Chapter 2 · Acids, Bases and Salts

2.1 Properties of Acids and Bases

Acids taste sour and turn blue litmus red; bases taste bitter, feel soapy and turn red litmus blue. Learn to detect them with indicators, predict how they react with metals, carbonates, oxides and each other, and see the $\mathrm{H^{+}}$ and $\mathrm{OH^{-}}$ ions behind it all.

Bite into a raw mango or squeeze a lemon onto your tongue and your face screws up: the taste is sour. Curd, tamarind, vinegar and amla taste sour for the same chemical reason — they contain acids. Now think of the bitter taste of a strong cup of coffee without sugar, or the slippery, soapy feel of soap solution between your fingers. Those substances contain bases. Acids and bases are opposites: mix them in the right amounts and each cancels the effect of the other. That is why a spoonful of an antacid soothes acidity in the stomach, and why a farmer spreads lime on soil that has become too sour.

Tasting and touching are, of course, a dangerous way to test a chemical — many acids and bases burn the skin. Chemists use indicators instead: dyes and natural extracts that change colour, or smell, in acids and bases. In this section you will learn to use indicators, then study what acids and bases do — with metals, with carbonates, with oxides and with each other — and finally find the single idea that explains all of it: in water, acids give hydrogen ions and bases give hydroxide ions.

Acids, Bases and Indicators

Long before anyone knew what acids were made of, people sorted substances by how they behave. Here is what acids have in common, and what bases have in common:

Acids
  • taste sour (never test this in the laboratory!)
  • turn blue litmus red
  • react with active metals such as zinc to give hydrogen
  • react with carbonates to give carbon dioxide
Bases
  • taste bitter and feel soapy to the touch
  • turn red litmus blue
  • turn phenolphthalein pink
  • cancel (neutralise) the properties of acids
📘 Definition 2.1 — Acids and bases (by their properties)
An acid is a substance that tastes sour and turns blue litmus red. A base is a substance that tastes bitter, feels soapy, turns red litmus blue, and reacts with an acid to cancel its acidic properties. A solution that is neither acidic nor basic — pure water, salt solution, sugar solution — is called neutral; it does not change the colour of either litmus.

Some acids you will meet again and again: hydrochloric acid $\mathrm{HCl}$, sulphuric acid $\mathrm{H_{2}SO_{4}}$ and nitric acid $\mathrm{HNO_{3}}$ (the mineral acids of the laboratory), and the natural acids in food — ethanoic (acetic) acid in vinegar, citric acid in lemons and oranges, lactic acid in curd, tartaric acid in tamarind, oxalic acid in tomatoes. Common bases include sodium hydroxide $\mathrm{NaOH}$ (caustic soda), potassium hydroxide $\mathrm{KOH}$, calcium hydroxide $\mathrm{Ca(OH)_{2}}$ (slaked lime; its solution is lime water), ammonium hydroxide $\mathrm{NH_{4}OH}$, and magnesium hydroxide $\mathrm{Mg(OH)_{2}}$ (milk of magnesia, an antacid).

📘 Definition 2.2 — Indicator
An indicator is a substance that shows, by a change in colour or smell, whether another substance is acidic or basic.
  • Natural indicators come from plants: litmus (a purple dye extracted from lichens), turmeric, and the juice of red cabbage or coloured flower petals (hydrangea, petunia, geranium).
  • Synthetic indicators are made in the laboratory: phenolphthalein and methyl orange.
  • Olfactory indicators change their smell: the smell of onion, vanilla essence or clove oil can no longer be detected in a basic solution, but stays in an acidic one.

The table below gives the colours you need to remember. Notice that some indicators tell you only one thing: phenolphthalein is colourless in both acids and neutral solutions, so it can only pick out bases; methyl orange is yellow in both bases and neutral solutions, so it can only pick out acids. Litmus and red cabbage juice show all three.

IndicatorIn an acidIn a neutral solutionIn a base
Blue litmus paperturns redstays bluestays blue
Red litmus paperstays redstays redturns blue
Litmus solution (purple)redpurpleblue
Phenolphthaleincolourlesscolourlesspink
Methyl orangeredyellowyellow
Turmericyellowyellowreddish-brown
Red cabbage juicered / pinkpurplegreen
Onion, vanilla, clove oilsmell stayssmell stayssmell disappears

A curry stain on a white shirt shows turmeric at work: rub it with soap, which is basic, and the yellow stain turns reddish-brown; rinse it well with plenty of water and it turns yellow again.

