Acids, Bases and Salts

Acids taste sour and turn blue litmus red. Bases taste bitter, feel soapy and turn red litmus blue. In the lab you never taste anything, so you rely on indicators: substances whose colour (or smell) changes with the acidic or basic nature of the solution they are put in.

Litmus is the one NCERT starts with. It is a purple dye extracted from lichen, a plant of the Thallophyta group. In a neutral solution it stays purple; an acid turns it red and a base turns it blue. It is used either as a solution or as litmus paper (red and blue). Two synthetic indicators are also on the syllabus. Methyl orange is red in an acid and yellow in a base. Phenolphthalein is colourless in an acid and pink in a base. Learn these three colour pairs by heart; a one-mark question that gives a colour change and asks you to name the indicator or the solution turns up almost every year.

Natural indicators are found in coloured plant material: red cabbage leaves, the petals of the china rose (gudhal), and turmeric. China rose extract, normally light pink, turns magenta (dark pink) in an acid and green in a base. Turmeric is yellow and turns reddish-brown in a base, which is why a curry stain on a white shirt turns reddish-brown when soap (which is basic) is rubbed on it and goes back to yellow on rinsing with water.

An olfactory indicator is a substance whose smell changes in an acid or a base. Finely chopped onion, vanilla essence and clove oil are the NCERT examples: their smell is destroyed (cannot be detected) in a basic solution such as NaOH but remains in dilute HCl. So if you have unlabelled test tubes and are told to use only smell, onion-soaked cloth or vanilla essence lets you pick out the base.

When a dilute acid reacts with an active metal, hydrogen gas is evolved and a salt is formed. The general form the examiner wants is: Acid + Metal → Salt + Hydrogen gas. Zinc granules with dilute sulphuric acid is the standard activity: bubbles of gas rise from the zinc, and when the gas is passed through soap solution the bubbles fill with hydrogen. Bring a burning candle near a gas-filled bubble and the gas burns with a pop sound. That pop is the test for hydrogen, and you should write it in every answer about acid-metal reactions.

Zn + H₂SO₄ → ZnSO₄ + H₂. The same thing happens with dilute HCl: Zn + 2HCl → ZnCl₂ + H₂. Note that the metal displaces hydrogen from the acid; the salt is named after the metal and the acid (zinc sulphate, zinc chloride).

Bases also react with some metals, and this is the part students forget. Zinc granules warmed with sodium hydroxide solution give hydrogen gas and sodium zincate: 2NaOH + Zn → Na₂ZnO₂ + H₂. NCERT is careful to say that such reactions are not possible with all metals, so do not generalise to a rule that every base reacts with every metal. The general form is Base + Metal → Salt + Hydrogen gas, and when a question asks you to show that both acids and bases can liberate hydrogen with a metal, zinc is the metal to choose and the two equations above are the ones to write.

A useful lab detail: aluminium also gives hydrogen with NaOH (forming sodium aluminate), which is why aluminium vessels should not be used to store strong alkalis. That is beyond the NCERT text but it is a sensible reasoning point if a question asks about storage.

Both a metal carbonate and a metal hydrogencarbonate react with an acid to give a salt, water and carbon dioxide. General form: Metal carbonate (or hydrogencarbonate) + Acid → Salt + Water + Carbon dioxide.

Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂, and NaHCO₃ + HCl → NaCl + H₂O + CO₂. The same pattern holds for calcium carbonate (marble, egg shell, limestone): CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. This is the equation behind the NCERT in-text question where a metal compound A gives brisk effervescence with dilute HCl and one product is calcium chloride; A is calcium carbonate.

The test for carbon dioxide is that it turns lime water milky. Lime water is calcium hydroxide solution, and the milkiness is a white precipitate of calcium carbonate: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. If you keep passing CO₂ in excess, the milkiness disappears because the insoluble carbonate is converted into soluble calcium hydrogencarbonate: CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂. Examiners like this two-step story: the milkiness appears, then clears. Give both equations.

