Carbon and its Compounds

Start with what carbon is. Its atomic number is 6, so its electronic configuration is 2, 4. It has four electrons in the outermost shell and needs four more to reach the stable configuration of neon (2, 8). There are three ways to reach a full shell: lose electrons, gain electrons, or share them. Carbon cannot lose four electrons to form C⁴⁺, because holding six protons with only two remaining electrons would need an enormous amount of energy. It cannot gain four electrons to form C⁴⁻ either, because six protons cannot hold ten electrons. So carbon shares. It shares its four valence electrons with other atoms of carbon or with other elements, and each shared pair is a covalent bond. This argument, in these three steps, is a 2- or 3-mark question and an Assertion-Reason favourite.

NCERT builds covalent bonding from the simplest cases. Two hydrogen atoms (1 electron each) share one pair to form H₂: a single bond, and each hydrogen now has the helium configuration. Two oxygen atoms (2, 6) each need two electrons, so they share two pairs: a double bond, O=O. Two nitrogen atoms (2, 5) share three pairs: a triple bond, N≡N. In methane, CH₄, the carbon atom shares one electron with each of four hydrogen atoms, forming four single C–H bonds. In water, oxygen shares one pair with each of two hydrogen atoms and keeps two lone pairs.

Covalent compounds have a recognisable set of physical properties, and the board keeps asking for the reason behind each. They have low melting and boiling points because the molecules are held together by weak intermolecular forces, even though the bonds inside each molecule are strong. They are generally poor conductors of electricity because bonding does not produce ions; there are no charged particles to carry the current. Many are gases or volatile liquids at room temperature. Contrast this with ionic compounds, which are solids with high melting points and conduct when molten or dissolved.

Two more points the examiner rewards. Carbon forms strong bonds with itself and with other elements because its atom is small, so the shared electrons are held close to both nuclei. And the older textbook's box on allotropes (diamond, graphite, fullerenes) is not in the rationalised chapter, so a one-line mention is enough: diamond has each carbon bonded to four others in a rigid three-dimensional lattice, graphite has layers of hexagonal rings and conducts electricity.

Carbon forms more compounds than all the other elements put together: millions are known, and the number keeps growing. NCERT gives two reasons, and every board answer to 'why does carbon form so many compounds' must name both.

The first is catenation: carbon atoms can bond to other carbon atoms to give long chains, branched chains and rings. The chains can be very long, and the carbon–carbon bonds can be single, double or triple. No other element does this to the same extent. Silicon, just below carbon in the same group, forms hydrogen compounds with chains of only seven or eight silicon atoms, and those compounds are very reactive. The difference is bond strength: the C–C bond is strong, so carbon chains are stable, while the Si–Si bond is weak.

The second is tetravalency. Carbon has a valency of four, so each carbon atom can bond with four other atoms. Those atoms can be carbon or a different element such as oxygen, hydrogen, nitrogen, sulphur, chlorine and so on. Changing the element attached changes the properties of the compound, which multiplies the possibilities enormously.

Why are these bonds strong? Carbon is a small atom, so the shared electron pairs sit close to the nucleus and are held tightly. Elements with larger atoms form much weaker bonds. This is why compounds of carbon are stable in the way that compounds of silicon are not.

Hydrocarbons are compounds of carbon and hydrogen only, and they are where the chapter starts counting. The simplest is methane, CH₄. Then comes ethane, C₂H₆, where two carbons are joined by a single bond and the remaining six valencies are taken by hydrogen. In an exam you should be able to draw any hydrocarbon in three ways: link the carbon skeleton first, then add hydrogens until every carbon has four bonds, then, if asked, convert it to an electron dot structure by replacing each bond with a shared pair of dots. A frequent 1-mark question asks how many covalent bonds ethane has: six C–H and one C–C, so seven.

Take ethane, C₂H₆, and ask what happens if we remove one hydrogen from each carbon. The two carbons then have one unused valency each. They satisfy those valencies by sharing a second pair of electrons between themselves, giving a double bond, C=C. That compound is ethene, C₂H₄. Remove one more hydrogen from each carbon and the carbons share a third pair: a triple bond, C≡C, in ethyne, C₂H₂.