✏️ Example 1 — Three unlabelled test tubes
Three test tubes contain distilled water, dilute hydrochloric acid and dilute sodium hydroxide solution, but the labels have fallen off. You are given only red litmus paper. How can you identify all three?
Step 1 Dip a strip of red litmus into a drop from each tube. The tube that turns it blue contains the base, sodium hydroxide.
Step 2 You now have a strip of blue litmus — the one just turned blue by the base. (Rinse it with distilled water first.) Dip it into the other two liquids.
Step 3 The liquid that turns the blue strip red is the acid, hydrochloric acid. The one that changes neither colour is distilled water (neutral).
Check Each liquid has been identified by a different result: base — red to blue; acid — blue to red; water — no change.
✏️ Example 2 — An indicator you can smell
A student keeps finely chopped onion in a closed plastic bag with strips of clean cloth overnight. Next day she puts one strip in dilute hydrochloric acid and another in dilute sodium hydroxide solution, and smells each. (a) What does she observe? (b) Why are such indicators useful to a visually impaired student? (c) Vanilla essence is added to a liquid and its smell disappears. What can you conclude?
(a) The onion smell can still be detected on the strip in hydrochloric acid, but not on the strip in sodium hydroxide solution.
(b) They are tested by smell instead of colour, so a student who cannot see a colour change can still tell an acid from a base.
(c) The liquid is basic: olfactory indicators lose their smell in bases.
Interactive 1 — Indicator Colour Lab

Ten test tubes hold ten common solutions. Pick an indicator and press ▶ Play: a dropper adds a few drops to each tube in turn. Before each drop lands, predict the colour (or, for the olfactory indicators, whether the smell survives). Which indicators tell you all three — acid, neutral and base?

in acids—
in neutral solutions—
in bases—
tubes tested—

Acids and Bases with Metals and Carbonates

Drop a few granules of zinc into dilute sulphuric acid in a test tube. Bubbles stream from the zinc, and the zinc slowly dissolves. Pass the gas into soap solution: soap bubbles fill with it and rise. Bring a burning candle (or splint) near a gas-filled bubble or the mouth of the tube, and the gas burns with a pop sound. That pop is the test for hydrogen. The zinc has displaced hydrogen from the acid: $$\mathrm{Zn(s) + H_{2}SO_{4}(aq) \rightarrow ZnSO_{4}(aq) + H_{2}(g)}$$ The same thing happens with dilute hydrochloric acid, and with other active metals such as magnesium and iron: $$\mathrm{Mg(s) + 2HCl(aq) \rightarrow MgCl_{2}(aq) + H_{2}(g)}$$ The compound formed, zinc sulphate or magnesium chloride, is a salt: the metal has taken the place of the hydrogen of the acid.

📐 Principle 2.1 — Acids with metals
A dilute acid reacts with an active metal (one above hydrogen in the reactivity series, such as Mg, Al, Zn, Fe) to give a salt and hydrogen gas: $$\text{acid} + \text{metal} \rightarrow \text{salt} + \text{hydrogen}$$ Metals below hydrogen, such as copper and silver, do not give hydrogen with dilute acids. (Nitric acid behaves differently; you will see why in Section 3.1.)

Some metals also react with strong bases. Warm zinc granules with sodium hydroxide solution and hydrogen is again given off, with a salt called sodium zincate: $$\mathrm{Zn(s) + 2NaOH(aq) \xrightarrow{\text{warm}} Na_{2}ZnO_{2}(aq) + H_{2}(g)}$$ Not every metal does this — only a few, such as zinc and aluminium.

Next, add dilute hydrochloric acid to a little sodium carbonate in a boiling tube, and pass the gas given off through lime water (calcium hydroxide solution). The acid fizzes, and the lime water turns milky. The gas is carbon dioxide: $$\mathrm{Na_{2}CO_{3}(s) + 2HCl(aq) \rightarrow 2NaCl(aq) + H_{2}O(l) + CO_{2}(g)}$$ Sodium hydrogencarbonate (baking soda) behaves the same way: $$\mathrm{NaHCO_{3}(s) + HCl(aq) \rightarrow NaCl(aq) + H_{2}O(l) + CO_{2}(g)}$$ The milkiness is a fine white precipitate of calcium carbonate: $$\mathrm{Ca(OH)_{2}(aq) + CO_{2}(g) \rightarrow CaCO_{3}(s) + H_{2}O(l)}$$ If you keep bubbling carbon dioxide through for a long time, the milkiness disappears, because the calcium carbonate turns into calcium hydrogencarbonate, which dissolves in water: $$\mathrm{CaCO_{3}(s) + H_{2}O(l) + CO_{2}(g) \rightarrow Ca(HCO_{3})_{2}(aq)}$$

📐 Principle 2.2 — Acids with carbonates and hydrogencarbonates
Every metal carbonate and every metal hydrogencarbonate reacts with a dilute acid to give a salt, carbon dioxide and water: $$\text{carbonate (or hydrogencarbonate)} + \text{acid} \rightarrow \text{salt} + \text{CO}_2 + \text{water}$$ The carbon dioxide is identified by the lime water test: it turns lime water milky.

Limestone, chalk and marble are all forms of calcium carbonate, $\mathrm{CaCO_{3}}$, and so are egg shells and sea shells. Drop a marble chip into dilute hydrochloric acid and it fizzes away: $$\mathrm{CaCO_{3}(s) + 2HCl(aq) \rightarrow CaCl_{2}(aq) + H_{2}O(l) + CO_{2}(g)}$$ This is why acid rain slowly eats away marble statues and monuments.

