Revision notes, a question bank, assertion and reason questions and answers to the NCERT questions. Tap a group to open it; opening one closes the others.
Carbon has 4 electrons in its outer shell (2, 4). To get a full shell it would have to gain 4 or lose 4 electrons. Gaining 4 would make the nucleus hold 10 electrons with only 6 protons, and losing 4 needs a large amount of energy. So carbon shares electrons. A shared pair of electrons is a covalent bond.
Saturated hydrocarbons have only single bonds. They are the alkanes, general formula CₙH₂ₙ₊₂. Unsaturated hydrocarbons have double or triple bonds: alkenes (CₙH₂ₙ, one double bond) and alkynes (CₙH₂ₙ₋₂, one triple bond).
Compounds with the same molecular formula but different structures are isomers. Butane has two isomers (butane and 2-methylpropane); pentane has three (pentane, 2-methylbutane, 2,2-dimethylpropane).
A hetero atom or group that decides the chemical properties of a compound is its functional group.
A series of compounds with the same functional group, where the formula of successive members differs by –CH₂– (14 u in mass), is a homologous series. Members show a gradation in physical properties (melting point, boiling point) but have similar chemical properties. Example: methane CH₄, ethane C₂H₆, propane C₃H₈ (alkanes); or methanol, ethanol, propanol (alcohols).
Carbon compounds burn in oxygen to give carbon dioxide, water, heat and light.
CH₄ + 2O₂ → CO₂ + 2H₂O + heat and lightC₂H₅OH + 3O₂ → 2CO₂ + 3H₂O + heat and lightAlcohols are oxidised to carboxylic acids by oxidising agents such as alkaline potassium permanganate or acidified potassium dichromate. These substances add oxygen to others.
CH₃CH₂OH → CH₃COOH (alkaline KMnO₄ + heat, or acidified K₂Cr₂O₇ + heat)Unsaturated hydrocarbons add hydrogen in the presence of a catalyst such as palladium or nickel to give saturated ones. This is hydrogenation, used to convert vegetable oils (unsaturated) into vanaspati ghee (saturated).
CH₂=CH₂ + H₂ → CH₃CH₃ (Ni)Animal fats contain saturated fatty acids, which are harmful for health; oils with unsaturated fatty acids should be chosen for cooking.
Saturated hydrocarbons are fairly unreactive, but in sunlight chlorine replaces hydrogen atoms one by one.
CH₄ + Cl₂ → CH₃Cl + HCl (sunlight)Soaps are sodium or potassium salts of long-chain carboxylic acids. Each molecule has a hydrophilic (water-loving) ionic head and a hydrophobic (water-hating) hydrocarbon tail.
Soap is made by heating a fat or oil with sodium hydroxide (saponification). The soap solution is basic: it turns red litmus blue and does not change blue litmus.
In water the tails cluster inward, away from water, with the heads outward, forming a micelle. The tails attach to the oily dirt, which is trapped in the centre of the micelle and suspended in water as an emulsion. Rinsing carries it away. Agitation (beating, scrubbing) helps to pull the dirt off the cloth and bring it into contact with soap.
Hard water contains calcium and magnesium salts. Soap reacts with them to form an insoluble, sticky scum, so a lot of soap is wasted and the lather is poor.
Detergents are sodium salts of sulphonic acids or ammonium salts with chlorides or bromides. They also have a long hydrocarbon tail and a charged head. Their charged ends do not form insoluble precipitates with the calcium and magnesium of hard water, so they work in hard water as well. Some detergents are not biodegradable and cause water pollution.
Pick a level and answer 10 questions with instant feedback. Each level has a bank of 20 questions, and New 10 questions draws a fresh set.
Read the Assertion (A) and the Reason (R), then pick one of the four standard options. You get 5 questions per set from a bank of 20 at each level, and New 5 questions draws a fresh set.
Answers to every in-text and exercise question of the NCERT chapter. Try each one first, then tap Show answer.
Carbon (4 outer electrons) shares two electrons with each oxygen atom (6 outer electrons), forming two double bonds. Each oxygen has two lone pairs.
O::C::O or O=C=O, with two lone pairs (:) on each oxygen, so every atom has 8 electrons around it.
Each sulphur atom has 6 outer electrons. In the S₈ ring each atom shares one pair with each of its two neighbouring sulphur atoms (two single bonds) and keeps two lone pairs, so every atom has 8 electrons around it. The eight atoms are arranged in a crown-shaped ring: S–S–S–S–S–S–S–S (closed).
Three: (i) pentane, CH₃CH₂CH₂CH₂CH₃; (ii) 2-methylbutane, CH₃CH(CH₃)CH₂CH₃; (iii) 2,2-dimethylpropane, C(CH₃)₄.
Formula: C₅H₁₀. Five carbon atoms form a ring, each joined by single bonds to two neighbouring carbons and to two hydrogen atoms, so each carbon has four bonds (4 shared pairs).
(i) CH₃–C(=O)–OH
(ii) CH₃CH₂CH₂CH₂CH₂Br (1-bromopentane)
(iii) CH₃–C(=O)–CH₂CH₃
(iv) CH₃CH₂CH₂CH₂CH₂–CHO
Yes, bromopentane has isomers: the Br can be on carbon 1, 2 or 3 (1-, 2-, 3-bromopentane), and the carbon chain itself can be branched (for example 1-bromo-2-methylbutane).
