Grade 5 (age 10-11)

grade 5 Acid, Base and Indicators

Vinegar and lemon juice are sour enough to make your mouth pucker, and soapy water feels slippery between your fingers. Solutions with such different properties are grouped as acidic, neutral, or basic; tasting or touching them is dangerous, so we use indicators that show the property with a colour. How strongly acidic or basic a solution is, is given as a pH from 0 to 14: 7 is neutral, below 7 is more strongly acidic the smaller it gets, and above 7 is more strongly basic the bigger it gets. Move the pH with the slider to see everyday solutions at that pH, and compare how litmus, phenolphthalein, and BTB solution change colour.

What are acids and bases?

pH7

🧪 Acids and bases

①Acidic solutions: many taste sour, like vinegar or lemon juice

②Basic solutions: many feel slippery, like soapy water or detergent

③Neutral solutions: neither acidic nor basic, like pure water

④The further below 7 the pH, the more strongly acidic; the further above 7, the more strongly basic

⑤In experiments, never taste solutions or touch them with bare hands

Finding out with indicators

🔬 What is an indicator?

①An indicator is a substance that changes colour depending on whether a solution is acidic or basic

②Litmus paper: blue paper turns red in an acid, and red paper turns blue in a base

③Phenolphthalein solution: colourless in acidic and neutral solutions, red (pink) in a basic one

④BTB solution: yellow in an acid, green when neutral, and blue in a base

Acids and bases in daily life

Acidic: vinegar, lemon juice, fizzy drinks / basic: soapy water, baking soda water, antacids

There are plenty of acidic and basic substances around us

💡 Acids and bases in daily life

①Antacids taken for heartburn are basic, so they weaken the acid in stomach juice

②The substances that make fish smell are basic, so a squeeze of lemon juice (acidic) reduces the smell

③Lime (basic) is spread on soil that has turned acidic, to make it better for farming

④Toilet cleaners contain acids, and drain cleaners for blocked pipes contain bases

Summary

The essentials of acids and bases

Acidic (pH < 7) / neutral (pH = 7) / basic (pH > 7)

Indicator colours tell acids and bases apart

🎯 Key points

①Solutions can be grouped as acidic, neutral, or basic

②A pH below 7 is acidic, 7 is neutral, and above 7 is basic

③The colour of an indicator reveals a solution's property

④Phenolphthalein turns red only in a base, and BTB turns yellow · green · blue in an acid · neutral · base

⑤Mixing an acidic solution with a basic one weakens both

Examples and Unit Test

Example 1

You add phenolphthalein solution to a solution and it turns red. Is the solution acidic, neutral, or basic?

Phenolphthalein is colourless in acidic and neutral solutions and turns red only in basic ones.

It turned red, so the solution is basic.

▸ Basic

Phenolphthalein cannot tell acidic from neutral, so BTB solution is used alongside it.

Example 2

A solution has a pH of 2. What happens if you dip blue litmus paper in it, and what is an example of such a solution?

A pH of 2 is below 7, so the solution is acidic.

Acidic solutions turn blue litmus paper red. Stomach juice and dilute hydrochloric acid are this strongly acidic.

▸ The blue litmus paper turns red / e.g. stomach juice, dilute hydrochloric acid

Vinegar and lemon juice are about pH 3, slightly weaker acids than pH 2.

Unit-test style

Which statement about acids, bases, and indicators is correct?

①Phenolphthalein turns red in an acid

②A pH below 7 is acidic

③BTB solution turns yellow in a base

④Soapy water is an acidic solution

⑤Litmus paper does not change colour

▸ ② A pH below 7 is acidic

pH 7 is neutral, below 7 is acidic, and above 7 is basic.

Phenolphthalein turns red in a base (①), BTB is blue in a base (③), soapy water is basic (④), and litmus paper changes colour in acids and bases (⑤).

← Previous

Dissolving and Solutions

Light and Lenses

Last updated 2026-10-17