur dioxide??
- Burning of coal and diesel releases Sulphur dioxide
- It is extremely suffocating and corrosive gas.
- Sulphur dioxide dissolves in rainwater to form sulphuric acid.
Write a short note on Oxides of Nitrogen??
- Exhaust of vehicles contain oxides of nitrogen that are poisonous
- Catalytic converters are installed in vehicles to convert them into harmless gas
- Oxides of nitrogen combined with water to form nitric acid.
Acid rain
- The rainwater containing dissolved oxides of nitrogen sulphur is known as acid rain
- These are corrosive they destroy marbles, limestone, metallic structure, plants and soil
Write about SPM??
- SPM means Suspended Particulate Matter
- Burning of fuels containing carbon and its compounds releases ash and fine unburnt carbon particles in air called as SPM
- It reduces the visibility to few metres and affects breathing of organism
Lead compounds:
- Exhaust of vehicles release as lead compounds in the atmosphere.
- These are poisonous gas in nature.
- Use of lead free petrol may reduce the lead pollution
Check out the other chapter notes
Physics
Chemistry
Chemical Effects of Electric Current
Biology