This gives the classification the whole chapter uses. Compounds of carbon that contain only single bonds between carbon atoms are saturated: every carbon is bonded to the maximum number of other atoms it can hold. Compounds with one or more double or triple bonds between carbon atoms are unsaturated. Saturated hydrocarbons are alkanes, with the general formula C_nH_2n+2. Hydrocarbons with a double bond are alkenes, C_nH_2n, and with a triple bond are alkynes, C_nH_2n−2. Notice that there is no alkene or alkyne with one carbon; the first alkene is ethene and the first alkyne is ethyne.

Unsaturated compounds are more reactive than saturated ones, because the extra shared pair in a double or triple bond is available for reaction: it opens up and lets other atoms add on. That single sentence is the reason behind addition reactions, the bromine water test and hydrogenation, all of which come later.

Exams test this section through formulae. Given a molecular formula, decide which series it belongs to: C₃H₈ fits C_nH_2n+2, so it is an alkane, saturated, and will not show addition. C₄H₈ fits C_nH_2n, so it is an alkene and will show addition. C₃H₄ fits C_nH_2n−2, so it is propyne with one triple bond. The 'odd one out' MCQ works the same way: among C₅H₁₂, C₆H₁₄, C₇H₁₄ and C₈H₁₈, only C₇H₁₄ fails the alkane formula, so it belongs to a different series.

Carbon skeletons need not be straight. NCERT introduces branching with butane, C₄H₁₀. You can arrange four carbons as a straight chain, CH₃–CH₂–CH₂–CH₃, or as a three-carbon chain with the fourth carbon attached to the middle one, CH₃–CH(CH₃)–CH₃. Both have the formula C₄H₁₀, but they are different compounds with different properties. Compounds with the same molecular formula but different structures are called structural isomers. Methane, ethane and propane have no isomers because there is only one way to join one, two or three carbons; isomerism begins at butane. Pentane, C₅H₁₂, has three structural isomers: the straight chain, a four-carbon chain with one methyl branch on the second carbon, and a three-carbon chain with two methyl branches on the middle carbon.

Carbon also forms rings. Cyclohexane, C₆H₁₂, is a ring of six carbons joined by single bonds, each carbon carrying two hydrogens. It is saturated even though it has fewer hydrogens than hexane, C₆H₁₄, because the ring uses up two valencies. Cyclopentane, C₅H₁₀, is a ring of five carbons and has 15 single bonds in all (5 C–C and 10 C–H). Benzene, C₆H₆, is a six-membered ring with alternate single and double bonds: 3 C=C double bonds, 3 C–C single bonds and 6 C–H bonds, so 9 single bonds and 3 double bonds in total. Benzene is an unsaturated cyclic hydrocarbon; cyclohexane is a saturated one.

Counting is the main exam skill here. For any open-chain alkane of n carbons there are n − 1 C–C bonds and 2n + 2 C–H bonds; for a saturated ring of n carbons there are n C–C bonds and 2n C–H bonds. Cyclohexane therefore has 6 + 12 = 18 single bonds. Draw rings as regular polygons with a carbon at each corner and be explicit about the hydrogens if the question says 'structure' rather than 'skeleton'.

Whenever you draw an isomer, check that every carbon has exactly four bonds and that the molecular formula still adds up. Most lost marks in isomer questions come from a carbon with five bonds or a hydrogen count that has drifted.

In a hydrocarbon, one or more hydrogen atoms can be replaced by other atoms or groups of atoms in a way that keeps the valency of carbon satisfied. The element that replaces hydrogen is called a heteroatom: oxygen, nitrogen, sulphur, chlorine, bromine and so on. The heteroatom, or the group containing it, gives the compound its characteristic properties regardless of the length of the carbon chain. Such a group is called a functional group. The name says it well: the functional group is where the chemistry happens.

The functional groups in NCERT, with the way they are written in a structure, are: halogen (–Cl, –Br, called halo group), alcohol (–OH), aldehyde (–CHO, a carbon double bonded to oxygen and single bonded to hydrogen, always at the end of a chain), ketone (a carbon double bonded to oxygen with carbon atoms on both sides, so never at the end of a chain), and carboxylic acid (–COOH, a carbon carrying both a double bonded oxygen and an –OH). Alkenes and alkynes are also treated as having a functional group: the C=C double bond and the C≡C triple bond.

Two consequences are tested. First, the aldehyde group cannot be in the middle of a chain because its carbon has already used three valencies (two on oxygen, one on hydrogen), leaving only one for the chain. Second, a ketone needs at least three carbons, so the first ketone is propanone, CH₃COCH₃. Propanal, CH₃CH₂CHO, and propanone are both C₃H₆O; they have the same molecular formula but different functional groups, so they are structural isomers. This pair was an Assertion-Reason item in 2025.