✏️ Example 3 — Which gas, and how do you know?
Two test tubes, A and B, contain dilute hydrochloric acid. Magnesium ribbon is dropped into A and a spatula of sodium hydrogencarbonate into B. Both fizz. (a) Name the gas from each tube and describe a test for it. (b) Write a balanced equation with state symbols for each reaction. (c) Which tube's gas would put out a burning splint?
(a) A: hydrogen — burns with a pop sound when a burning splint is brought near. B: carbon dioxide — turns lime water milky.
(b) A $$\mathrm{Mg(s) + 2HCl(aq) \rightarrow MgCl_{2}(aq) + H_{2}(g)}$$ Check: Mg $1=1$; H $2=2$; Cl $2=2$. ✓
(b) B $$\mathrm{NaHCO_{3}(s) + HCl(aq) \rightarrow NaCl(aq) + H_{2}O(l) + CO_{2}(g)}$$ Check: Na $1=1$; H $1+1=2$; C $1=1$; O $3=1+2$; Cl $1=1$. ✓
(c) Tube B: carbon dioxide does not burn and does not support burning, so the splint goes out. (In A the hydrogen itself burns, with a pop.)
✏️ Example 4 — An egg without its shell
A raw egg is left in a glass of dilute hydrochloric acid. Bubbles form on the shell, and after some hours the shell has gone, leaving the egg held only by its soft membrane. (a) What is the shell made of, and which gas forms the bubbles? (b) Write the balanced equation. (c) Taking $\mathrm{Ca}=40\ \mathrm{u}$, $\mathrm{C}=12\ \mathrm{u}$ and $\mathrm{O}=16\ \mathrm{u}$, what mass of carbon dioxide is released when $5.0\ \mathrm{g}$ of calcium carbonate reacts completely?
(a) Calcium carbonate, $\mathrm{CaCO_{3}}$; the gas is carbon dioxide.
(b) $$\mathrm{CaCO_{3}(s) + 2HCl(aq) \rightarrow CaCl_{2}(aq) + H_{2}O(l) + CO_{2}(g)}$$ Check: Ca $1=1$; C $1=1$; O $3=1+2$; H $2=2$; Cl $2=2$. ✓
Ratio One unit of $\mathrm{CaCO_{3}}$ ($40+12+3\times16=100\ \mathrm{u}$) gives one molecule of $\mathrm{CO_{2}}$ ($12+2\times16=44\ \mathrm{u}$).
(c) Mass of $\mathrm{CO_{2}}=5.0\times\tfrac{44}{100}=2.2\ \mathrm{g}$.
Interactive 2 — Gas Lab: Pop or Milky?

Choose what goes into the flask and how you will test the gas, then press ▶ Play. Predict first: will the gas burn with a pop, put the flame out, turn lime water milky — or will there be no gas at all? Try “keep bubbling into lime water” with a carbonate and watch the milkiness come and go.

type of reaction—
gas evolved—
test result—

Acids with Bases, and the Oxides of Metals and Non-metals

Put about $2\ \mathrm{mL}$ of dilute sodium hydroxide solution in a test tube and add two drops of phenolphthalein: the solution turns pink. Now add dilute hydrochloric acid drop by drop, shaking after each drop. Suddenly the pink colour disappears — the base has been used up. Add one drop of sodium hydroxide again and the pink comes back. The acid and the base cancel each other's effect, forming a salt and water: $$\mathrm{NaOH(aq) + HCl(aq) \rightarrow NaCl(aq) + H_{2}O(l)}$$

📘 Definition 2.3 — Neutralisation reaction
The reaction between an acid and a base to give a salt and water is called a neutralisation reaction: $$\text{base} + \text{acid} \rightarrow \text{salt} + \text{water}$$ A salt is the compound formed from the metal (or ammonium) part of the base and the non-metal part of the acid, for example $\mathrm{NaCl}$, $\mathrm{K_{2}SO_{4}}$, $\mathrm{CaCl_{2}}$. Neutralisation gives out heat (it is exothermic).

Neutralisation is a double displacement reaction (Section 1.2): the two compounds exchange partners. It is used every day — antacids such as milk of magnesia neutralise excess acid in the stomach, farmers add quicklime or slaked lime to acidic soil, and factories neutralise acidic waste before releasing it. You will see these uses, with numbers on the pH scale, in Section 2.2.

Metal oxides behave like bases too. Add a little black copper(II) oxide to dilute hydrochloric acid and stir: the black powder dissolves and the colourless solution turns blue-green, the colour of copper(II) chloride: $$\mathrm{CuO(s) + 2HCl(aq) \rightarrow CuCl_{2}(aq) + H_{2}O(l)}$$ The products are a salt and water, exactly as in neutralisation. Non-metal oxides do the opposite: carbon dioxide, an oxide of a non-metal, reacts with the base calcium hydroxide in the lime water test to give a salt and water, as an acid would.

