Chapter 2 · Acids, Bases and Salts

2.2 Strength of Acids and Bases: the pH Scale

Some acids sting at a touch, others are safe in your food. The pH scale gives every solution a number from 0 to 14: below 7 acidic, 7 neutral, above 7 basic. See what the number means, how strong and weak acids differ, why diluting never turns an acid into a base, and why pH matters in your stomach, teeth, soil and the rain.

The sourness of a lemon is harmless, but a drop of concentrated sulphuric acid on your skin is a serious burn. Both are acids, so “acid” alone does not tell you how much an acid acts like one. Likewise, soap solution and caustic soda are both bases, yet you can wash with one and not the other. We need a way to say how acidic or how basic a solution is — and a number is the natural tool.

Chemists use the pH scale. It runs from 0 to 14 and is read with a colour: a few drops of universal indicator turn a different colour at every pH. In this section you will learn to read the scale, to tell a strong acid from a weak one (and a strong acid from a concentrated one), to predict what happens when an acid is diluted, and to see why pH matters in your stomach, on your teeth, in a farmer's field and in the rain.

The pH Scale and Universal Indicator

In Section 2.1 you used indicators that sort a liquid into three groups: acid, neutral, base. A single indicator that changes colour gradually across the whole range can do much more. A mixture of dyes called universal indicator does exactly that. Add a few drops to a solution and match the colour with a chart, or dip a strip of pH paper, which is paper soaked in universal indicator.

📘 Definition 2.5 — pH scale and universal indicator

The pH scale is a scale of numbers, usually from 0 to 14, that tells how acidic or basic a solution is. The pH of a solution is a measure of the concentration of hydronium ions $\mathrm{H_{3}O^{+}}$ in it: the more hydronium ions, the lower the pH. A universal indicator is a mixture of dyes that shows a different colour at different pH values, so that it can be used to read the pH approximately.

📐 Principle 2.5 — Reading the pH scale

$\mathrm{pH}\lt 7$: the solution is acidic. The smaller the pH, the more acidic it is.
$\mathrm{pH}=7$: the solution is neutral (pure water, common salt solution).
$\mathrm{pH}\gt 7$: the solution is basic (alkaline). The larger the pH, the more basic it is.

Pure water at room temperature has pH $7$, because it has the same small number of $\mathrm{H_{3}O^{+}}$ and $\mathrm{OH^{-}}$ ions.

The usual colours of universal indicator are given in the table. They are approximate: different makers use slightly different dye mixtures, so always compare with the chart that comes with your bottle or paper.

pH range	Colour of universal indicator	Nature	Examples
0 – 2	red	strongly acidic	stomach acid, dilute $\mathrm{HCl}$, lemon juice
3 – 4	orange	acidic	vinegar, tomato juice
5 – 6	yellow	weakly acidic	black coffee, rain water, milk
7	green	neutral	pure water, salt solution
8 – 9	blue	weakly basic	baking soda solution, toothpaste
10 – 11	indigo	basic	milk of magnesia, ammonia solution
12 – 14	violet	strongly basic	lime water, sodium hydroxide solution

Notice that the scale describes solutions. A dry solid has no pH until it is dissolved in water. Notice also that the reading is a single number whichever indicator you use: litmus, phenolphthalein and methyl orange only change colour at their own pH values (litmus near 7, phenolphthalein near 8 to 10, methyl orange between about 3 and 4.4), so each gives only a rough sorting.

✏️ Example 7 — Reading the scale

Five solutions have these pH values: P $=1.5$, Q $=6.8$, R $=7.0$, S $=8.3$, T $=12.4$. (a) Classify each as acidic, neutral or basic. (b) Give the colour of universal indicator in each. (c) Which is the most acidic and which the most basic? (d) Which would phenolphthalein turn pink?

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(a) P and Q have pH below 7: acidic. R: neutral. S and T have pH above 7: basic.

(b) P: red. Q: yellow-green (almost neutral, just on the acid side). R: green. S: blue. T: violet.

(d) Phenolphthalein is pink in bases, from about pH 8.2 upwards: S (8.3, only just) and T.

✏️ Example 8 — Which is the strongest acid?

Four colourless liquids give these results with universal indicator: A — orange, B — red, C — green, D — yellow. Arrange them in order of increasing acidity and say which one is neutral.