Learn to spot the group from a written formula. CH₃CHO is an aldehyde. C₂H₅COOH is a carboxylic acid. CH₃COCH₂CH₃ is a ketone (butanone). CH₃CH₂Br carries a halo group. A compound of formula C₂H₆O that reacts with sodium is the alcohol, ethanol; a compound of formula C₂H₄O₂ that fizzes with sodium carbonate is the acid, ethanoic acid. These identifications are the entry point to every 5-mark identification question.

A homologous series is a series of carbon compounds in which the same functional group substitutes for hydrogen in a carbon chain, and successive members differ from each other by a –CH₂– unit. The alkanes are the simplest example: CH₄, C₂H₆, C₃H₈, C₄H₁₀. Alcohols form another: CH₃OH, C₂H₅OH, C₃H₇OH, C₄H₉OH. So do aldehydes (HCHO, CH₃CHO, C₂H₅CHO), carboxylic acids (HCOOH, CH₃COOH, C₂H₅COOH), alkenes and alkynes.

The examiner expects these characteristics, and the definition question usually asks for two or three of them:

• Successive members differ by a –CH₂– unit, so their molecular masses differ by 14 u (12 for carbon plus 2 for hydrogen).
• All members can be represented by one general formula, such as C_nH_2n+2 for alkanes or C_nH_2n+1OH for alcohols.
• All members contain the same functional group, so their chemical properties are similar. The chemistry is decided by the functional group, not the chain length.
• Physical properties such as melting point, boiling point and solubility change gradually along the series as molecular mass increases, because the intermolecular forces grow with size. Methane, ethane, propane and butane are gases; pentane onwards are liquids; the very long ones are waxy solids.

Two things trap students. The first is the phrase 'chemical properties show gradation'. They do not; chemical properties stay similar, and it is the physical properties that show gradation. The second is thinking that a homologous series is defined by the formula alone. C₄H₈ (an alkene) and C₅H₁₂ (an alkane) differ by CH₄, not CH₂, and have different functional groups, so they are not homologues.

Typical short questions: write the next homologue of propanol (butanol, C₄H₉OH); write the first two members of the series with a –Br group (bromomethane CH₃Br and bromoethane C₂H₅Br); give the third member of the aldehyde series (propanal). Always give both the name and the formula.

Naming follows a fixed two-step recipe that NCERT gives and that the board tests through 'name the following' and 'draw the structure of' questions.

Step 1. Count the carbon atoms in the longest chain and take the root word: meth (1), eth (2), prop (3), but (4), pent (5), hex (6). A saturated chain takes the suffix -ane; a double bond changes it to -ene and a triple bond to -yne. So C₃H₈ is propane, C₃H₆ is propene, C₃H₄ is propyne.

Step 2. Add the functional group. Halogens are added as a prefix: chloro-, bromo-. The others are added as a suffix that replaces the final -e of the hydrocarbon name: -ol for alcohol, -al for aldehyde, -one for ketone, -oic acid for carboxylic acid. Hence CH₃OH is methanol, C₂H₅OH is ethanol, HCHO is methanal, CH₃CHO is ethanal, CH₃COCH₃ is propanone, CH₃COOH is ethanoic acid, CH₃CH₂Cl is chloroethane, CH₃Br is bromomethane.

Some names you should be able to write and draw on demand: butanone, CH₃COCH₂CH₃, a four-carbon ketone; hexanal, CH₃(CH₂)₄CHO, a six-carbon aldehyde; bromopentane, C₅H₁₁Br, which has structural isomers because the bromine can sit on different carbons of the chain and the chain itself can branch; propanoic acid, C₂H₅COOH; hex-1-yne, HC≡C–CH₂–CH₂–CH₂–CH₃. Esters are named from the alcohol part and then the acid part: the ester of ethanol and ethanoic acid is ethyl ethanoate, CH₃COOC₂H₅.

Two reminders. The carbon of –CHO and –COOH counts in the chain: ethanoic acid has two carbons in total, not two plus one. And an aldehyde or acid group is always on carbon 1, so you never need a position number for it, whereas a ketone or a double bond in a longer chain may need one (butan-2-one, but-1-ene). At Class 10 level, the board rarely asks for positional numbers, but writing but-1-ene rather than butene is never penalised.