📐 Principle 2.3 — Metal oxides are basic, non-metal oxides are acidic
  • Metal oxides react with acids to give a salt and water: $\text{metal oxide} + \text{acid} \rightarrow \text{salt} + \text{water}$. They are called basic oxides.
  • Non-metal oxides react with bases to give a salt and water: $\text{non-metal oxide} + \text{base} \rightarrow \text{salt} + \text{water}$. They are called acidic oxides.
(A few metal oxides, such as $\mathrm{Al_{2}O_{3}}$ and $\mathrm{ZnO}$, react with both acids and bases; you will meet them in Section 3.1.)
✏️ Example 5 — Completing neutralisation and oxide equations
Complete and balance, with state symbols, naming the salt formed: (a) potassium hydroxide + nitric acid; (b) calcium hydroxide + hydrochloric acid; (c) magnesium oxide + sulphuric acid; (d) sodium hydroxide + carbon dioxide.
(a) $$\mathrm{KOH(aq) + HNO_{3}(aq) \rightarrow KNO_{3}(aq) + H_{2}O(l)}$$ Salt: potassium nitrate.
(b) $\mathrm{Ca(OH)_{2}}$ has two $\mathrm{OH}$ groups, so it needs two $\mathrm{HCl}$: $$\mathrm{Ca(OH)_{2}(aq) + 2HCl(aq) \rightarrow CaCl_{2}(aq) + 2H_{2}O(l)}$$ Salt: calcium chloride. Check: Ca $1=1$; O $2=2$; H $2+2=4$; Cl $2=2$. ✓
(c) $$\mathrm{MgO(s) + H_{2}SO_{4}(aq) \rightarrow MgSO_{4}(aq) + H_{2}O(l)}$$ Salt: magnesium sulphate (a basic oxide behaving like a base).
(d) $$\mathrm{2NaOH(aq) + CO_{2}(g) \rightarrow Na_{2}CO_{3}(aq) + H_{2}O(l)}$$ Salt: sodium carbonate (an acidic oxide behaving like an acid). Check: Na $2=2$; O $2+2=3+1$; H $2=2$; C $1=1$. ✓

What Acids Have in Common: Ions in Water

Why do hydrochloric acid, sulphuric acid and citric acid all behave alike? Every acid contains hydrogen — but so do glucose $\mathrm{C_{6}H_{12}O_{6}}$ and alcohol $\mathrm{C_{2}H_{5}OH}$, which are not acids. A simple circuit settles the question. Fix two iron nails in a rubber cork inside a beaker, connect them through a bulb to a $6\ \mathrm{V}$ battery, and pour in different solutions. With dilute hydrochloric acid or sulphuric acid the bulb glows; with glucose or alcohol solution it does not. The acid solutions contain ions that carry the current; glucose and alcohol do not give ions in water.

A second experiment shows that water is essential. Dry hydrogen chloride gas (made by adding concentrated sulphuric acid to sodium chloride, and dried by passing it through a guard tube of calcium chloride) does not change the colour of dry blue litmus paper. Moisten the litmus and it turns red at once. Hydrogen chloride shows acidic behaviour only when it is dissolved in water, because only then does it form hydrogen ions: $$\mathrm{HCl(aq) \rightarrow H^{+}(aq) + Cl^{-}(aq)}$$ A hydrogen ion cannot exist alone; it attaches itself to a water molecule to form the hydronium ion, $\mathrm{H_{3}O^{+}}$: $$\mathrm{H^{+} + H_{2}O \rightarrow H_{3}O^{+}}$$ So $\mathrm{H^{+}(aq)}$ and $\mathrm{H_{3}O^{+}}$ mean the same thing, and the full change is $$\mathrm{HCl + H_{2}O \rightarrow H_{3}O^{+} + Cl^{-}}$$

📐 Principle 2.4 — Acids give H⁺ ions, bases give OH⁻ ions
  • Acids produce hydrogen ions, $\mathrm{H^{+}(aq)}$ (that is, hydronium ions $\mathrm{H_{3}O^{+}}$), when dissolved in water. These ions are responsible for their acidic properties.
  • Bases that dissolve in water produce hydroxide ions, $\mathrm{OH^{-}(aq)}$, for example $\mathrm{NaOH(s) \xrightarrow{\text{water}} Na^{+}(aq) + OH^{-}(aq)}$.
  • Neutralisation is the joining of these two ions into water: $\mathrm{H^{+}(aq) + OH^{-}(aq) \rightarrow H_{2}O(l)}$.
📘 Definition 2.4 — Alkali
A base that is soluble in water is called an alkali, and its solution is alkaline. Examples: $\mathrm{NaOH}$, $\mathrm{KOH}$, $\mathrm{Ca(OH)_{2}}$, $\mathrm{NH_{4}OH}$. All alkalis are bases, but not all bases are alkalis: copper(II) oxide and copper(II) hydroxide are bases that do not dissolve in water. Alkalis are soapy to touch, bitter and corrosive — never taste or touch them.

Dilution is exothermic. Mixing a concentrated acid (or a solid alkali such as sodium hydroxide) with water gives out a great deal of heat. If water is poured into concentrated sulphuric acid, the heat produced can boil the water at once, splashing acid out of the container and possibly cracking the glass. So the rule is always: add the acid to water, slowly, with constant stirring — never water to acid. Adding acid (or base) to water also lowers the concentration of $\mathrm{H_{3}O^{+}}$ (or $\mathrm{OH^{-}}$) ions per unit volume; this is what dilution means.