Limestone, chalk and marble are all forms of calcium carbonate, which is why marble statues and buildings are damaged by acid rain. The gas evolved here is CO₂, not hydrogen, so do not mention a pop sound in a carbonate question. That is one of the most common mix-ups in the answer scripts.

When an acid and a base react, each cancels the effect of the other; the reaction is called neutralisation and the products are a salt and water. Acid + Base → Salt + Water. NaOH + HCl → NaCl + H₂O is the equation every marking scheme accepts, and adding a second example such as KOH + HNO₃ → KNO₃ + H₂O or Ca(OH)₂ + 2HCl → CaCl₂ + 2H₂O earns the second example mark.

The NCERT activity uses phenolphthalein. A few drops are added to dilute NaOH: the solution turns pink. Dilute HCl is added drop by drop with stirring; the pink disappears when the base is exactly used up. Add NaOH again and the pink returns. Two things the examiner looks for in this activity answer: the colour goes pink with base and colourless with acid, and the colour change is reversible because it depends only on which of acid or base is present in excess.

Neutralisation is exothermic; that is why the beaker gets warm. In ionic terms the reaction is simply H⁺ + OH⁻ → H₂O, which will help you in section 2.2.

Everyday neutralisations you should be able to quote: an antacid such as milk of magnesia (magnesium hydroxide) neutralises excess HCl in the stomach; baking soda (mild base) rubbed on a bee sting neutralises the methanoic acid injected; lime (quicklime or slaked lime) or chalk added to acidic soil; a farmer may add organic matter to soil that has turned basic. The NCERT exercise Q3 is a proportion problem based on neutralisation: if 10 mL of NaOH solution is exactly neutralised by 8 mL of HCl, then 20 mL of the same NaOH needs 16 mL of that HCl.

A metal oxide reacts with an acid to give a salt and water, exactly as a base does. Because of this, metallic oxides are called basic oxides. The NCERT activity uses copper(II) oxide: a small amount of the black powder is added to dilute HCl and the mixture is stirred. The black solid dissolves and the solution turns bluish-green because copper(II) chloride is formed.

CuO + 2HCl → CuCl₂ + H₂O. General form: Metal oxide + Acid → Salt + Water. Other examples you can quote: MgO + 2HCl → MgCl₂ + H₂O and CaO + 2HCl → CaCl₂ + H₂O.

Why does this count as evidence that metal oxides are basic? Because the pattern (salt plus water with an acid) is the pattern of a base, and a base is by definition what neutralises an acid. In a reasoning question this is the sentence to write: metallic oxides give salt and water with acids, which is the characteristic property of bases, hence they are basic in nature.

Colour observations are commonly tested here. Copper(II) oxide is black; the copper(II) chloride solution is bluish-green. A question may describe the observation and ask you to identify the compound, or give the equation and ask for the colour change. Be precise: black solid to bluish-green solution.

Non-metallic oxides do the opposite of metallic oxides: they react with bases to give a salt and water, so they are acidic in nature. The NCERT example is carbon dioxide with calcium hydroxide (lime water): Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. You have already met this as the lime-water test; here the point is what it tells you about CO₂.

Because carbon dioxide reacts with a base the way an acid does, it behaves as an acidic oxide. Other non-metal oxides on the same side of the fence are sulphur dioxide and nitrogen dioxide, which is why they dissolve in rain to make acid rain (rain water with pH less than 5.6). Sulphur dioxide with sodium hydroxide gives sodium sulphite and water: 2NaOH + SO₂ → Na₂SO₃ + H₂O. With carbon dioxide, sodium hydroxide gives sodium carbonate: 2NaOH + CO₂ → Na₂CO₃ + H₂O.

