CDS Chemistry · Acids, Bases and Salts

The pH Scale and Indicators

What pH measures, the 0 to 14 scale, the pH of everyday substances in order, and the indicators that show it.

Why this matters

Seven CDS questions, all EASY or MODERATE. Two ask what a pH value means, three ask for the pH of real substances (in order, or which is a mild base), and two ask about indicators.

Concept 1 of 3: The pH scale

pH counts hydrogen ions backwards. More H⁺ means a LOWER pH. Below 7 is acidic, 7 is neutral, above 7 is basic. Each step of one unit is a factor of ten in H⁺.

Definition

The scale:

pH is the negative logarithm of the hydrogen-ion concentration.
It runs from 0 to 14 at room temperature: below 7 acidic, 7 neutral, above 7 basic.
pH 0 means [H⁺] = 1 mol/L: a highly acidic solution. pH 14 is highly basic.
One unit of pH is a tenfold change in [H⁺]: pH 3 has ten times the H⁺ of pH 4.

Definition of pH

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

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

Worked example

A solution has a hydrogen-ion concentration of

10^{-3}

mol/L. Find its pH and say whether it is acidic or basic.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

CDS · 2020 · CDS (I) 2020 — General Knowledge · Q4Easy

Example 1 · Acids, Bases and Salts · pH Scale and Indicators

A solution having pH equal to zero is known as

pH 0 is the most acidic, not neutral

pH 0 means [H⁺] = 1 mol/L, the strong-acid end of the scale. Neutral is 7, not 0.

Drill 1 more on the ph scale

Concept 2 of 3: pH of everyday substances

CDS asks you to put real substances in order of pH, or to say whether something is a mild or strong base. Learn a few landmarks and the rest falls into place: stomach acid at the bottom, blood just above 7, milk of magnesia around 10.

Definition

The landmarks:

Gastric juice about 1.5–2, lemon juice about 2.2, vinegar about 3, coffee about 5.
Pure water 7. Blood about 7.4, slightly basic.
Milk of magnesia (Mg(OH)₂) about 10: a mild base, used as an antacid because it neutralises stomach acid without harming the stomach.
Sodium hydroxide solution 13–14: a strong base.
Acidic soil is corrected by adding a base such as lime or dolomite (calcium magnesium carbonate), which raises the pH.

Substance	Approx. pH	Nature
Gastric juice	1.5–2	Strongly acidic
Lemon juice	2.2	Acidic
Coffee	5	Weakly acidic
Pure water	7	Neutral
Blood	7.4	Slightly basic
Milk of magnesia	10	Mild base (antacid)Q CDS 2022 (II): milk of magnesia is a mild base.
Sodium hydroxide solution	13–14	Strong base

CDS 2023 (I) order of increasing pH: lemon juice < coffee < blood < milk of magnesia.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

CDS · 2023 · CDS (I) 2023 — General Knowledge · Q51Moderate

Example 2 · Acids, Bases and Salts · pH Scale and Indicators

Which one of the following is the correct order of pH for the given substances?

Liming soil RAISES its pH

Lime and dolomite are bases. Spreading them neutralises soil acidity, so the pH goes up. They do not add nitrogen or phosphorus.

Drill 2 more on ph of everyday substances

Concept 3 of 3: Acid-base indicators

An indicator is a dye whose colour depends on pH. Simple indicators only say acid or base. Universal indicator is a mixture, so it shows a different colour at almost every pH.

Definition

The indicators asked about:

Litmus is a natural dye from lichens: red in acid, blue in base.
Phenolphthalein: colourless in acid, pink in base.
Methyl orange: red in acid, yellow in base.
Turmeric: yellow, turning reddish-brown in a base such as soap.
Universal indicator is a mixture of several indicators. It shows different colours at different H⁺ concentrations, so it gives the pH. It is not used to find the end point of a titration, which needs one indicator with a sharp change.

Indicator	In acid	In base
Litmus (from lichen)	Red	BlueQ CDS 2022 (I): litmus comes from a lichen.
Phenolphthalein	Colourless	Pink
Methyl orange	Red	Yellow
Turmeric	Yellow	Reddish-brown
Universal indicator	Red to yellow as pH rises	Green at 7, then blue to violetQ CDS 2021 (II): it is a mixture and shows many colours, but it is not used for titrations.

Practice this concept4 quick reps

The same idea in a real exam question:

CDS · 2021 · CDS (II) 2021 — General Knowledge · Q7Moderate

Example 3 · Acids, Bases and Salts · pH Scale and Indicators

Which of the following statements about universal indicator is/are true ? 1. It is a mixture of several indicators. 2. It shows different colours at different concentrations of hydrogen ions in solution. 3. It helps to determine the strength of given acid and base in titration. Select the correct answer using the code given below :

Universal indicator is not a titration indicator

Universal indicator changes colour gradually across the whole scale, so it cannot mark a sharp end point. Titrations use one indicator such as phenolphthalein or methyl orange.

Drill 1 more on acid-base indicators

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)

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

Reference tables (2)

pH of everyday substances7 rows

Substance	Approx. pH	Nature
Gastric juice	1.5–2	Strongly acidic
Lemon juice	2.2	Acidic
Coffee	5	Weakly acidic
Pure water	7	Neutral
Blood	7.4	Slightly basic
Milk of magnesia	10	Mild base (antacid)Q CDS 2022 (II): milk of magnesia is a mild base.
Sodium hydroxide solution	13–14	Strong base

CDS 2023 (I) order of increasing pH: lemon juice < coffee < blood < milk of magnesia.

Acid-base indicators5 rows

Indicator	In acid	In base
Litmus (from lichen)	Red	BlueQ CDS 2022 (I): litmus comes from a lichen.
Phenolphthalein	Colourless	Pink
Methyl orange	Red	Yellow
Turmeric	Yellow	Reddish-brown
Universal indicator	Red to yellow as pH rises	Green at 7, then blue to violetQ CDS 2021 (II): it is a mixture and shows many colours, but it is not used for titrations.

Watch out for (3)

pH 0 is the most acidic, not neutral
→ The pH scale
Liming soil RAISES its pH
→ pH of everyday substances
Universal indicator is not a titration indicator
→ Acid-base indicators

Test yourself on Acids, Bases and Salts

15 past CDS questions from this chapter, timed at 15 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.

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