✏️ Example 6 — Explaining the experiments with ions
(a) Why does a solution of glucose not light the bulb, although glucose contains hydrogen? (b) Why does dry HCl gas not change dry blue litmus? (c) Write the ions formed when (i) $\mathrm{H_{2}SO_{4}}$ and (ii) $\mathrm{Ca(OH)_{2}}$ dissolve in water. (d) A student pours water into a beaker of concentrated sulphuric acid. What may happen, and what should she have done?
(a) Glucose does not split into ions in water, so its solution has nothing to carry the current. Containing hydrogen is not enough — an acid must release $\mathrm{H^{+}}$ ions in water.
(b) Without water, HCl does not form $\mathrm{H^{+}}$ ions, so it cannot act as an acid.
(c) (i) $\mathrm{H_{2}SO_{4}(aq) \rightarrow 2H^{+}(aq) + SO_{4}^{2-}(aq)}$; (ii) $\mathrm{Ca(OH)_{2}(aq) \rightarrow Ca^{2+}(aq) + 2OH^{-}(aq)}$. Check charges: $+2-2=0$ and $+2-2=0$. ✓
(d) The heat released can boil the water locally, so hot acid spurts out and may burn her; the beaker may crack. She should have added the acid slowly to the water, stirring all the time.
⚠️ Common mistakes with acids and bases

1. “Every compound with hydrogen is an acid.” Glucose, alcohol and methane contain hydrogen but give no $\mathrm{H^{+}}$ ions in water, so they are not acids.

2. Expecting phenolphthalein to show an acid. Phenolphthalein stays colourless in an acid and in a neutral solution; “colourless” does not prove that a liquid is acidic.

3. Mixing up the gases. Acid + metal gives hydrogen (pop test). Acid + carbonate or hydrogencarbonate gives carbon dioxide (lime water test). Acid + base gives no gas at all.