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Step 1 Convert colours to pH: red (0–2) < orange (3–4) < yellow (5–6) < green (7).

Step 2 Lower pH means more acidic, so the most acidic is the red one (B) and the least acidic is the green one (C).

Answer Increasing acidity: C (green, neutral) < D (yellow) < A (orange) < B (red).

Strong and Weak Acids and Bases

Take two beakers with the same volume of dilute hydrochloric acid and dilute ethanoic (acetic) acid of the same concentration. Drop a strip of magnesium ribbon into each. The ribbon in hydrochloric acid fizzes briskly and dissolves quickly; in ethanoic acid it fizzes gently and lasts much longer. Test both with universal indicator: hydrochloric acid is red (pH about 1), ethanoic acid is orange (pH about 3). What is different?

Recall from Section 2.1 that the properties of an acid come from the hydrogen ions $\mathrm{H^{+}(aq)}$ (hydronium ions) it gives in water. Hydrochloric acid splits almost completely into ions; ethanoic acid splits only slightly, so most of its molecules stay as they are: $$\mathrm{HCl(aq) \rightarrow H^{+}(aq) + Cl^{-}(aq)}\qquad\qquad \mathrm{CH_{3}COOH(aq) \rightleftharpoons H^{+}(aq) + CH_{3}COO^{-}(aq)}$$ The double arrow $\rightleftharpoons$ tells you that the change goes both ways and only a small part of the acid has turned into ions at any moment.

📘 Definition 2.6 — Strong and weak acids and bases

A strong acid gives (almost) all its hydrogen ions to water; a weak acid gives only a few. Likewise a strong base gives (almost) all its hydroxide ions in water, and a weak base gives only a few.

Strong acids: hydrochloric $\mathrm{HCl}$, sulphuric $\mathrm{H_{2}SO_{4}}$, nitric $\mathrm{HNO_{3}}$. Weak acids: ethanoic $\mathrm{CH_{3}COOH}$, citric, carbonic $\mathrm{H_{2}CO_{3}}$, methanoic $\mathrm{HCOOH}$.
Strong bases: sodium hydroxide $\mathrm{NaOH}$, potassium hydroxide $\mathrm{KOH}$. Weak bases: ammonium hydroxide $\mathrm{NH_{4}OH}$, magnesium hydroxide $\mathrm{Mg(OH)_{2}}$ (very sparingly soluble).

📐 Principle 2.6 — Strength, concentration and dilution

At the same concentration, a strong acid has a lower pH than a weak acid, and a strong base has a higher pH than a weak base. Strong acids react faster with metals and carbonates.
Strength belongs to the substance (how completely it splits into ions). Concentration is how much of it is dissolved in a given volume. A dilute solution of a strong acid can have a higher pH than a concentrated solution of a weak acid, though the strong acid is still the stronger one.
Diluting an acid with water lowers the concentration of $\mathrm{H_{3}O^{+}}$ ions, so its pH rises towards 7 but never goes above 7. Diluting a base lowers its pH towards 7 but never below 7. Dilution does not turn a strong acid into a weak acid.

Why does the pH stop at 7? Water itself always contains a tiny number of $\mathrm{H_{3}O^{+}}$ and $\mathrm{OH^{-}}$ ions. However much water you add, the solution can never have fewer $\mathrm{H_{3}O^{+}}$ ions than water has, so it can never be less acidic than water. That is also why “neutralising an acid by dilution” does not work: to make it truly neutral you must add a base.

✏️ Example 9 — Strong or weak?

A student prepares equal volumes of $0.1\ \mathrm{mol/L}$ solutions of acids X and Y. X has pH $1$ and Y has pH $3$; magnesium ribbon fizzes strongly in one and weakly in the other. (a) Which acid is stronger? (b) In which test tube does the ribbon fizz more? (c) Suggest one possible acid for X and one for Y. (d) X is now diluted with a very large amount of water. Can its pH exceed 7?

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(a) The concentrations are equal, so the lower pH shows more hydrogen ions: X (pH 1) is the stronger acid.

(b) More $\mathrm{H^{+}}$ ions mean a faster reaction, so the ribbon fizzes more strongly in X.

(d) No. Dilution raises the pH towards 7 (after very large dilution it is close to 7, slightly below), but pure water is neutral, so the pH never goes above 7.

