4. Carbon and its Compounds

Carbon has 4 valence electrons. Each oxygen has 6. Carbon shares two pairs of electrons with each oxygen atom. So there are two double bonds in CO₂.

Each sulphur atom has 6 valence electrons. In S₈, each sulphur atom shares one pair of electrons with each of its two neighbours. The remaining four electrons stay as two lone pairs.

Answer: Three

They are n-pentane, iso-pentane (2-methylbutane) and neo-pentane (2,2-dimethylpropane).

(i) Catenation: Carbon can form bonds with other carbon atoms. This gives long chains, branched chains and rings.

(ii) Tetravalency: Carbon has a valency of four. So it can bond with four other atoms of carbon or other elements like hydrogen, oxygen, nitrogen and chlorine.

Formula of cyclopentane: C₅H₁₀

Yes, structural isomers are possible for bromopentane. The bromine atom can be attached to different carbon atoms. Examples: 1-bromopentane, 2-bromopentane and 3-bromopentane. The carbon chain can also be branched.

(i) Bromoethane

(ii) Methanal (formaldehyde)

(iii) Hexyne (1-hexyne)

In this reaction, oxygen is added to ethanol. Addition of oxygen is oxidation.

Ethyne burns completely in pure oxygen. It gives a very hot clean flame, which can melt metals for welding.

Air has less oxygen. So ethyne burns incompletely in air. It gives a sooty flame which is not hot enough for welding.

Add a pinch of sodium hydrogencarbonate to both liquids.

A carboxylic acid gives brisk effervescence. Carbon dioxide gas is given out, which turns lime water milky. An alcohol shows no reaction.

Substances that are capable of adding oxygen to others are called oxidising agents. Examples: alkaline potassium permanganate and acidified potassium dichromate.

No. Detergents form lather easily with both hard and soft water. They do not form scum with hard water. So we cannot check hardness of water using a detergent. Soap is used for this test.

Soap molecules form micelles around the oily dirt. But the dirt sticks to the cloth. Agitation helps to loosen the dirt from the cloth and bring it into the water. Then the micelles carry it away. So agitation is necessary.

Answer: (b) 7 covalent bonds.

There are 6 C–H bonds and 1 C–C bond.

Answer: (c) ketone.

Answer: (b) the fuel is not burning completely.

Incomplete burning gives unburnt carbon (soot), which blackens the vessel.

A covalent bond is formed by sharing a pair of electrons between two atoms.

In CH₃Cl, carbon has 4 valence electrons. It needs 4 more to complete its octet. It shares one electron each with three hydrogen atoms and one with a chlorine atom. Each hydrogen gets a duplet and chlorine gets an octet.

No ions are formed. So the bonds are covalent. Such compounds have low melting and boiling points and are poor conductors of electricity.

A series of compounds in which the same functional group replaces hydrogen in a carbon chain is called a homologous series.

Example: Alkanes – CH₄, C₂H₆, C₃H₈, C₄H₁₀.

• Each member differs from the next by a –CH₂ unit (14 u in mass).
• All members have the same general formula C_nH_2n+2.
• They have similar chemical properties. Their physical properties change gradually with molecular mass.

A soap molecule has two ends. The ionic end is hydrophilic (attracted to water). The long hydrocarbon tail is hydrophobic (repelled by water).

In water, the tails stay away from water and come together inside. The ionic ends face the water outside. This cluster of molecules is called a micelle.

No, micelles will not form in ethanol. The hydrocarbon tail dissolves in ethanol. So there is no need for the molecules to cluster together.

• They burn in air and give a large amount of heat and light.
• They have a high calorific value.
• They are easily available and easy to store and handle.
• Their burning can be easily controlled.

Hard water contains calcium and magnesium salts. Soap reacts with these salts and forms an insoluble substance. This white curdy precipitate is called scum. So soap does not form lather easily with hard water.

Soap solution is basic. So red litmus paper turns blue. Blue litmus paper shows no change.

Addition of hydrogen to unsaturated hydrocarbons in the presence of catalysts like nickel or palladium is called hydrogenation.

Industrial application: It is used to convert vegetable oils into vegetable ghee (saturated fats).

Answer: C₃H₆ and C₂H₂

These are unsaturated hydrocarbons with double and triple bonds. So they undergo addition reactions.

Bromine water test: Add bromine water to both. An unsaturated hydrocarbon removes the red-brown colour of bromine water. A saturated hydrocarbon does not.

Flame test: Saturated hydrocarbons burn with a clean blue flame. Unsaturated hydrocarbons burn with a yellow sooty flame.