4. Calling every base an alkali. Only water-soluble bases are alkalis; $\mathrm{CuO}$ is a base but not an alkali.

5. Water into acid. Always add acid to water, slowly, while stirring.

Exercises

I
Indicators.
  • 1 What colour does (a) blue litmus turn in an acid, (b) red litmus turn in a base, (c) phenolphthalein turn in a base, (d) methyl orange turn in an acid?
  • 2 Name two natural indicators, two synthetic indicators and three olfactory indicators.
  • 3 A liquid turns methyl orange yellow. Can you conclude that it is a base? Explain.
  • 4 A liquid leaves phenolphthalein colourless. Give two possibilities for its nature.
  • 5 Why does a turmeric (curry) stain on a shirt turn reddish-brown when it is washed with soap?
  • 6 You are given blue litmus paper only. How would you identify which of three test tubes contains an acid, a base and distilled water?
  • 7 The smell of clove oil cannot be detected after it is mixed with a solution X. What is the nature of X? Why is such an indicator useful?
  • 8 Red cabbage juice is added to (a) lemon juice, (b) sugar solution, (c) lime water. Give the colour in each case.
1 (a) red; (b) blue; (c) pink; (d) red.
2 Natural: litmus, turmeric (also red cabbage, coloured petals). Synthetic: phenolphthalein, methyl orange. Olfactory: onion, vanilla essence, clove oil.
3 No. Methyl orange is yellow in neutral solutions as well as in bases; yellow shows only that the liquid is not acidic.
4 It is either acidic or neutral (phenolphthalein turns pink only in bases).
5 Soap solution is basic, and turmeric turns reddish-brown in a base. (Rinsing with plenty of water removes the soap and the stain turns yellow again.)
6 Test each liquid with blue litmus: the one that turns it red is the acid. Use that red strip on the other two: the one that turns it blue is the base; the one that changes neither is distilled water.
7 X is basic. Olfactory indicators can be used by a visually impaired person, who tests by smell instead of colour.
8 (a) red / pink (acid); (b) purple (neutral); (c) green (base).
II
Acids with metals, carbonates and hydrogencarbonates. Write balanced equations with state symbols where asked.
  • 1 Write the general word equation for the reaction of a dilute acid with an active metal, and give the test for the gas formed.
  • 2 Write balanced equations for (a) iron + dilute sulphuric acid; (b) aluminium + dilute hydrochloric acid.
  • 3 Zinc is warmed with sodium hydroxide solution. Write the equation and name the salt formed.
  • 4 Copper turnings are placed in dilute hydrochloric acid. What do you observe? Why?
  • 5 Write balanced equations for (a) potassium carbonate + dilute nitric acid; (b) magnesium carbonate + dilute hydrochloric acid.
  • 6 Describe what you see when carbon dioxide is passed through lime water (a) for a short time, (b) for a long time. Write an equation for each change.
  • 7 Why should curd and sour substances not be kept in brass or copper vessels?
  • 8 Taking $\mathrm{Zn}=65\ \mathrm{u}$ and $\mathrm{H}=1\ \mathrm{u}$, what mass of hydrogen is given off when $6.5\ \mathrm{g}$ of zinc dissolves completely in dilute sulphuric acid?
  • 9 Applied. Taking $\mathrm{Na}=23\ \mathrm{u}$, $\mathrm{H}=1\ \mathrm{u}$, $\mathrm{C}=12\ \mathrm{u}$ and $\mathrm{O}=16\ \mathrm{u}$, a tablet containing $0.84\ \mathrm{g}$ of sodium hydrogencarbonate is dropped into excess dilute acid. What mass of carbon dioxide is released?
1 acid + metal → salt + hydrogen. Hydrogen burns with a pop sound when a burning splint is brought near.
2 (a) $\mathrm{Fe(s) + H_{2}SO_{4}(aq) \rightarrow FeSO_{4}(aq) + H_{2}(g)}$; (b) $$\mathrm{2Al(s) + 6HCl(aq) \rightarrow 2AlCl_{3}(aq) + 3H_{2}(g)}$$ Check (b): Al $2=2$; H $6=6$; Cl $6=6$. ✓
3 $$\mathrm{Zn(s) + 2NaOH(aq) \rightarrow Na_{2}ZnO_{2}(aq) + H_{2}(g)}$$ Salt: sodium zincate.
4 No bubbles and no change: copper is below hydrogen in the reactivity series and cannot displace hydrogen from dilute acids.
5 (a) $$\mathrm{K_{2}CO_{3}(s) + 2HNO_{3}(aq) \rightarrow 2KNO_{3}(aq) + H_{2}O(l) + CO_{2}(g)}$$ (b) $$\mathrm{MgCO_{3}(s) + 2HCl(aq) \rightarrow MgCl_{2}(aq) + H_{2}O(l) + CO_{2}(g)}$$
6 (a) It turns milky: $\mathrm{Ca(OH)_{2}(aq) + CO_{2}(g) \rightarrow CaCO_{3}(s) + H_{2}O(l)}$. (b) The milkiness disappears: $$\mathrm{CaCO_{3}(s) + H_{2}O(l) + CO_{2}(g) \rightarrow Ca(HCO_{3})_{2}(aq)}$$ calcium hydrogencarbonate is soluble.
7 Curd and sour foods contain acids, which react slowly with the metal of the vessel (and with its corrosion layer), forming salts that make the food taste bad and can be harmful.
8 One Zn atom ($65\ \mathrm{u}$) gives one $\mathrm{H_{2}}$ molecule ($2\ \mathrm{u}$). Mass of hydrogen $=6.5\times\tfrac{2}{65}=0.20\ \mathrm{g}$.
9 $\mathrm{NaHCO_{3}}=23+1+12+48=84\ \mathrm{u}$ gives one $\mathrm{CO_{2}}=44\ \mathrm{u}$. Mass $=0.84\times\tfrac{44}{84}=0.44\ \mathrm{g}$.
III
Neutralisation and oxides.
  • 1 Define a neutralisation reaction and give one example with a balanced equation.
  • 2 Phenolphthalein is added to dilute NaOH solution, then dilute HCl is added drop by drop. Describe the colour changes, and explain what happens when a drop of NaOH is added again at the end.
  • 3 Write balanced equations and name the salt for (a) sodium hydroxide + sulphuric acid; (b) magnesium hydroxide + hydrochloric acid.
  • 4 Copper(II) oxide is added to dilute hydrochloric acid. What do you observe? Write the equation. What does this tell you about the nature of metal oxides?
  • 5 Why is the lime water test, the reaction of $\mathrm{CO_{2}}$ with $\mathrm{Ca(OH)_{2}}$, evidence that carbon dioxide is an acidic oxide?
  • 6 Calcium oxide reacts with hydrochloric acid. Write the balanced equation and classify calcium oxide.
  • 7 In a test, $20\ \mathrm{mL}$ of a sodium hydroxide solution is exactly neutralised by $10\ \mathrm{mL}$ of a hydrochloric acid solution. What volume of the same acid is needed for $50\ \mathrm{mL}$ of the same sodium hydroxide solution?
  • 8 Challenging. Sodium hydrogencarbonate solution is added to a solution of sulphuric acid. Write the balanced equation. Is this a neutralisation? Explain.
1 The reaction of an acid with a base to give a salt and water: $\mathrm{NaOH(aq) + HCl(aq) \rightarrow NaCl(aq) + H_{2}O(l)}$.
2 The solution turns pink with phenolphthalein; as acid is added it stays pink until the base is used up, then becomes colourless. One more drop of NaOH makes the solution basic again and the pink returns.
3 (a) $$\mathrm{2NaOH(aq) + H_{2}SO_{4}(aq) \rightarrow Na_{2}SO_{4}(aq) + 2H_{2}O(l)}$$ sodium sulphate. (b) $$\mathrm{Mg(OH)_{2}(s) + 2HCl(aq) \rightarrow MgCl_{2}(aq) + 2H_{2}O(l)}$$ magnesium chloride.
4 The black powder dissolves and the solution turns blue-green. $\mathrm{CuO(s) + 2HCl(aq) \rightarrow CuCl_{2}(aq) + H_{2}O(l)}$. Metal oxides react with acids to give salt and water: they are basic.
5 $\mathrm{CO_{2}}$ reacts with a base, calcium hydroxide, to give a salt ($\mathrm{CaCO_{3}}$) and water — exactly the way an acid reacts with a base.
6 $\mathrm{CaO(s) + 2HCl(aq) \rightarrow CaCl_{2}(aq) + H_{2}O(l)}$. Calcium oxide is a basic oxide.
7 The ratio is $10\ \mathrm{mL}$ acid per $20\ \mathrm{mL}$ base, i.e. half the volume. For $50\ \mathrm{mL}$ of base: $50\times\tfrac{10}{20}=25\ \mathrm{mL}$ of acid.
8 $$\mathrm{2NaHCO_{3}(aq) + H_{2}SO_{4}(aq) \rightarrow Na_{2}SO_{4}(aq) + 2H_{2}O(l) + 2CO_{2}(g)}$$ Check: Na $2=2$; H $2+2=4$; C $2=2$; O $6+4=4+2+4$; S $1=1$. ✓ It is an acid–carbonate reaction rather than a simple neutralisation, but the acid is used up and a salt and water form, so baking soda does neutralise the acid — which is why it works as an antacid.
IV
Ions, alkalis and safety.
  • 1 Which ion is responsible for the properties of acids? Write the formula of the hydronium ion and show how it forms.
  • 2 Why do solutions of HCl and $\mathrm{H_{2}SO_{4}}$ conduct electricity, while solutions of glucose and alcohol do not?
  • 3 Dry HCl gas does not change the colour of dry litmus paper. Explain.
  • 4 What is an alkali? Name two alkalis and one base that is not an alkali.
  • 5 Write the ions produced in water by (a) $\mathrm{HNO_{3}}$, (b) $\mathrm{KOH}$, (c) $\mathrm{NH_{4}OH}$.
  • 6 Write the ionic equation for every neutralisation reaction between an acid and an alkali.
  • 7 Why must a concentrated acid be added to water and not water to the acid? Why should the mixture be stirred?
1 The hydrogen ion, $\mathrm{H^{+}(aq)}$. It joins a water molecule: $\mathrm{H^{+} + H_{2}O \rightarrow H_{3}O^{+}}$ (hydronium ion).
2 HCl and $\mathrm{H_{2}SO_{4}}$ produce ions ($\mathrm{H^{+}}$, $\mathrm{Cl^{-}}$, $\mathrm{SO_{4}^{2-}}$) in water, which carry the current. Glucose and alcohol do not form ions.
3 Without water, HCl does not separate into $\mathrm{H^{+}}$ ions, so it shows no acidic properties.
4 A base that dissolves in water. Alkalis: $\mathrm{NaOH}$, $\mathrm{KOH}$ (also $\mathrm{Ca(OH)_{2}}$, $\mathrm{NH_{4}OH}$). A base that is not an alkali: $\mathrm{CuO}$ (or $\mathrm{Cu(OH)_{2}}$).
5 (a) $\mathrm{H^{+}}$ and $\mathrm{NO_{3}^{-}}$; (b) $\mathrm{K^{+}}$ and $\mathrm{OH^{-}}$; (c) $\mathrm{NH_{4}^{+}}$ and $\mathrm{OH^{-}}$.
6 $\mathrm{H^{+}(aq) + OH^{-}(aq) \rightarrow H_{2}O(l)}$.
7 Dissolving the acid in water releases a lot of heat. When acid is added slowly to a large amount of water, the water absorbs the heat safely; water added to acid can boil instantly and splash hot acid out. Stirring spreads the heat evenly so no part gets too hot.
🎯 Exam corner BoardBoard · A–RBoard · Case-basedJEE Main / NEET