⚠️ Strong is not the same as concentrated

“Strong” describes how completely an acid splits into ions; “concentrated” describes how much acid is in the solution. Ethanoic acid can be concentrated yet weak (glacial ethanoic acid); hydrochloric acid can be dilute yet strong. A very dilute strong acid is far less dangerous than a concentrated one — but it is still a strong acid, and it still has a lower pH than a weak acid of the same concentration.

🚀 Beyond the board syllabus (JEE Main · NEET · JEE Advanced) — pH as a logarithm

In JEE Main, NEET and JEE Advanced (as part of Class 11 ionic equilibrium); not in the Class 10 CBSE syllabus.

Chemists define $\mathrm{pH}=-\log_{10}[\mathrm{H^{+}}]$, where $[\mathrm{H^{+}}]$ is the concentration in $\mathrm{mol/L}$. So $0.1\ \mathrm{mol/L}$ $\mathrm{HCl}$ (fully split) has $\mathrm{pH}=1$, and $0.001\ \mathrm{mol/L}$ $\mathrm{HCl}$ has $\mathrm{pH}=3$. Each fall of one pH unit means ten times more hydrogen ions. Diluting a strong acid ten times raises its pH by one. For a base, $\mathrm{pH}+\mathrm{pOH}=14$ at $298\ \mathrm{K}$, so $0.01\ \mathrm{mol/L}$ $\mathrm{NaOH}$ has $\mathrm{pOH}=2$ and $\mathrm{pH}=12$. The Class 11 course derives all this properly (Class 11 Chemistry, Ch. 6 — Equilibrium).

Watching pH Change During Neutralisation

In Section 2.1 you saw phenolphthalein lose its pink colour when enough acid had been added to a base. We can follow the same change on the pH scale. Put $25\ \mathrm{mL}$ of an acid in a flask, add a few drops of indicator, and run sodium hydroxide solution in from a long graduated tube called a burette, a little at a time, swirling the flask. At first the pH rises slowly. Then, as the last of the acid is used up, it shoots up in a few drops from the acid side to the base side. The point where the acid has been exactly cancelled — and the indicator changes colour — is the end point of the neutralisation.

The shape of this pH curve tells you about the acid. With a strong acid such as hydrochloric acid, the solution at the end point contains only sodium chloride and water, which is neutral: pH $7$. With a weak acid such as ethanoic acid, the solution at the end point contains sodium ethanoate, whose solution is slightly basic (you will see why in Section 2.3), and the curve starts higher and finishes above 7.

$25.0\ \mathrm{mL}$ of $0.10\ \mathrm{mol/L}$ acid is neutralised by $0.10\ \mathrm{mol/L}$ $\mathrm{NaOH}$ from the burette. Choose the acid and an indicator, then press ▶ Play (or drag the volume slider, or add single drops near the end point). Predict first: where does the colour change, and what is the pH there for a weak acid?

Acid:Indicator:NaOH added:

NaOH added—

pH of the flask—

indicator colour—

stage—

pH in Everyday Life

Plants and animals are pH-sensitive. Your body works best when the pH of your blood stays between about 7.35 and 7.45, slightly basic. Many crop plants grow well in soil of pH between about 6 and 7.5, though a few, such as tea and blueberries, prefer acidic soil. Farmers test the soil with pH paper, and add lime (quicklime $\mathrm{CaO}$, slaked lime $\mathrm{Ca(OH)_{2}}$ or chalk $\mathrm{CaCO_{3}}$) to acidic soil, or organic matter such as compost to basic soil. Rivers and ponds also need a pH close to 7: fish die if the water becomes too acidic.

The stomach. The wall of your stomach makes hydrochloric acid, so the fluid in it has a pH of about 1.5 to 3.0. This acid helps in digestion and kills germs that enter with food, and a lining of mucus protects the stomach wall from it. Eating spicy, oily food or being anxious can make the stomach produce too much acid, causing pain and a burning feeling called acidity (indigestion). Weak bases called antacids relieve it by neutralising the excess acid. Examples: milk of magnesia $\mathrm{Mg(OH)_{2}}$, baking soda $\mathrm{NaHCO_{3}}$, and magnesium or aluminium hydroxide tablets.

