NDA Chemistry · Acids, Bases and Salts

The pH Scale and Common Substances

What pH measures (hydrogen-ion concentration), the 0–14 range, and the pH values of everyday solutions — from gastric juice to milk of magnesia.

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Why this matters

Eight PYQs, split between two ideas: how the pH scale works (lower pH = more H+ = more acidic) and the memorised pH values of common substances. The single most-tested trap is the inverse relationship — higher H+ means LOWER pH, not higher.

Concept 1 of 3

What the pH scale measures

Intuition

pH is a number from 0 to 14 that says how acidic or basic a solution is. The key relationship is inverse: the more hydrogen ions (H+) a solution has, the LOWER its pH. Below 7 is acidic, exactly 7 is neutral, above 7 is basic.

Definition

The pH scale, defined:

pH measures the hydrogen-ion (H+) concentration of a solution; the 'p' stands for the German 'potenz' (power).
The scale runs from 0 to 14: pH < 7 acidic, pH = 7 neutral, pH > 7 basic (alkaline).
The relationship is inverse — the higher the H+ concentration, the lower the pH.
pH is defined as the negative logarithm (base 10) of the H+ concentration.

Definition of pH

\text{pH} = -\log_{10}[\text{H}^{+}]

$[\text{H}^{+}]$ hydrogen-ion concentration (mol/L)

Worked example

Solution X has a higher hydrogen-ion concentration than solution Y. Which has the lower pH, and which is more acidic?

Practice this concept5 quick reps

From the bank · past-year question

Example 1Acids, Bases and SaltsEASY

Which one of the following statements is NOT correct?

[Q78 · Apr · 2025]

Higher H+ means LOWER pH

The statement 'the higher the hydrogen-ion concentration, the higher its pH' is NOT correct — it is the inverse. More H+ means a lower pH and a more acidic solution.

Concept 2 of 3

pH values of common substances

Intuition

The bank asks you to recall or rank the pH of everyday solutions. Gastric juice is strongly acidic (about pH 1.5), pure water is 7, milk of magnesia is basic (about pH 10). Learn the landmarks.

Definition

The pH landmarks the bank tests:

Gastric juice ≈ 1.5–2 — strongly acidic; the highest H+ concentration of common body/household fluids.
Lemon juice ≈ 2–3; vinegar ≈ 3 — acidic.
Pure water = 7 — neutral.
Human blood / body works in the narrow range 7.0–7.8 (slightly basic).
Milk of magnesia ≈ 10 — basic (an antacid).
Sodium hydroxide solution ≈ 13–14 — strongly basic.
Acid rain is rain with pH below 5.6 (normal rain is about 5.6 due to dissolved CO2).

Substance	Approx. pH	Nature
Gastric juice	1.5–2	Strongly acidic — highest H+ Gastric juice has the lowest pH and therefore the highest H+ concentration of the common options.
Lemon juice	2–3	Acidic
Pure water	7	Neutral
Human body / blood	7.0–7.8	Slightly basic (narrow range) The human body operates in the pH range 7.0–7.8 — the bank's answer.
Milk of magnesia	10	Basic (antacid) Milk of magnesia (magnesium hydroxide) has pH about 10.
Sodium hydroxide solution	13–14	Strongly basic
Acid rain	below 5.6	Acidic — rain turns acidic below pH 5.6 For rain to be called 'acid rain', its pH must fall below 5.6.

Practice this conceptself-check · 5 quick reps

From the bank · past-year question

Example 2Acids, Bases and SaltsMODERATE

Which one of the following gives the highest amount of hydrogen ions

\left(\text{H}^{+}\right)

[Q65 · Apr · 2018]

Acid rain threshold is pH 5.6, not 7

Normal rain is already slightly acidic (about pH 5.6) from dissolved CO2. Rain is only called acid rain when its pH falls BELOW 5.6 — not below 7.

Drill 3 more on ph values of common substances

Concept 3 of 3

Indicators and the acid-base nature of household items

Intuition

Some natural dyes change colour with pH — turmeric is the classic. The bank also asks whether everyday items are acidic or basic: toothpaste is basic (to neutralise mouth acid), and a salt like FeCl3 makes an acidic solution by hydrolysis.

Definition

Indicator and household-pH facts:

Turmeric is a natural indicator: yellow in neutral/acidic, reddish-brown in alkaline (soap). A turmeric stain scrubbed with soap then washed runs yellow → reddish-brown → yellow.
Toothpaste is basic — it neutralises the acid produced by mouth bacteria.
FeCl3 in water gives a solution with pH < 7 (acidic) — it is the salt of a strong acid (HCl) and a weak base (Fe(OH)3), so it hydrolyses to give an acidic solution.
Neutral salts like NaCl and KCl give pH ≈ 7; NaOH gives a basic solution.

Item / indicator	Behaviour
Turmeric stain + soap then water	Yellow → reddish-brown → yellow Turmeric goes reddish-brown in alkaline soap and back to yellow when the soap is washed away.
Toothpaste	Basic (neutralises mouth acid)
FeCl3 solution	pH < 7 (acidic, by hydrolysis) FeCl3 = strong-acid + weak-base salt, so it hydrolyses to an acidic solution (pH < 7).
NaCl, KCl solution	pH ≈ 7 (neutral)
NaOH solution	Basic (pH > 7)

Practice this concept4 quick reps

From the bank · past-year question

Example 3Acids, Bases and SaltsMODERATE

The solution of which one of the following will have pH less than 7?

[Q103 · Sep · 2018]

FeCl3 is acidic, not neutral

FeCl3 looks like a simple chloride salt but its solution is acidic (pH < 7) — it is the salt of strong acid HCl and weak base Fe(OH)3, so it hydrolyses. NaCl and KCl, in contrast, are neutral.

Drill 2 more on indicators and the acid-base nature of household items

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

What the pH scale measures
Definition of pH
$\text{pH} = -\log_{10}[\text{H}^{+}]$

Reference tables (2)

pH values of common substances7 rows

Substance	Approx. pH	Nature
Gastric juice	1.5–2	Strongly acidic — highest H+ Gastric juice has the lowest pH and therefore the highest H+ concentration of the common options.
Lemon juice	2–3	Acidic
Pure water	7	Neutral
Human body / blood	7.0–7.8	Slightly basic (narrow range) The human body operates in the pH range 7.0–7.8 — the bank's answer.
Milk of magnesia	10	Basic (antacid) Milk of magnesia (magnesium hydroxide) has pH about 10.
Sodium hydroxide solution	13–14	Strongly basic
Acid rain	below 5.6	Acidic — rain turns acidic below pH 5.6 For rain to be called 'acid rain', its pH must fall below 5.6.

Indicators and the acid-base nature of household items5 rows

Item / indicator	Behaviour
Turmeric stain + soap then water	Yellow → reddish-brown → yellow Turmeric goes reddish-brown in alkaline soap and back to yellow when the soap is washed away.
Toothpaste	Basic (neutralises mouth acid)
FeCl3 solution	pH < 7 (acidic, by hydrolysis) FeCl3 = strong-acid + weak-base salt, so it hydrolyses to an acidic solution (pH < 7).
NaCl, KCl solution	pH ≈ 7 (neutral)
NaOH solution	Basic (pH > 7)

Watch out for (3)

Higher H+ means LOWER pH
→ What the pH scale measures
Acid rain threshold is pH 5.6, not 7
→ pH values of common substances
FeCl3 is acidic, not neutral
→ Indicators and the acid-base nature of household items

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