E1. Board · 1 mark Which substance gives a gas that turns lime water milky when dilute hydrochloric acid is added to it? (A) zinc granules   (B) sodium carbonate   (C) sodium hydroxide   (D) copper turnings

Idea Lime water turns milky with carbon dioxide, which comes from a carbonate or hydrogencarbonate. Zinc gives hydrogen; NaOH is neutralised without any gas; copper does not react.
Answer (B).

E2. Board · A–R Assertion (A): Dry hydrogen chloride gas does not turn dry blue litmus paper red. Reason (R): Hydrogen chloride produces hydrogen ions only in the presence of water. Choose: (A) Both A and R are true and R is the correct explanation of A. (B) Both A and R are true but R is not the correct explanation of A. (C) A is true but R is false. (D) A is false but R is true.

Idea Acidic behaviour is due to $\mathrm{H^{+}(aq)}$ ions; without water, HCl forms none, so dry litmus does not change. R explains A exactly.
Answer (A).

E3. Board · 3 marks A white solid P, collected from crushed sea shells, fizzes when dilute nitric acid is poured on it. The gas Q turns lime water milky, but when Q is bubbled through for a long time the liquid becomes clear again. Identify P and Q, and write balanced equations for (a) the reaction of P with nitric acid, (b) the milkiness, and (c) its disappearance.

Identify P is calcium carbonate, $\mathrm{CaCO_{3}}$; Q is carbon dioxide, $\mathrm{CO_{2}}$.
(a) $$\mathrm{CaCO_{3}(s) + 2HNO_{3}(aq) \rightarrow Ca(NO_{3})_{2}(aq) + H_{2}O(l) + CO_{2}(g)}$$
(b) $\mathrm{Ca(OH)_{2}(aq) + CO_{2}(g) \rightarrow CaCO_{3}(s) + H_{2}O(l)}$
Answer (c) $$\mathrm{CaCO_{3}(s) + H_{2}O(l) + CO_{2}(g) \rightarrow Ca(HCO_{3})_{2}(aq)}$$ (soluble calcium hydrogencarbonate).