Tooth decay. The mouth always has bacteria that feed on the sugar and food particles left on your teeth, and make acids. When the pH in the mouth falls below 5.5, the acid starts to corrode tooth enamel, which is calcium phosphate, the hardest substance in the body, and cavities form. Cleaning your teeth after meals removes the food. Toothpastes are generally basic and neutralise the excess acid in the mouth, which stops the decay.

Stings. The sting of a honeybee or an ant, and the hair of a nettle leaf, inject methanoic acid (formic acid, $\mathrm{HCOOH}$) into the skin, causing pain and burning. Rubbing the spot with a mild base such as baking soda neutralises it. Nettle stings are traditionally eased with the leaf of the dock plant, which grows beside it; a calamine-type lotion works too. Never use a strong base such as sodium hydroxide: it would burn the skin.

Acid rain. Rain water is naturally slightly acidic, with a pH of about 5.6, because carbon dioxide in the air dissolves in it to form carbonic acid.

📘 Definition 2.7 — Acid rain

Acid rain is rain with a pH lower than 5.6. It forms when oxides of sulphur and nitrogen, released by burning fossil fuels and by vehicles, dissolve in rain drops to form sulphuric and nitric acids: $$\mathrm{SO_{2}(g) + H_{2}O(l) \rightarrow H_{2}SO_{3}(aq)} \qquad \mathrm{2SO_{2}(g) + O_{2}(g) + 2H_{2}O(l) \rightarrow 2H_{2}SO_{4}(aq)}$$ $$\mathrm{4NO_{2}(g) + O_{2}(g) + 2H_{2}O(l) \rightarrow 4HNO_{3}(aq)}$$ Acid rain corrodes marble and metals (the Taj Mahal's marble is slowly yellowing), harms forests and lakes, and washes nutrients out of soil. These oxides are acidic oxides (Section 2.1).

📐 Principle 2.7 — Living things need a narrow pH range

Chemical changes in living cells are controlled by enzymes that work only in a narrow pH range. A large change in the pH of blood, soil or water therefore harms the living things in it. This is why the body keeps its pH nearly constant, why antacids must be mild bases, and why acid rain and acidic waste are controlled.

✏️ Example 10 — Two antacid tablets

One antacid tablet contains $0.29\ \mathrm{g}$ of magnesium hydroxide and another contains $0.84\ \mathrm{g}$ of sodium hydrogencarbonate. (a) Write the balanced equation for each tablet reacting with the stomach acid ($\mathrm{HCl}$). (b) With $\mathrm{Mg}=24\ \mathrm{u}$, $\mathrm{O}=16\ \mathrm{u}$, $\mathrm{H}=1\ \mathrm{u}$, $\mathrm{Na}=23\ \mathrm{u}$, $\mathrm{C}=12\ \mathrm{u}$, $\mathrm{Cl}=35.5\ \mathrm{u}$, find the mass of $\mathrm{HCl}$ each tablet can neutralise. (c) Why is a mild base chosen rather than sodium hydroxide?

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(a) $$\mathrm{Mg(OH)_{2}(s) + 2HCl(aq) \rightarrow MgCl_{2}(aq) + 2H_{2}O(l)}$$ $$\mathrm{NaHCO_{3}(s) + HCl(aq) \rightarrow NaCl(aq) + H_{2}O(l) + CO_{2}(g)}$$ Check: Mg tablet — Mg $1=1$; O $2=2$; H $2+2=4$; Cl $2=2$. NaHCO₃ — Na $1=1$; H $1+1=2$; C $1=1$; O $3=1+2$; Cl $1=1$. ✓

(b) Mg $\mathrm{Mg(OH)_{2}}=24+2\times17=58\ \mathrm{u}$ reacts with $2\,\mathrm{HCl}=2\times36.5=73\ \mathrm{u}$. Mass of $\mathrm{HCl}=0.29\times\tfrac{73}{58}=0.365\ \mathrm{g}$.

(b) Na $\mathrm{NaHCO_{3}}=23+1+12+48=84\ \mathrm{u}$ reacts with $1\,\mathrm{HCl}=36.5\ \mathrm{u}$. Mass of $\mathrm{HCl}=0.84\times\tfrac{36.5}{84}=0.365\ \mathrm{g}$. Both tablets neutralise the same amount: $0.365\ \mathrm{g}$.