Put the two oxide sections side by side and you have a small table worth memorising: metallic oxides (CuO, MgO, CaO) are basic and react with acids; non-metallic oxides (CO₂, SO₂) are acidic and react with bases. Both reactions produce salt and water. This is the NCERT summary point and it is a favourite one-mark item phrased as: which of these oxides will react with NaOH?

NCERT returns to these acid-base properties of oxides in Chapter 3 (Metals and Non-metals), so expect an occasional cross-chapter question that gives an oxide and asks for its nature and one equation.

All acids contain hydrogen, but not every hydrogen-containing compound is an acid. The NCERT activity tests dilute HCl, dilute H₂SO₄, glucose solution and alcohol with a bulb-and-electrodes circuit: the bulb glows with the acids but not with glucose or alcohol. So acids produce ions in solution (they conduct), while glucose and alcohol, although they contain hydrogen, do not give H⁺ ions and are not acids.

The property common to all acids is that they produce hydrogen ions, H⁺(aq), in water. The property common to all bases is that they produce hydroxide ions, OH⁻(aq), in water. Bases that are soluble in water are called alkalis (NaOH, KOH); every alkali is a base but not every base is an alkali. NCERT's example of an insoluble base is copper oxide; magnesium hydroxide is only sparingly soluble.

The conduction activity is the standard answer to why an aqueous solution of an acid conducts electricity: the acid dissociates in water to give H⁺ ions (as H₃O⁺) and anions, and these mobile ions carry the current. It is also the answer to NCERT exercise Q7: distilled water contains no ions and so does not conduct, while rain water has dissolved acidic gases (CO₂, SO₂) and other impurities that give ions, so it conducts.

The dry HCl activity is the most-asked experiment in the chapter. Dry HCl gas is prepared by heating a little sodium chloride with concentrated sulphuric acid: 2NaCl + H₂SO₄ → Na₂SO₄ + 2HCl. The gas is dried by passing it through a guard tube containing calcium chloride. A dry blue litmus paper held in the gas shows no colour change; a moist blue litmus paper turns red. Conclusion: hydrogen ions are produced only in the presence of water; dry HCl gas contains no H⁺(aq), so it is not acidic. This is why NCERT says acids show acidic behaviour only in aqueous solution. The guard tube is there to keep the gas dry, and the question that asks you why the guard tube is used is really asking you to say exactly that.

Hydrogen ions cannot exist alone in water; they attach to water molecules and exist as hydronium ions, H₃O⁺. So the honest way to write the dissociation of HCl is HCl + H₂O → H₃O⁺ + Cl⁻, though H⁺(aq) is accepted as a shorthand. Bases dissociate to give hydroxide ions: NaOH(s) → Na⁺(aq) + OH⁻(aq); KOH(s) → K⁺(aq) + OH⁻(aq); Mg(OH)₂(s) → Mg²⁺(aq) + 2OH⁻(aq).

Dissolving an acid or a base in water is highly exothermic. When concentrated acid is diluted, the acid must be added slowly to water with constant stirring. If water is added to concentrated acid instead, the heat released is so large that the mixture can boil locally and splash out of the container, and the glass may crack because of the sudden rise in temperature. This is the safety rule behind the warning label on concentrated acid bottles, and it is a routine assertion-reason item. Diluting a strong base also releases heat, so the same care is needed.

Dilution and ion concentration is the third idea here. Mixing an acid (or base) with water decreases the concentration of H₃O⁺ (or OH⁻) ions per unit volume; the number of ions does not change, but the volume they are spread over increases. Diluting an acid therefore makes it weaker in effect and raises its pH toward 7. Adding more base to a base raises the OH⁻ concentration per unit volume, though NCERT points out that beyond a point the increase is small because there is a limit to how much dissolves.

Two NCERT in-text questions come straight from here: why does dry HCl gas not change the colour of dry litmus (no water, so no H⁺ ions), and how does the concentration of hydronium ions change on dilution (it decreases). Learn to answer both in two sentences.