Carbon, in all its forms, burns in oxygen to give carbon dioxide along with heat and light. Most carbon compounds burn the same way, releasing a lot of heat, which is why they are used as fuels. The equations NCERT gives are: C + O₂ → CO₂ + heat and light; CH₄ + 2O₂ → CO₂ + 2H₂O + heat and light; and CH₃CH₂OH + 3O₂ → 2CO₂ + 3H₂O + heat and light. Balance them yourself in the exam rather than relying on memory: count carbons, then hydrogens, then oxygens.

The flame tells you about the compound. Saturated hydrocarbons burn with a clean blue flame in good supply of air. Unsaturated hydrocarbons burn with a yellow, sooty flame, because the higher proportion of carbon in them does not burn completely and the unburnt carbon particles glow yellow. A saturated hydrocarbon burning in a limited supply of air also gives a sooty flame, which is why the outside of a cooking vessel blackens when the burner holes are blocked and the fuel does not burn completely. This is NCERT exercise question 3 and a regular 1-mark MCQ.

Burning coal and petroleum releases sulphur and nitrogen oxides as well, which cause pollution. Coal and petroleum formed from the remains of living organisms over millions of years and are therefore called fossil fuels.

A related in-text question asks why oxygen and ethyne are burnt together for welding rather than ethyne and air. Air is mostly nitrogen, so the supply of oxygen is limited, ethyne burns incompletely with a sooty flame and the temperature is low. With pure oxygen the ethyne burns completely and releases a very high temperature, enough to melt metal.

Why are carbon compounds good fuels? They contain a lot of energy per unit mass, they burn readily in air with a clean flame, they release a great deal of heat, and the products, carbon dioxide and water, do not leave a residue. That four-point list answers NCERT exercise 9 in full.

Combustion is a complete oxidation, but carbon compounds can also be oxidised gently and selectively. NCERT's example is the conversion of an alcohol to a carboxylic acid: ethanol is converted to ethanoic acid by alkaline potassium permanganate or by acidified potassium dichromate, on heating. The equation is written with the oxidising agent above the arrow: CH₃CH₂OH → (alkaline KMnO₄ + heat, or acidified K₂Cr₂O₇ + heat) → CH₃COOH.

Substances such as alkaline KMnO₄ and acidified K₂Cr₂O₇ that are capable of adding oxygen to other substances are called oxidising agents. That one sentence is an in-text question and a 1-mark board question.

The activity behind this is a favourite for the 3-mark 'observation' question. Take about 3 mL of ethanol in a test tube, warm it gently in a water bath, and add a 5% solution of alkaline potassium permanganate drop by drop. At first the purple colour disappears as each drop is added, because the permanganate is being used up to oxidise the ethanol. After enough has been added the colour no longer disappears, because all the ethanol has been oxidised and the excess permanganate remains. A 5% solution means 5 g of KMnO₄ dissolved in 95 g of water.

Why is this an oxidation? Compare the formulae. Ethanol is C₂H₆O and ethanoic acid is C₂H₄O₂. The product has gained an oxygen atom and lost two hydrogen atoms. Addition of oxygen or removal of hydrogen is oxidation, so the change is an oxidation reaction. Give both the gain of oxygen and the loss of hydrogen when the question is for 2 marks.

The same reasoning extends to other alcohols: warm propanol with alkaline KMnO₄ and the purple colour fades as propanoic acid forms. A question that gives you an unknown alcohol and an oxidising agent wants the corresponding acid, named with the same carbon count.

Unsaturated hydrocarbons add hydrogen in the presence of a catalyst such as palladium or nickel to give saturated hydrocarbons. The double or triple bond opens and each carbon takes on a hydrogen: CH₂=CH₂ + H₂ → (Ni catalyst) → CH₃–CH₃. The reaction is called hydrogenation, and it is an addition reaction because two reactants combine into one product with nothing left over. A catalyst is a substance that causes a reaction to occur or proceed at a different rate without itself being affected.

The industrial application is the hydrogenation of vegetable oils. Vegetable oils generally have long unsaturated carbon chains, while animal fats have saturated chains. When a vegetable oil is heated with hydrogen in the presence of nickel, the double bonds are hydrogenated, the oil becomes solid at room temperature and the product is sold as vanaspati or hydrogenated ghee. NCERT adds the health point: oils containing unsaturated fatty acids are healthier for cooking than saturated fats, and animal fats have saturated chains that are harmful for health. This is NCERT exercise 12 and a regular case-based passage.