(c) Sodium hydroxide is a strong, corrosive base: it would burn the stomach wall and could make the stomach contents far too basic. A mild base neutralises the excess acid and cannot raise the pH much above 7.

✏️ Example 11 — Sting, soil and rain

(a) A red ant stings a child's arm. Name the substance injected, give a first-aid remedy, and write a balanced equation using baking soda. (b) A field's soil has pH $4.8$. What is its nature and what should the farmer add? (c) A rain sample has pH $4.3$. Is it acid rain? Name the gases likely to be responsible.

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(a) Methanoic acid. Rub on a paste of baking soda (a mild base): $$\mathrm{HCOOH(aq) + NaHCO_{3}(s) \rightarrow HCOONa(aq) + H_{2}O(l) + CO_{2}(g)}$$ Check: C $2=2$; H $2+1=1+2$; O $2+3=2+1+2$; Na $1=1$. ✓

(b) The soil is acidic (pH below 7, and below the 6–7.5 needed by most crops). The farmer should add lime (quicklime, slaked lime or chalk) to neutralise the acid.

⚠️ Common mistakes about pH

1. “A solution with pH 0 has no acid, or pH 14 has no acid ions at all.” The scale is just a way to compare; even the most basic solution contains some $\mathrm{H_{3}O^{+}}$.

2. Reading the scale backwards. A lower pH means more acidic. Lemon juice (pH $\approx 2$) is more acidic than milk (pH $\approx 6.5$).

3. “Dilute it enough and an acid becomes a base.” Dilution brings the pH close to 7 and no further.

4. Mixing up strong and concentrated. Strength is about ions; concentration is about quantity.

5. Forgetting that pH is for solutions. Dry hydrogen chloride gas and dry sodium hydroxide pellets have no pH until they dissolve.

Exercises

Reading the pH scale.

1 What does the pH scale measure, and what is its usual range?
2 Classify as acidic, neutral or basic: lemon juice (pH $2.2$), pure water ($7.0$), washing soda solution ($11.0$), milk ($6.5$), blood ($7.4$), tomato juice ($4.1$).
3 Give the colour of universal indicator at (a) pH 1, (b) pH 3, (c) pH 7, (d) pH 13.
4 Arrange in order of increasing acidity: milk (pH $6.5$), tomato juice ($4.1$), vinegar ($2.9$), black coffee ($5.0$).
5 A solution gives a red colour with universal indicator and stays colourless with phenolphthalein. State its nature and the likely range of its pH.
6 Solutions X and Y have pH $9$ and $13$. Which is more basic? Both are tested with turmeric: what do you expect?
7 A student wants the pH of solid sodium hydroxide pellets and presses pH paper on the dry pellets. What is wrong with this?
8 The pH of human blood is about $7.4$. Is blood acidic or basic? Why must its pH stay almost constant?

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1 How acidic or basic a solution is (the concentration of hydronium ions); usually from 0 to 14.

2 Acidic: lemon juice, milk, tomato juice. Neutral: pure water. Basic: washing soda solution, blood (slightly).

3 (a) red; (b) orange; (c) green; (d) violet.

4 Lower pH is more acidic, so: milk < black coffee < tomato juice < vinegar.

5 It is acidic, with a pH roughly between 0 and 2 (red with universal indicator); phenolphthalein is colourless in all acids.

6 Y (pH 13) is more basic. Turmeric turns reddish-brown in both, since both are bases (it changes colour above about pH 8.6).

7 A dry solid has no pH: it must be dissolved in water to give hydroxide ions first. Moist paper in contact with pellets would also be damaged and the pellets are corrosive.

8 Slightly basic. Enzymes and cell processes work only in a narrow pH range, so a large change would be harmful.

Strong and weak acids and bases; dilution.