E4. Board · Case-based In a practical class, three colourless liquids P, Q and R are tested. P turns blue litmus red and methyl orange red. Q turns red litmus blue and phenolphthalein pink. R changes the colour of neither red nor blue litmus. (i) State the nature of P, Q and R, and suggest one possible substance for each. (2 marks) (ii) What colour would turmeric solution show with Q? (1 mark) (iii) P and Q are mixed in exactly the right proportion. Name the type of reaction and its products in general terms. (1 mark)

(i) P is acidic (e.g. dilute hydrochloric acid); Q is basic (e.g. sodium hydroxide solution); R is neutral (e.g. distilled water or common salt solution).
(ii) Reddish-brown — turmeric changes colour in a base.
Answer (iii) Neutralisation: acid + base → salt + water.

E5. JEE Main / NEET Foundation. Which of the following is a base but not an alkali? (A) $\mathrm{NaOH}$   (B) $\mathrm{KOH}$   (C) $\mathrm{Cu(OH)_{2}}$   (D) $\mathrm{Ca(OH)_{2}}$

Idea An alkali is a water-soluble base. NaOH, KOH and $\mathrm{Ca(OH)_{2}}$ dissolve (calcium hydroxide only slightly, enough to make lime water); copper(II) hydroxide is insoluble.
Answer (C).

E6. Board · 2 marks A metal M gives off a gas that burns with a pop sound both when it is added to dilute hydrochloric acid and when it is warmed with sodium hydroxide solution. Identify M and write balanced equations for the two reactions.

Idea The gas is hydrogen. A metal that gives hydrogen with an acid and with an alkali is zinc (aluminium also does).
Answer M = zinc. $$\mathrm{Zn(s) + 2HCl(aq) \rightarrow ZnCl_{2}(aq) + H_{2}(g)}$$ $$\mathrm{Zn(s) + 2NaOH(aq) \rightarrow Na_{2}ZnO_{2}(aq) + H_{2}(g)}$$
🔬 Research–Teaching NexusResearch-tutored · optional, two or three evenings

Research question. Which kitchen and garden plants make the best home-made acid–base indicators, and can you use them to sort ten household substances as accurately as litmus does?

Why it matters. Plant pigments that change colour with acidity are used all over the world: gardeners know that hydrangea flowers come out blue in acidic soils and pink in alkaline ones; food scientists use the colour of red-cabbage and purple-sweet-potato pigments as natural food colours; and researchers develop cheap paper test strips coloured with plant extracts for testing water and soil where laboratory chemicals are expensive or unavailable. A good indicator must give a clear, different colour for acids, neutral and basic samples.

What to do.

  1. Make three extracts: chop red cabbage and soak it in hot water; mix a pinch of turmeric in a little water; crush dark-coloured petals (hibiscus, rose or petunia) in warm water. Strain each one.
  2. Collect ten samples: lemon juice, vinegar, tamarind water, tap water, sugar solution, salt solution, baking soda solution, soap solution, washing soda solution and lime water (from a paan shop's chuna, diluted).
  3. Put a little of each sample in a row of small transparent cups and add a few drops of one extract. Photograph the row against a white background. Repeat for each extract.
  4. If you can get litmus paper from school, test every sample with it too, as a reference.
  5. For each extract, count how many samples it sorts correctly into acid / neutral / base compared with litmus.

Tools: Red cabbage, turmeric, flower petals, small cups or test tubes, a dropper, a phone camera, and litmus paper if available.

Hand in: A colour chart (photographs) for each indicator, a results table, and a short paragraph naming your best indicator with reasons.

Going further. Soak strips of filter paper in your best extract, dry them in the shade, and make your own indicator paper. Does it still work after a week? Does sunlight fade it?

Instructor note: red cabbage usually wins — red/pink in lemon, vinegar and tamarind, purple in water, sugar and salt solutions, and blue-green to green in baking soda, soap, washing soda and lime water (yellowish in very strong alkali). Turmeric only distinguishes bases (reddish-brown) from everything else; baking soda may give only a faint change because it is a weak base. Hibiscus and rose extracts behave like red cabbage but with paler colours. Tap water may be very slightly basic. Common errors: extracts too dilute, samples contaminated by a dirty dropper.

🧪 Self-check quiz

0/5 correct

Q1. A few drops of phenolphthalein are added to dilute sodium hydroxide solution. The solution becomes

Q2. Zinc granules react with dilute sulphuric acid. The gas evolved

Q3. Carbon dioxide reacts with calcium hydroxide to give a salt and water. This shows that oxides of non-metals are

Q4. Which of these is an olfactory indicator?

Q5. Dilute hydrochloric acid reacts with sodium carbonate to give

← Chapter 2 — Acids, Bases and Salts2.2 Strength of Acids and Bases: the pH Scale →