The pH scale measures the hydrogen ion concentration of a solution. The p comes from the German word potenz, meaning power. The scale runs from 0 (very acidic) to 14 (very alkaline). Neutral solutions have pH 7. A pH below 7 means an acidic solution; the lower the pH, the higher the H⁺ concentration and the stronger the acid. A pH above 7 means a basic solution; the higher the pH, the higher the OH⁻ concentration and the stronger the base. pH is measured with universal indicator (a mixture of several indicators that shows a range of colours) or pH paper, and NCERT gives a colour chart to read the value off.

Strength depends on how completely the substance ionises in water. Strong acids (HCl, H₂SO₄, HNO₃) give a high concentration of H⁺ ions; weak acids (acetic acid, formic acid, carbonic acid) give fewer H⁺ ions at the same concentration. Strong bases (NaOH, KOH) ionise fully; ammonium hydroxide is a weak base. That is why acetic acid of the same concentration as HCl has a higher pH, fizzes more slowly with magnesium ribbon (NCERT exercise Q10) and is called a weak acid.

Approximate pH values worth knowing (from the NCERT figure): 1 M HCl about 0, gastric juice about 1.2, lemon juice about 2.2, pure water 7, human blood about 7.4, milk of magnesia about 10, 1 M NaOH about 14. A question may list five solutions with pH values (as NCERT exercise Q9 does: A 4, B 1, C 11, D 7, E 9) and ask you to pick the most acidic (lowest pH, B) and the most basic (highest pH, C), then arrange them in increasing H⁺ concentration: C, E, D, A, B.

One more relation the examiner rewards: each unit decrease in pH means a ten-fold increase in H⁺ concentration. You are not asked to calculate with logarithms in Class 10, but you should know that pH 2 is ten times more acidic than pH 3, and you should be able to say that a solution of pH 6 has less hydrogen ion concentration than a solution of pH 2.

Living organisms are pH-sensitive. The human body works within a pH range of about 7.0 to 7.8, and most biochemical processes fail outside it. Rain with pH below 5.6 is acid rain; when it flows into rivers it lowers their pH, and aquatic life finds it hard to survive. Plants need a specific pH range of soil for healthy growth, so a farmer tests the soil and, if it is too acidic, adds quicklime (calcium oxide), slaked lime (calcium hydroxide) or chalk (calcium carbonate) to neutralise it. That is the answer to the NCERT in-text question about when a farmer would use lime.

Digestive system: the stomach produces hydrochloric acid that helps digest food without harming the stomach. When too much acid is produced there is pain and irritation (acidity), and the cure is an antacid, a mild base such as magnesium hydroxide (milk of magnesia) that neutralises the excess acid. Write the word neutralise; it is the marking point. This is also why NCERT exercise Q4 answers that indigestion is treated with an antacid.

Tooth decay: tooth enamel is made of calcium hydroxyapatite, a crystalline form of calcium phosphate, and is the hardest substance in the body. It does not dissolve in water but is corroded when the pH in the mouth falls below 5.5. Bacteria in the mouth produce acids by degrading sugar and food particles left after eating. The remedy is to clean the mouth after eating and to use toothpaste, which is generally basic, so that the excess acid is neutralised and decay is prevented.

Self defence by animals and plants: a bee sting leaves an acid (methanoic acid) that causes pain and irritation; rubbing a mild base such as baking soda on the stung area gives relief by neutralising it. The nettle plant has stinging hairs that inject methanoic acid; the traditional remedy is to rub the leaf of the dock plant, which often grows nearby. A case-based question on this section usually asks for the acid involved, the remedy and the name of the process (neutralisation) with one equation, so keep NaHCO₃ + HCOOH → HCOONa + H₂O + CO₂ ready as an illustrative equation even though NCERT does not write it.

