Acids, bases and alkalis

Learn it by playing

Answer these questions to earn energy, then fish and explore. No account needed.

For teachers: ready-to-use lesson slides, revision notes, diagrams for Acids, bases and alkalis (KS3 Science, Chemistry) — use them in your lesson, or run the topic as an interactive class activity your students play as a live game.

Lesson notes

Acids and Their Properties

**Acids** are a group of chemicals with a sharp taste, found in foods like lemons and vinegar.
Our stomach contains **hydrochloric acid** to help digest food.
Strong acids can be hazardous and labelled with symbols: **irritant**, **toxic**, or **corrosive**.
Common lab acids include **hydrochloric acid (HCl)**, **sulfuric acid (H₂SO₄)**, and **nitric acid (HNO₃)**.

The pH Scale

The **pH scale** ranges from 0 to 14 and measures how acidic or alkaline a substance is.
**Acids** have a pH **less than 7**; lower pH means stronger acid.
A substance with pH **7** is **neutral** (e.g., pure water).
**Alkalis** have a pH **greater than 7**; higher pH means stronger alkali.

Bases and Alkalis

A **base** is a substance that neutralises an acid.
If a base is **soluble** in water, it is called an **alkali**.
Common alkalis include **soap** and **bleach**.
Lab alkalis include **sodium hydroxide (NaOH)**, **potassium hydroxide (KOH)**, and **calcium hydroxide (Ca(OH)₂)**.

Neutralisation

**Neutralisation** is a chemical reaction where an acid and a base react to produce a neutral solution (pH 7).
It raises the pH of an acid or lowers the pH of an alkali to 7.

Hazard Symbols

**Irritant** – causes skin/eye irritation.
**Toxic** – harmful to health.
**Corrosive** – damages materials it contacts.

Particle arrangement in solids, liquids, and gases. Not directly about acids/alkalis but useful for understanding solubility of bases.

Slides

Step through every slide for this topic — plus flashcards and revision notes — with a free account.

Practice questions

Free preview — 8 of 40 questions. Sign up to see them all.

1.Which of the following is true about acids?
Easy
- ASome foods are acidic.
- BAcids have a pH greater than 7.
- CAcids are always safe to handle.
- DAcids have a pH of exactly 7.
2.What is the pH of a neutral substance?
Easy
- A0
- B7
- C14
- D1
3.What does a base do to an acid?
Easy
- ANeutralises it
- BMakes it stronger
- CIncreases its pH above 7
- DTurns it into an alkali
4.Which of the following is an example of an alkali?
Medium
- AHydrochloric acid
- BPure water
- CSodium hydroxide
- DLemon juice
5.What is the pH range for acids?
Medium
- A0 to 6
- B7 to 14
- C0 to 14
- D8 to 14
6.Which of the following is a hazard symbol associated with strong acids?
Medium
- AFlammable
- BCorrosive
- CRadioactive
- DOxidising
7.A student tests a substance with pH paper and gets a pH of 9. What is the substance?
Hard
- AA strong acid
- BA weak acid
- CA weak alkali
- DA strong alkali
8.What is the difference between a base and an alkali?
Hard
- AAn alkali is a soluble base.
- BA base is a soluble alkali.
- CThey are the same thing.
- DBases have pH less than 7, alkalis have pH greater than 7.

Revision notes

## Acids, Bases and Alkalis - **Acids**: pH < 7; strong acids have lower pH (e.g., pH < 3). Examples: hydrochloric acid (HCl), sulfuric acid (H₂SO₄), nitric acid (HNO₃). Found in lemons, vinegar, stomach acid, batteries. - **Hazard symbols**: strong acids may be **irritant**, **toxic**, or **corrosive**. - **Neutral**: pH = 7; pure water is neutral. - **Base**: substance that neutralises an acid (neutralisation reaction raises pH to 7). - **Alkali**: a base that is **soluble** in water; pH > 7. Strong alkalis have higher pH. Examples: sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide (Ca(OH)₂). Common alkalis: soap, bleach. Alkalis can also be irritant, toxic, corrosive. - **pH scale**: 0 (strong acid) to 14 (strong alkali); 7 is neutral. ## Key Points - Acids: pH < 7; alkalis: pH > 7. - Neutralisation: acid + base → neutral (pH 7). - Soluble base = alkali.

Acid: A substance with a pH less than 7. Strong acids have lower pH values.
Base: A substance that neutralises an acid.
Alkali: A base that is soluble in water; has a pH greater than 7.
pH scale: A scale from 0 to 14 used to show how acidic or alkaline a substance is.
Neutral: A substance with a pH of 7, e.g., pure water.
Neutralisation: A chemical reaction where an acid is neutralised by a base, raising the pH to 7.
Irritant: A hazard label meaning can cause irritation to skin and eyes.
Toxic: A hazard label meaning can cause harmful health effects.
Corrosive: A hazard label meaning can damage substances it contacts.

Flashcards

What is the pH range of acids?

Acids have a pH of less than 7.

What is the pH of a neutral substance?

Neutral substances have a pH of 7.

What is the pH of pure water?

Pure water is neutral and has a pH of 7.

What is a base?

A base is a substance that neutralises an acid.

What is an alkali?

An alkali is a base that is soluble in water.

What is the pH range of alkalis?

Alkalis have a pH greater than 7.

What is neutralisation?

Neutralisation is a chemical reaction where an acid is neutralised by a base, raising the pH to 7.

Name three strong acids used in a lab.

Hydrochloric acid (HCl), sulfuric acid (H2SO4), and nitric acid (HNO3).

Name three alkalis used in a lab.

Sodium hydroxide (NaOH), potassium hydroxide (KOH), and calcium hydroxide (Ca(OH)2).

What hazard symbols might a strong acid have?

Irritant, toxic, and corrosive.

What pH value does a strong acid typically have?

A strong acid typically has a pH of less than 3.

Give two examples of common acids found in everyday life.

Lemons (citric acid) and vinegar (acetic acid).

Give two examples of common alkalis found in everyday life.

Soap and bleach.

What is the pH of a weak alkali compared to a strong alkali?

A weak alkali has a lower pH than a strong alkali, but still above 7.

What is the pH of a weak acid compared to a strong acid?

A weak acid has a higher pH than a strong acid, but still below 7.

True or false: Acids have a pH of less than 8.

False. Acids have a pH of less than 7.

True or false: If a base is insoluble in water, it is called an alkali.

False. An alkali is a base that is soluble in water.

What is the chemical formula for hydrochloric acid?

HCl

What is the chemical formula for sulfuric acid?

H2SO4

What is the chemical formula for nitric acid?

HNO3

What is the chemical formula for sodium hydroxide?

NaOH

What is the chemical formula for potassium hydroxide?

KOH

What is the chemical formula for calcium hydroxide?

Ca(OH)2

Unlock all 40 questions, flashcards & more

Create a free account to see every question, the slides, flashcards and revision notes for this topic.

Past papers

Past-paper practice for this topic is coming soon.

Coming soon