Wednesday, September 1, 2021

4. Carbon and its Compounds | Class 10 CBSE | Web Notes - Part 3




Carbon (all allotropic forms) & most carbon compounds burn in oxygen to give CO2 releasing heat and light. These are oxidation reactions.

C + O2 → CO2 + heat & light

CH4 + 2O2 → CO2 + 2H2O + heat & light

CH3CH2OH + 3O2 → 2CO2 + 3H2O + heat & light

Saturated hydrocarbons generally give a clean flame.

Unsaturated carbon compounds give a yellow flame with black smoke or sooty deposit (carbon). E.g. Camphor & Naphthalene are unsaturated hydrocarbons. So they burn with yellow flame and leave residues.

Alcohol is saturated and burns with clean blue flame.

Light a Bunsen burner and adjust the air hole at the base to get different types of flames/presence of smoke.

If there is no sufficient supply of air, it results in incomplete combustion of even saturated hydrocarbons giving a yellow, sooty flame. In presence of sufficient supply of air with oxygen, it gives blue flame.

The gas/kerosene stove has inlets for sufficient supply of air, the fuel is burnt to give a clean blue flame.

Blackening of the bottom of cooking vessel indicates that the air holes are blocked and fuel is getting wasted.

Coal and petroleum have some nitrogen & sulphur. Their combustion forms oxides of sulphur & nitrogen. They are major air pollutants.
Why do substances burn with or without a flame?

A flame is produced only when gaseous substances burn. So a candle or LPG burns with a flame.

Wood, coal or charcoal burn with a flame at first due to the volatile substances in them. After that they just glow red and gives out heat.

Atoms of gas substance are heated and glow to produce flame. Each element produces characteristic colour. E.g. heating a copper wire in flame gives bluish green flame.

Yellow colour of a candle flame is due to the incomplete combustion of carbon particles. When light falls on them, they scatter yellow colour.

Formation of coal and petroleum (fossil fuels)

Fossil fuels were formed from biomass by biological and geological processes.

Millions of years ago, trees, ferns and other plants were crushed into the earth due to earthquakes or volcanic eruptions. They were pressed down by layers of earth and rock. They slowly decayed into coal.

Dead marine tiny plants and animals sank to the sea bed and were covered by silt. Due to bacterial action, they turned into oil & gas under high pressure. The silt was compressed into rock. The oil & gas seeped into porous rock parts, and got trapped like water in a sponge.


Carbon compounds are easily oxidised on combustion.

Alcohols can be oxidised to carboxylic acids. Here, oxidising agents like alkaline potassium permanganate (KMnO4) or acidified potassium dichromate (K2Cr2O7) are used. (oxidising agents: The substances that can add oxygen to others).

E.g. Take 3 mL ethanol in a test tube and warm gently in a water bath. Add a 5% solution of alkaline KMnO4 drop by drop. Purple colour of KMnO4 disappears initially.
When more KMnO4 is added, the colour persists because all the alcohol gets consumed and the reaction stops.

Addition Reaction

Unsaturated hydrocarbons add hydrogen in the presence of catalysts such as palladium or nickel to give saturated hydrocarbons. (Catalysts: The substances that influence the rate of a reaction without changing itself).

This reaction is commonly used in the hydrogenation of vegetable oils using a nickel catalyst.

Vegetable oils generally have long unsaturated carbon chains (fatty acids). So they are healthy.

Animal fats generally contain saturated fatty acids which are harmful to health.

Substitution Reaction

Saturated hydrocarbons are unreactive and inert in the presence of most reagents.

However, in the presence of sunlight, hydrocarbons undergo a substitution reaction very fast. E.g.

CH4 + Cl2 → CH3Cl + HCl (in the presence of sunlight)

Here, chlorine replaces the hydrogen atoms one by one.

Higher homologues of alkanes can form many products.

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