1 Define a strong acid and a weak acid, and give one example of each.
2 Equal volumes of $0.1\ \mathrm{mol/L}$ hydrochloric acid and $0.1\ \mathrm{mol/L}$ ethanoic acid are tested. Which has the lower pH? In which does magnesium ribbon react faster? Explain.
3 Which has the higher pH at the same concentration: NaOH solution or $\mathrm{NH_{4}OH}$ solution? Why?
4 Explain the difference between a strong acid and a concentrated acid with one example each of: a dilute strong acid; a concentrated weak acid.
5 $10\ \mathrm{mL}$ of an acid of pH $2$ is mixed with $90\ \mathrm{mL}$ of water. Does the pH increase, decrease or stay the same? Is the solution still acidic?
6 Can the pH of dilute hydrochloric acid be made $9$ by adding enough water? Explain.
7 A student says, “diluting a strong base makes it a weak base”. Correct her statement.
8 In the conductivity set-up of Section 2.1, a bulb glows more brightly with $0.1\ \mathrm{mol/L}$ HCl than with $0.1\ \mathrm{mol/L}$ ethanoic acid. Why?

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1 A strong acid gives (almost) all its hydrogen ions in water, e.g. $\mathrm{HCl}$; a weak acid gives only a few, e.g. $\mathrm{CH_{3}COOH}$.

2 HCl has the lower pH and the faster reaction: it is a strong acid, so the same concentration gives many more $\mathrm{H^{+}}$ ions.

3 NaOH: it is a strong base, so it gives many more $\mathrm{OH^{-}}$ ions than weakly ionised $\mathrm{NH_{4}OH}$.

4 Strength is how completely an acid gives ions; concentration is the amount per volume. Example: very dilute $\mathrm{HCl}$ is a dilute strong acid; glacial ethanoic acid is a concentrated weak acid.

5 The pH increases (about 10 times fewer ions per volume), but it stays below 7, so the solution is still acidic.

6 No. The solution can never have fewer $\mathrm{H_{3}O^{+}}$ ions than pure water, so its pH only approaches 7 from below.

7 Dilution lowers the concentration, not the strength: the base is still strong. Its pH falls towards 7 but remains above 7.

8 HCl splits completely into ions, ethanoic acid only slightly; more ions carry more current.

III

pH in everyday life.

1 What causes acidity (indigestion)? Name two antacids and say what they do.
2 Aluminium hydroxide is used in an antacid. Write its balanced equation with hydrochloric acid.
3 With $\mathrm{Mg}=24$, $\mathrm{O}=16$, $\mathrm{H}=1$, $\mathrm{Cl}=35.5\ \mathrm{u}$, what mass of $\mathrm{HCl}$ is neutralised by $1.16\ \mathrm{g}$ of $\mathrm{Mg(OH)_{2}}$?
4 With $\mathrm{Na}=23$, $\mathrm{H}=1$, $\mathrm{C}=12$, $\mathrm{O}=16$, $\mathrm{Cl}=35.5\ \mathrm{u}$, what mass of $\mathrm{NaHCO_{3}}$ is needed to neutralise $0.73\ \mathrm{g}$ of $\mathrm{HCl}$?
5 At what pH does tooth decay begin? Why does it begin, and how can it be prevented?
6 Why is toothpaste usually basic?
7 Name the acid in a bee sting. Why is baking soda applied, and why is sodium hydroxide not?
8 The soil in a field has pH $5.2$. Name two substances that can be added, say why, and write a balanced equation for quicklime with nitric acid.
9 Applied. What is acid rain? Write a balanced equation for the action of sulphuric acid in acid rain on a marble statue ($\mathrm{CaCO_{3}}$).

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1 Excess hydrochloric acid in the stomach. Antacids such as milk of magnesia and baking soda are mild bases that neutralise it.

2 $$\mathrm{Al(OH)_{3}(s) + 3HCl(aq) \rightarrow AlCl_{3}(aq) + 3H_{2}O(l)}$$ Check: Al $1=1$; O $3=3$; H $3+3=6$; Cl $3=3$. ✓

3 $\mathrm{Mg(OH)_{2}}=58\ \mathrm{u}$ reacts with $2\,\mathrm{HCl}=73\ \mathrm{u}$. Mass $=1.16\times\tfrac{73}{58}=1.46\ \mathrm{g}$.

4 $\mathrm{NaHCO_{3}}=84\ \mathrm{u}$ per $\mathrm{HCl}=36.5\ \mathrm{u}$. Mass $=0.73\times\tfrac{84}{36.5}=1.68\ \mathrm{g}$.

5 Below pH 5.5. Bacteria turn sugar and food particles into acids that corrode the enamel. Brush after meals and use a basic toothpaste.