Addition also underlies the chemical test for unsaturation, which the board asks as 'give a test to distinguish between ethane and ethene' or 'between butter and cooking oil'. Add a few drops of bromine water to each sample and shake. The unsaturated compound (ethene, or the unsaturated cooking oil) decolourises the reddish-brown bromine water because bromine adds across the double bond. The saturated compound (ethane, or butter, which is largely saturated) does not decolourise it. Write the observation for both samples, not just the positive one.

To answer NCERT exercise 13, pick out the unsaturated formulae: C₃H₆ (propene) and C₂H₂ (ethyne) undergo addition; C₂H₆, C₃H₈ and CH₄ are alkanes and do not.

Saturated hydrocarbons are fairly unreactive. They do not react with most reagents, and they cannot show addition because there is no double or triple bond to open. But in the presence of sunlight, chlorine adds to hydrocarbons rapidly, replacing hydrogen atoms one by one. The reaction with methane is CH₄ + Cl₂ → (in the presence of sunlight) → CH₃Cl + HCl. Because one atom (chlorine) takes the place of another (hydrogen), this is a substitution reaction. If more chlorine is available the substitution continues to give CH₂Cl₂, CHCl₃ and CCl₄; at Class 10 the first step is enough unless the question asks for the series.

Chlorine attacks higher alkanes the same way, so the reaction gives a mixture of products with the chlorine on different carbons. The condition, sunlight, is what the examiner reads first. Without it, chlorine and methane do not react at room temperature.

The 3-mark comparison question is 'distinguish between addition and substitution reactions with one example each'. Put it in a table or in two parallel sentences. Addition: shown by unsaturated compounds; the double or triple bond breaks and atoms add to both carbons; two reactants give one product; example, ethene plus hydrogen with nickel gives ethane. Substitution: shown by saturated compounds; a hydrogen atom is replaced by another atom or group; two reactants give two products; example, methane plus chlorine in sunlight gives chloromethane and hydrogen chloride. That covers all the marking points.

The substitution reaction also explains why a compound of formula C₄H₁₀ reacts with chlorine in sunlight (it is butane, an alkane) but does not decolourise bromine water. Reactions are decided by saturation, so decide first whether the compound is saturated, then choose the reaction.

Ethanol, CH₃CH₂OH, is a colourless liquid at room temperature, with a boiling point of 351 K (78 °C), and is commonly called alcohol. It is soluble in water in all proportions and is a good solvent, which is why it is used in medicines such as tincture of iodine, cough syrups and many tonics. In the exam, 'properties of ethanol' means two things: these physical facts and the two chemical reactions below.

Reaction with sodium. Like water, ethanol reacts with sodium metal to give hydrogen gas and sodium ethoxide: 2Na + 2CH₃CH₂OH → 2CH₃CH₂ONa + H₂. Bubbles of a gas that burns with a pop sound are the evidence for hydrogen. This is the test that identifies an alcohol (and also a carboxylic acid, which reacts the same way), and it is the 'reacts with sodium to give a gas that burns with a pop' clue in identification questions.

Dehydration to ethene. Heating ethanol at 443 K with excess concentrated sulphuric acid removes a molecule of water and gives ethene: CH₃CH₂OH → (hot conc. H₂SO₄, 443 K) → CH₂=CH₂ + H₂O. Concentrated sulphuric acid acts as a dehydrating agent here, because it removes water from ethanol. The three things marked are the temperature, the word 'excess concentrated' and the role of the acid.

NCERT also discusses the effect of alcohol on living beings. Even small amounts of dilute ethanol cause drunkenness, and drinking pure ethanol is lethal. Ethanol is also used industrially, and to prevent misuse of industrial alcohol it is made unfit for drinking by adding poisonous substances such as methanol; dyes are added to colour it blue so it is recognised. This is called denatured alcohol. Methanol is dangerous because even small quantities cause death: it is oxidised to methanal in the liver, which reacts rapidly with cell components and causes the protoplasm to coagulate; it also affects the optic nerve and causes blindness.

Ethanol is also a good fuel. Sugarcane juice can be fermented to give ethanol, and mixing it with petrol (gasohol) makes a cleaner fuel that produces only CO₂ and water on burning.

