high importancemedium difficulty6 min

PH scale

The pH scale is a number scale from 0 to 14 that shows how acidic, neutral, or basic a solution is.

In one line: pH low means acid, pH high means base.

Practice This Concept

Understand it

The pH scale helps compare the strength of acids and bases.
A pH below 7 shows acidic nature, pH 7 shows neutrality, and pH above 7 shows basic nature.
The lower the pH, the stronger the acidity; the higher the pH above 7, the stronger the basic nature.
This scale is very important in laboratories, body fluids, soil, and water testing.

Example

A solution with pH 3 is acidic, while a solution with pH 10 is basic.

Rule to remember

pH = -log10[H+].

For school-level use, pH below 7 is acidic, 7 is neutral, and above 7 is basic.

See it worked out

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

Worked example

1If solution A has pH 4 and solution B has pH 6, solution A is more acidic because the pH is lower.
2That means it has a higher concentration of hydrogen ions.

Method you can reuse

1Read the pH number carefully.
2Decide whether it is below 7, equal to 7, or above 7.
3Then compare strength: lower pH means stronger acid, higher pH means stronger base.

Visual tip

A horizontal pH line from 0 to 14 with colour shading is useful for showing acidic, neutral, and basic zones.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

What does a pH value of 8 tell us about a solution?

Think first, then show answer

A pH value of 8 tells us that the solution is mildly basic, because it lies just above neutral on the pH scale.

Common wrong answer

A common wrong answer is to say pH 8 is acidic because it is close to 7.

Why it loses marks: Any value above 7 is basic.

Don't mix this up

Students often think pH 6 and pH 3 are only slightly different. In reality, a lower pH means much stronger acidity.

Exam tip

When comparing pH values, always state which sample is more acidic or more basic before giving the reason.

PH scale diagram

Open full screen to read every label clearly.

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 often compare two pH values, ask the nature of a solution, or connect pH with body fluids, soil, or toothpaste.

1
Enough for 1 mark
The pH scale is a number scale from 0 to 14 that shows how acidic, neutral, or basic a solution is.
2
Add for 2 marks
Example: A solution with pH 3 is acidic, while a solution with pH 10 is basic.
3
Add for 3 marks
The pH scale helps compare the strength of acids and bases.

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