A salt is the compound formed when an acid reacts with a base; it has a positive ion from the base and a negative ion from the acid. Salts with the same positive or negative radical belong to the same family. NaCl and Na₂SO₄ belong to the family of sodium salts; NaCl and KCl belong to the family of chloride salts. If a question lists a few salts and asks you to group them into families, look at the metal ion first, then the acid radical. Remember that potassium sulphate, sodium sulphate, calcium sulphate and magnesium sulphate all come from sulphuric acid, while sodium chloride and potassium chloride come from hydrochloric acid.

The pH of a salt depends on the strength of the acid and base it is made from. A salt of a strong acid and a strong base is neutral, pH 7 (NaCl from HCl and NaOH; K₂SO₄; NaNO₃). A salt of a strong acid and a weak base is acidic, pH below 7 (NH₄Cl from HCl and NH₄OH; CuSO₄). A salt of a weak acid and a strong base is basic, pH above 7 (Na₂CO₃ from carbonic acid and NaOH; CH₃COONa from acetic acid and NaOH; NaHCO₃). Sodium carbonate solution turning red litmus blue is the standard observation that shows a salt need not be neutral.

The NCERT activity has you dissolve NaCl, KNO₃, Na₂CO₃, NH₄Cl, NaHCO₃, CH₃COONa and similar salts and test them with litmus and pH paper, then write down the acid and base each one came from. A one-mark MCQ will show you a salt and ask whether its solution is acidic, basic or neutral; the rule above answers every such item.

Also note the colours and solubility facts that turn up: copper sulphate crystals are blue, sodium and potassium salts are generally white and soluble, and all the common salts in this chapter are ionic solids.

Sodium chloride, common salt, is formed by the neutralisation of hydrochloric acid with sodium hydroxide. It is dissolved in sea water along with other salts, and is obtained by evaporating sea water. Large solid deposits of it, called rock salt, formed when seas of earlier times dried up; rock salt is mined like coal and is often brown because of impurities. NCERT reminds you that common salt was the subject of Gandhiji's Dandi march. Common salt is the raw material for four important chemicals: sodium hydroxide, bleaching powder, baking soda and washing soda.

Sodium hydroxide is made by the chlor-alkali process: electrolysis of a concentrated aqueous solution of sodium chloride, called brine. The name comes from the products, chlor for chlorine and alkali for sodium hydroxide. 2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g). Chlorine gas is given off at the anode, hydrogen gas at the cathode, and sodium hydroxide solution is formed near the cathode. All three products are useful, and the marking scheme for a 3-mark question is usually one mark for the equation, one for the electrode at which each gas forms, and one for the uses.

Uses to quote. Hydrogen: fuels, margarine, ammonia for fertilisers. Chlorine: water treatment, swimming pools, PVC, disinfectants, CFCs, pesticides. Sodium hydroxide: degreasing metals, soaps and detergents, paper making, artificial fibres. Hydrogen and chlorine together give hydrochloric acid, used for cleaning steel, in ammonium chloride, medicines and cosmetics. Chlorine with slaked lime gives bleaching powder (next section).

Do not confuse this with the electrolysis of molten NaCl (which gives sodium metal and chlorine, and belongs to Chapter 3). In the chlor-alkali process the electrolyte is brine, an aqueous solution, and the product at the cathode is hydrogen, not sodium.

Bleaching powder, CaOCl₂ (calcium oxychloride), is produced by the action of chlorine on dry slaked lime: Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O. The chlorine comes from the chlor-alkali process, so the two topics are often examined together. Uses: bleaching cotton and linen in the textile industry, bleaching wood pulp in paper factories and washed clothes in the laundry; as an oxidising agent in many chemical industries; and for disinfecting drinking water to make it germ-free. The mild chlorine smell of bleaching powder is because it slowly releases chlorine in moist air, which is also why it should be stored dry.

Baking soda is sodium hydrogencarbonate, NaHCO₃. It is made from sodium chloride: NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃. It is a mild, non-corrosive base. On heating (or during baking) it decomposes: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. The carbon dioxide is what makes bread and cakes rise and become soft and spongy.