Ethanoic acid, CH₃COOH, is commonly called acetic acid and belongs to the carboxylic acid group. A 5 to 8% solution of it in water is vinegar, used as a preservative in pickles. Pure ethanoic acid melts at 290 K (17 °C), so it often freezes in cold climates in winter; that is why it is called glacial acetic acid. Carboxylic acids are weak acids: unlike mineral acids such as HCl they ionise only partly in water, so a solution of ethanoic acid has a higher pH than a solution of hydrochloric acid of the same concentration.

Esterification. Ethanoic acid reacts with ethanol in the presence of a few drops of concentrated sulphuric acid as catalyst, on warming, to give the ester ethyl ethanoate and water: CH₃COOH + CH₃CH₂OH → (acid catalyst) → CH₃COOC₂H₅ + H₂O. Esters are sweet-smelling substances used in making perfumes and as flavouring agents. The sweet smell is the observation the examiner wants in an activity question.

Saponification. On treatment with sodium hydroxide, which is a base, an ester is converted back to the alcohol and the sodium salt of the carboxylic acid: CH₃COOC₂H₅ + NaOH → C₂H₅OH + CH₃COONa. This reaction is called saponification because it is used to prepare soap: soaps are sodium or potassium salts of long-chain carboxylic acids, made by treating esters (fats and oils) with alkali.

Reaction with a base. Like a mineral acid, ethanoic acid neutralises sodium hydroxide to give a salt and water: NaOH + CH₃COOH → CH₃COONa + H₂O. The salt is sodium ethanoate (sodium acetate).

Reaction with carbonates and hydrogencarbonates. Ethanoic acid reacts with sodium carbonate and sodium hydrogencarbonate to give the salt, carbon dioxide and water: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂; CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂. The brisk effervescence of CO₂, which turns lime water milky, is the chemical test that distinguishes a carboxylic acid from an alcohol: ethanol gives no gas with sodium carbonate.

Exercise 7 asks how to differentiate ethanol and ethanoic acid. Physical: ethanoic acid has a pungent vinegar-like smell and freezes at 290 K; ethanol has a pleasant smell and boils at 351 K. Chemical: ethanoic acid turns blue litmus red and gives CO₂ with NaHCO₃; ethanol does neither.

A soap molecule has two ends with different properties. The long hydrocarbon chain is hydrophobic: it does not dissolve in water but dissolves in oil and grease. The ionic end, –COO⁻Na⁺, is hydrophilic: it dissolves in water. Soaps are the sodium or potassium salts of long-chain carboxylic acids, for example sodium stearate, C₁₇H₃₅COONa.

Micelle formation. When soap is added to water, the molecules arrange themselves so that the hydrophobic tails are away from the water and the hydrophilic heads face the water. In the bulk of the water this produces a spherical cluster called a micelle, with the hydrocarbon tails pointing inward and the ionic heads on the outside. Micelles form in water because there is a medium that the ionic end can dissolve in while the tail cannot; in a solvent such as ethanol the hydrocarbon tail dissolves as well, so no micelle forms. That is the answer to NCERT exercise 8.

Cleaning action. Most dirt is oily and does not dissolve in water. When soap solution is applied to a dirty cloth, the hydrophobic tails of the soap molecules dissolve in the oily dirt while the ionic heads stay in the water. The soap molecules surround the oil droplet and form a micelle with the dirt trapped at the centre. The micelles stay suspended in water as a colloid; they do not come together and precipitate because the charged heads on their surfaces repel each other. Rinsing with water carries the micelles, and the dirt with them, away from the cloth. Agitation (beating the cloth or the action of a washing machine) helps to loosen the trapped dirt and remove it completely. Soap solution also scatters light, which is why it looks cloudy.

Hard water and scum. Hard water contains calcium and magnesium salts. The Ca²⁺ and Mg²⁺ ions react with soap to form insoluble calcium and magnesium salts of the fatty acid, which appear as a white, sticky precipitate called scum. Soap therefore does not lather well in hard water, and a good deal of soap is wasted forming scum before any lather appears. Soap is basic in nature, so it turns red litmus blue and leaves blue litmus unchanged (exercise 11).

Detergents. Detergents are generally ammonium or sulphonate salts of long-chain carboxylic acids. Their charged end does not form insoluble precipitates with calcium and magnesium ions, so they remain effective in hard water. They are used in shampoos and in products for cleaning clothes. Because a detergent lathers in both hard and soft water, you cannot use it to test whether water is hard; only soap shows the difference.