(i) Bromoethane (ii) Methanal (iii) Hexyne (1-hexyne): six carbon atoms with a triple bond.
In the conversion CH₃CH₂OH → CH₃COOH, oxygen is added to ethanol (and hydrogen is lost) by an oxidising agent such as alkaline KMnO₄ or acidified K₂Cr₂O₇. Gain of oxygen means oxidation.
Ethyne burns with a sooty flame in air because the combustion is incomplete, and the nitrogen of air absorbs heat. With pure oxygen the combustion is complete and the flame is very hot (above 3000 °C), as needed to melt and weld metals.
Add a little sodium hydrogencarbonate (or carbonate) to each. The carboxylic acid gives brisk effervescence of CO₂, which turns lime water milky; the alcohol gives none. (A blue litmus paper also turns red only with the acid.)
Substances that give oxygen to other substances (and are themselves reduced), such as alkaline potassium permanganate and acidified potassium dichromate.
No. Detergents form lather and clean in both hard and soft water as they do not form scum. To test hardness we use soap: hard water gives a scum and little lather.
Soap forms micelles around the oily dirt, but the dirt is stuck to the fibres. Agitation loosens the dirt, helps the soap reach into the fabric and helps the micelles to carry the dirt into the water, so the clothes become clean.
Six C–H bonds and one C–C bond: 7 in all.
Answer: (b)
Butanone has a C=O (carbonyl) group between two carbon atoms.
Answer: (c) ketone
Soot (unburnt carbon) forms in incomplete combustion.
Answer: (b)
Carbon has 4 outer electrons, hydrogen 1 and chlorine 7. Carbon shares one electron with each of three hydrogen atoms and one with chlorine, forming four single covalent bonds. Carbon thus has 8 electrons around it, each hydrogen has 2, and chlorine has 8 (three lone pairs and one shared pair). The bonds are formed by sharing, not transfer, of electrons.
(a) CH₃COOH: the C of CH₃ shares 4 pairs (3 with H, 1 with the carboxyl C); the carboxyl carbon shares a double bond with O (two pairs) and a single bond with the O of –OH; O–H has one shared pair; the O atoms have two lone pairs (C=O oxygen) and two lone pairs (–OH oxygen).
(b) H₂S: H : S : H, sulphur shares one pair with each hydrogen and keeps two lone pairs.
(c) CH₃–CO–CH₃: the central carbon has a double bond to O (two shared pairs, oxygen with two lone pairs) and single bonds to the two CH₃ carbons; each CH₃ carbon has three C–H pairs.
(d) F₂: F : F, one shared pair, and each fluorine has three lone pairs.
A series of carbon compounds with the same functional group and similar chemical properties, in which successive members differ by a –CH₂– group (14 u). Example: alcohols CH₃OH, C₂H₅OH, C₃H₇OH, C₄H₉OH. Their physical properties (melting and boiling points) change gradually with increasing mass.
Physical: ethanol is a liquid with a pleasant smell that does not freeze easily; ethanoic acid has a sharp vinegar-like smell and freezes into crystals at about 290 K (17 °C).
Chemical: ethanoic acid turns blue litmus red and gives brisk effervescence of CO₂ with sodium carbonate or hydrogencarbonate; ethanol is neutral and gives none. (Both react with sodium to give hydrogen.)
The hydrocarbon tails of soap are hydrophobic and the ionic heads hydrophilic. In water the tails cluster together away from water, with the heads outward, forming a micelle. In ethanol the tails dissolve readily, so there is no reason for the molecules to cluster, and micelles will not form.
They burn in oxygen with the release of a large amount of heat and light, they are easily available, can be easily stored and transported, and many burn cleanly (for example methane, LPG); also the products are largely carbon dioxide and water.
Hard water contains calcium and magnesium salts. Soap (sodium or potassium salt of a fatty acid) reacts with them to give insoluble calcium and magnesium salts of the fatty acid. These separate as a sticky white precipitate called scum, which does not clean and reduces lather.
Soap solution is basic, so red litmus turns blue. Blue litmus does not change.
Hydrogenation is the addition of hydrogen to unsaturated hydrocarbons in the presence of a catalyst (nickel or palladium) to give saturated compounds. Industrially it is used to convert vegetable oils (unsaturated) into solid vanaspati ghee (saturated).
The unsaturated ones: C₃H₆ (propene, a double bond) and C₂H₂ (ethyne, a triple bond). C₂H₆, C₃H₈ and CH₄ are saturated alkanes.
Add a few drops of bromine water (or shake with alkaline KMnO₄): an unsaturated hydrocarbon decolourises the brown bromine water (or the purple permanganate), while a saturated hydrocarbon does not.
In water the soap molecules form micelles, in which the hydrocarbon tails point inward and the ionic heads face outward into the water. The tails dissolve in the oily dirt and surround it, so the dirt is trapped in the centre of the micelles. The micelles do not stick together because of the repelling charges on their surfaces, so they stay suspended as an emulsion. On rinsing, the dirt is carried away with the water.