The pH scale and indicators

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Lesson notes

The pH Scale

The **pH scale** measures how acidic or alkaline a substance is.
pH values range from 0 (strong acid) to 14 (strong alkali).
A **neutral** substance has a pH of **7** (e.g., pure water).
Substances with pH **less than 7** are **acids**.
Substances with pH **greater than 7** are **alkalis**.

Indicators

**Indicators** change colour to show whether a substance is acidic or alkaline.
**Blue litmus paper** turns **red** in acids.
**Red litmus paper** turns **blue** in alkalis.
**Phenolphthalein** is **colourless** in acids and neutral, but turns **pink** in alkalis.
**Methyl orange** is **red** in acids and **yellow** in alkalis.
**Red cabbage** indicator gives a range of colours: pink/red in acids, purple in neutral, blue/green in alkalis.

Universal Indicator

**Universal indicator** is a mixture of dyes that changes colour gradually across the pH scale.
It gives a distinct colour for each pH value, allowing estimation of pH.
For example, it turns **red** at pH 0–2, **orange** at pH 3–4, **yellow** at pH 5–6, **green** at pH 7, **blue** at pH 8–10, and **purple** at pH 11–14.

Electronic pH Meter

An **electronic pH meter** measures pH directly and gives a numerical reading.
It is more **accurate** than using an indicator because it does not rely on colour interpretation.

Common Acids and Alkalis

Common **acids** include **lemon juice** (citric acid) and **stomach acid** (hydrochloric acid).
Common **alkalis** include **soap** and **bleach**.

Particle arrangement in solids, liquids, and gases. Not directly about pH, but useful for understanding states of matter in chemistry.

Slides

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Practice questions

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1.What colour does blue litmus paper turn in an acid?
Easy
- ARed
- BBlue
- CGreen
- DYellow
2.What is the pH of a neutral substance?
Easy
- A7
- B0
- C14
- D1
3.Which indicator turns from colourless to pink in an alkali?
Easy
- APhenolphthalein
- BMethyl orange
- CRed litmus
- DUniversal indicator
4.A student adds red litmus paper to a solution and it turns blue. What does this indicate about the solution?
Medium
- AIt is an alkali
- BIt is an acid
- CIt is neutral
- DIt is pure water
5.Methyl orange is red in acids and yellow in alkalis. What colour would methyl orange be in a solution of pH 9?
Medium
- AYellow
- BRed
- COrange
- DPurple
6.Which of the following is an advantage of using an electronic pH meter over an indicator?
Medium
- AIt gives a numerical pH value directly
- BIt is cheaper
- CIt changes colour
- DIt can only be used for acids
7.Universal indicator turns orange when added to a substance. What is the approximate pH of the substance?
Hard
- A4
- B7
- C9
- D12
8.A student has four unlabelled solutions: lemon juice, soap, pure water, and vinegar. Which solution would turn universal indicator green?
Hard
- APure water
- BLemon juice
- CSoap
- DVinegar

Revision notes

## pH Scale and Indicators - **pH scale**: measures how acidic or alkaline a substance is. - pH < 7: acidic - pH = 7: neutral (e.g., pure water) - pH > 7: alkaline ### Common Indicators | Indicator | Acid | Alkali | Neutral | |-----------|------|--------|---------| | **Blue litmus paper** | Turns red | Stays blue | Stays blue | | **Red litmus paper** | Stays red | Turns blue | Stays red | | **Phenolphthalein** | Colourless | Turns pink | Colourless | | **Methyl orange** | Red | Yellow | Orange (approx.) | | **Red cabbage** | Pink/red | Blue/green | Purple | | **Universal indicator** | Red (low pH) → yellow/green → blue/purple (high pH) | | Green (pH 7) | - **Universal indicator** is a mixture of dyes that gives a distinct colour for each pH. - **Electronic pH meter** gives a numerical pH reading – no colour interpretation needed. ### Key Facts - Acids: pH < 7; Alkalis: pH > 7; Neutral: pH = 7. - Examples of acids: lemon juice, stomach acid, vinegar. - Examples of alkalis: soap, bleach, oven cleaner. - Neutral substance: pure water (pH 7). ### Quick Check - Blue litmus in acid → red. - Red litmus in alkali → blue. - Phenolphthalein in alkali → pink; in acid → colourless. - Methyl orange in acid → red; in alkali → yellow. - Universal indicator in pure water → green. - Universal indicator orange → pH around 4 (acidic).

pH scale: A scale from 0 to 14 that measures how acidic or alkaline a substance is.
Acid: A substance with a pH less than 7.
Alkali: A substance with a pH greater than 7.
Neutral: A substance with a pH of exactly 7, e.g., pure water.
Indicator: A substance that changes colour to show whether a substance is acidic or alkaline.
Litmus paper: An indicator that turns red in acid and blue in alkali. Blue litmus turns red in acid; red litmus turns blue in alkali.
Phenolphthalein: An indicator that is colourless in acid and turns pink in alkali.
Methyl orange: An indicator that is red in acid and yellow in alkali.
Universal indicator: A mixture of dyes that changes colour gradually across the pH scale, giving a distinct colour for each pH.
Electronic pH meter: A device that measures pH and gives a numerical reading, avoiding colour interpretation.

Flashcards

What is the pH range for acidic substances?

pH less than 7.

What is the pH range for alkaline substances?

pH greater than 7.

What is the pH of a neutral substance?

pH 7.

Give an example of a neutral substance.

Pure water.

What colour does blue litmus paper turn in an acid?

Red.

What colour does red litmus paper turn in an alkali?

Blue.

What colour is phenolphthalein in an acid?

Colourless.

What colour does phenolphthalein turn in an alkali?

Pink.

What colour does red cabbage indicator turn in an acid?

Pink/red.

What colour does red cabbage indicator turn in a neutral substance?

Purple.

What colour does red cabbage indicator turn in an alkali?

Blue or green.

What colour is methyl orange in an acid?

Red.

What colour is methyl orange in an alkali?

Yellow.

What is universal indicator?

A mixture of dyes that changes colour gradually based on pH.

What is an advantage of using an electronic pH meter over an indicator?

It gives a numerical pH value, so you don't have to interpret a colour.

True or false: A substance with pH 4 is alkaline.

False (pH 4 is acidic).

True or false: Blue litmus paper turns red in lemon juice.

True (lemon juice is acidic).

True or false: Red litmus paper turns blue in stomach acid.

False (stomach acid is acidic, so red litmus stays red).

True or false: Phenolphthalein turns pink in vinegar.

False (vinegar is acidic, phenolphthalein stays colourless).

True or false: Universal indicator turns green in pure water.

True (pure water is neutral, pH 7, universal indicator is green).

True or false: Methyl orange turns red in milk.

False (milk is slightly acidic, methyl orange would be red only in strong acid; milk pH ~6.6, methyl orange is orange/yellow).

Give two examples of common acids.

Examples: lemon juice, vinegar (or stomach acid).

Give two examples of common alkalis.

Examples: soap, baking soda (or any alkali).

If universal indicator turns orange, what might the pH be?

Around pH 4-5 (orange corresponds to acidic).

If a substance is neutralised, what colour will universal indicator turn?

Green (pH 7).

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