6 A basic toothpaste neutralises the acid in the mouth and keeps the pH above 5.5.

7 Methanoic acid. Baking soda is a mild base that neutralises it; sodium hydroxide is a strong, corrosive base that would burn the skin.

8 Quicklime, slaked lime or chalk: they are bases that neutralise soil acid and raise the pH. $$\mathrm{CaO(s) + 2HNO_{3}(aq) \rightarrow Ca(NO_{3})_{2}(aq) + H_{2}O(l)}$$ Check: Ca $1=1$; N $2=2$; H $2=2$; O $1+6=6+1$. ✓

9 Rain of pH below 5.6, due to dissolved oxides of sulphur and nitrogen. $$\mathrm{CaCO_{3}(s) + H_{2}SO_{4}(aq) \rightarrow CaSO_{4}(aq) + H_{2}O(l) + CO_{2}(g)}$$ Check: Ca $1=1$; C $1=1$; O $3+4=4+1+2$; H $2=2$; S $1=1$. ✓

Neutralisation curves.

1 $25.0\ \mathrm{mL}$ of $0.10\ \mathrm{mol/L}$ HCl is titrated with $0.10\ \mathrm{mol/L}$ NaOH. What volume of NaOH reaches the end point, and what is the pH there?
2 The same HCl is titrated with $0.20\ \mathrm{mol/L}$ NaOH. What volume is needed?
3 State the nature of the flask contents (acidic, neutral, basic) before, at and after the end point in Question 1.
4 Phenolphthalein is used in the flask in Question 1. What colour change marks the end point?
5 Challenging. Ethanoic acid is titrated with sodium hydroxide. Why is phenolphthalein a better indicator than methyl orange?
6 Challenging. $20\ \mathrm{mL}$ of $0.10\ \mathrm{mol/L}$ HCl and $20\ \mathrm{mL}$ of $0.10\ \mathrm{mol/L}$ ethanoic acid are each titrated with $0.10\ \mathrm{mol/L}$ NaOH. Compare (a) the starting pH, (b) the volume of NaOH needed.

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1 Same concentration, same amount of reaction ($1\,\mathrm{NaOH}$ per $1\,\mathrm{HCl}$): $25.0\ \mathrm{mL}$. The solution contains only NaCl and water, so pH $7$.

2 The NaOH is twice as concentrated, so half the volume: $25.0\times\tfrac{0.10}{0.20}=12.5\ \mathrm{mL}$.

3 Before: acidic (acid in excess). At: neutral. After: basic (excess NaOH).

4 Colourless to a faint pink that stays.

5 At the end point the solution (sodium ethanoate) is slightly basic, about pH 8.7. Phenolphthalein changes colour around pH 8–10, so it changes right there; methyl orange has already turned yellow at pH about 4.4, long before the end point.

6 (a) HCl has the lower starting pH (about 1, against about 3 for ethanoic acid). (b) Both need $20\ \mathrm{mL}$ of NaOH: the amount of acid is the same, only the strength differs.

🎯 Exam corner BoardBoard · A–RBoard · Case-basedNEET

E1. Board · 1 mark Tooth enamel starts to corrode when the pH in the mouth falls below (A) $7.0$ (B) $6.5$ (C) $5.5$ (D) $4.0$

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Idea Enamel (calcium phosphate) is attacked by acids produced by bacteria once the pH of the mouth drops to a critical value.

Answer (C) $5.5$.

E2. Board · A–R Assertion (A): Adding a very large amount of water to dilute hydrochloric acid makes its pH greater than 7. Reason (R): Dilution decreases the concentration of hydronium ions. 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.

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Idea R is true: dilution lowers $[\mathrm{H_{3}O^{+}}]$, so the pH rises. But water itself is neutral, so the pH only approaches 7 and never exceeds it; A is false.

Answer (D).

E3. Board · 3 marks A boy is stung by a bee. (a) Which acid is injected? (b) Name a household substance that gives relief, and give a balanced equation using it. (c) Why should he not use sodium hydroxide solution?

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(a) Methanoic (formic) acid, $\mathrm{HCOOH}$.

(b) Baking soda paste: $$\mathrm{HCOOH(aq) + NaHCO_{3}(s) \rightarrow HCOONa(aq) + H_{2}O(l) + CO_{2}(g)}$$

Answer (c) Sodium hydroxide is a strong, corrosive base and would itself burn the skin.

