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Base

A base is a substance that gives hydroxide ions in water and shows basic behaviour such as turning red litmus blue.

In one line: Base: red to blue, pH rises.

Practice This Concept

Understand it

Bases are the opposite side of the acid-base pair in school chemistry.
They usually have pH above 7 and often feel slippery or soapy when dissolved in water.
Common examples include sodium hydroxide and soap solution.
Bases are important in cleaning, manufacture, and neutralisation reactions.

Example

Soap solution is basic. It turns red litmus blue and has a pH above 7.

Rule to remember

Bases give OH^- ions in aqueous solution.

In school tests, bases turn red litmus blue and usually have pH above 7.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

A student tests soap solution with red litmus.

1The litmus turns blue, so the sample is identified as a base.
2This matches the basic behaviour seen in common indicators.

Method you can reuse

1First check whether the sample turns red litmus blue or has a pH above 7.
2Then connect that observation with the idea of hydroxide ions in water.
3Finally name the sample as a base.

Visual tip

A labelled bottle or beaker sketch can show a red litmus strip changing to blue when dipped into a basic solution.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

A clear solution turns red litmus blue and has pH 12. What does this show?

Think first, then show answer

The solution is basic because it turns red litmus blue and a pH of 12 clearly lies on the basic side of the scale.

Common wrong answer

A common wrong answer is to call a slippery cleaning liquid an acid.

Why it loses marks: Slipperiness does not mean acidity; litmus and pH must be checked.

Don't mix this up

Students often think every slippery liquid is an acid or that bases only appear in laboratories. Many everyday cleaning solutions are basic.

Exam tip

When you write about a base, mention hydroxide ions, litmus change, and one example together for a strong answer.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Questions usually ask for the correct litmus change, the meaning of pH above 7, or the identification of a basic household solution.

1
Enough for 1 mark
A base is a substance that gives hydroxide ions in water and shows basic behaviour such as turning red litmus blue.
2
Add for 2 marks
Example: Soap solution is basic. It turns red litmus blue and has a pH above 7.
3
Add for 3 marks
Bases are the opposite side of the acid-base pair in school chemistry.

MCQ Quiz

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