Baking powder is not the same as baking soda. Baking powder is a mixture of baking soda and a mild edible acid such as tartaric acid. When baking powder is heated or mixed in water, the acid reacts with the sodium hydrogencarbonate: NaHCO₃ + H⁺ → CO₂ + H₂O + sodium salt of the acid. The tartaric acid has a second job: it neutralises the sodium carbonate formed, which would otherwise give the cake a bitter taste. A cake made with baking soda alone tastes bitter and rises less evenly; this is the CBSE 2019 reasoning question.

Uses of baking soda: an ingredient of baking powder; an antacid to neutralise excess stomach acid; and in soda-acid fire extinguishers, where its reaction with acid releases CO₂ that smothers the fire. Adding a pinch of baking soda to milk slightly raises its pH so that the milk takes longer to set as curd (NCERT exercise Q12).

Washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O. It is obtained from baking soda in two steps. First sodium hydrogencarbonate is heated to give sodium carbonate: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. Then the anhydrous sodium carbonate (soda ash) is recrystallised from water, taking up ten molecules of water of crystallisation: Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O. When a question asks how washing soda is prepared from baking soda, both equations are expected; the second is the one students skip.

Sodium carbonate is a basic salt (weak acid, strong base), so its solution turns red litmus blue and has pH above 7. The CBSE 2019 question that asked you to name the sodium compound that is basic and can be converted into washing soda wanted sodium hydrogencarbonate as the answer, with the two equations above.

Uses of washing soda: in glass, soap and paper industries; in the manufacture of sodium compounds such as borax; as a cleaning agent for domestic purposes; and for removing permanent hardness of water. The last use is the one asked most often. Hard water contains soluble calcium and magnesium salts (chlorides and sulphates); adding sodium carbonate precipitates them as insoluble carbonates, for example CaCl₂ + Na₂CO₃ → CaCO₃ + 2NaCl, so the water no longer forms scum with soap.

Keep the three sodium compounds separated in your head: NaHCO₃ is baking soda; Na₂CO₃ (anhydrous) is soda ash; Na₂CO₃·10H₂O is washing soda. A 2-mark difference question on baking soda versus washing soda wants formula, water of crystallisation (none versus ten) and one use each.

Crystals of some salts look dry but contain a fixed number of water molecules as part of their crystal structure. This is water of crystallisation: the fixed number of water molecules present in one formula unit of a salt. Examples: copper sulphate pentahydrate CuSO₄·5H₂O (five), washing soda Na₂CO₃·10H₂O (ten), gypsum CaSO₄·2H₂O (two).

The NCERT activity heats blue copper sulphate crystals in a dry boiling tube. Water droplets appear on the cooler walls of the tube and the crystals turn white (anhydrous copper sulphate). Adding two or three drops of water to the white powder brings the blue colour back. Both observations are tested: colour blue to white on heating (loss of water of crystallisation), and white to blue on adding water (water of crystallisation regained). CuSO₄·5H₂O ⇌ CuSO₄ + 5H₂O, forward on heating, backward on adding water.

Plaster of Paris is calcium sulphate hemihydrate, CaSO₄·½H₂O. It is prepared by heating gypsum at 373 K: CaSO₄·2H₂O →(373 K) CaSO₄·½H₂O + 1½H₂O. The temperature matters; if gypsum is heated above 373 K it loses all its water and forms anhydrous calcium sulphate (dead burnt plaster), which does not set. The half molecule of water in the formula means two formula units of CaSO₄ share one molecule of water.

Plaster of Paris is a white powder. Mixed with water it sets into a hard solid mass of gypsum within minutes: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O. That is why doctors use it to support fractured bones in the right position, and why it is used for making toys, materials for decoration and for making surfaces smooth. It must be stored in a moisture-proof container because even moisture from the air converts it into gypsum, after which it is useless as a plaster (NCERT exercise Q13).