E4. Board · Case-based The table gives the pH of four samples. P: stomach fluid, 1.8. Q: milk, 6.5. R: blood, 7.4. S: toothpaste, 9.0. (i) Which sample is the most acidic, and what colour does universal indicator show in it? (2 marks) (ii) Which sample or samples would turn phenolphthalein pink? (1 mark) (iii) Explain, with reference to the pH, why a person who brushes with S after meals is less likely to get cavities. (1 mark)

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(i) P has the lowest pH, so it is the most acidic (the acid is $\mathrm{HCl}$); universal indicator is red.

(ii) Phenolphthalein turns pink above pH $8.2$: only S (pH $9.0$). R at $7.4$ is too low.

Answer (iii) The basic toothpaste neutralises acid made by bacteria in the mouth and keeps the pH above $5.5$, so enamel is not corroded.

E5. NEET Foundation. A strong acid solution has pH $3$. When it is diluted to ten times its volume with water, the pH becomes about (A) $2$ (B) $3.5$ (C) $4$ (D) $13$

💡 Show solution▶

Idea With $\mathrm{pH}=-\log[\mathrm{H^{+}}]$, ten times less concentrated means $[\mathrm{H^{+}}]$ falls from $10^{-3}$ to $10^{-4}\ \mathrm{mol/L}$. A rise of one pH unit; the result stays below 7.

Answer (C) $4$.

E6. Board · 2 marks A farmer finds that the soil of his field has a pH of $4.5$ and crops grow badly. Suggest a treatment and write a balanced equation for the action of slaked lime on sulphuric acid in the soil.

💡 Show solution▶

Idea The soil is acidic, so add a base such as lime to neutralise the acid and raise the pH towards 7.

Answer $$\mathrm{Ca(OH)_{2}(s) + H_{2}SO_{4}(aq) \rightarrow CaSO_{4}(s) + 2H_{2}O(l)}$$ Check: Ca $1=1$; S $1=1$; O $2+4=4+2$; H $2+2=4$. ✓

🔬 Research–Teaching NexusResearch-tutored · optional, two or three evenings

Research question. How does the pH of the water and soil in your neighbourhood vary from place to place, and is there a pattern linked to what grows there or to the weather?

Why it matters. Water and soil quality are monitored all over the world. Scientists who study acid rain have tracked the pH of rain and lake water for decades, and in several countries lakes and forests recovered after sulphur dioxide emissions were reduced. In India, soil health cards tell farmers the pH of their land so that they can choose crops and decide how much lime to add. Careful, repeated measurements with simple tools can reveal real patterns.

What to do.

Choose six sampling sites: for example tap water, a pond or river, rain water collected in a clean bowl, garden soil, a flower bed and a roadside patch of soil.
For soil, stir one spoon of soil in five spoons of distilled water, let it settle and use the clear liquid. Use pH paper, or red cabbage juice compared with a colour chart you make using lemon juice, water and baking soda solution.
Test each sample three times and record the mean pH to one decimal place. Photograph the colours against white paper.
Repeat after rain, and again after a dry spell.
Plot a bar chart of pH by site and note which plants grow well at each place.

Tools: pH paper or red cabbage juice, distilled water, small clear cups, spoons, a phone camera and a notebook.

Hand in: A table of readings, the bar chart, and a paragraph that answers the research question and says how reliable your colour-matching method is.

Going further. Add a pinch of baking soda to a cup of acidic rain water, drop by drop. How many drops are needed to reach pH 7? This is a very simple model of how lime treats acidic soil and lakes.

Instructor note: expect tap water around pH 7–8, fresh rain around 5.5–6.5, soil between 5.5 and 8 depending on the region (more acidic in high-rainfall areas, more basic in dry areas with lime-rich soil). Colour matching is rough (±0.5 pH unit), so look for differences of a pH unit or more. Common errors: dirty cups, tap water used to extract soil instead of distilled water, reading the colour before it settles.

🧪 Self-check quiz

0/5 correct

Q1. The pH of pure water at room temperature is

Q2. Universal indicator turns red in a solution of pH

Q3. Which of these is a weak acid?

Q4. Dilute hydrochloric acid is diluted again and again with a large amount of water. Its pH

Q5. The pH of the rain is found to be 4.5. This rain is

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