How to set up and read a pH scale

The pH scale turns “how acidic or alkaline is this?” into a single number, and reading it confidently is a core skill in Acids, bases and salts. SPM does not just ask what pH a solution has, it asks you to explain why, in terms of hydrogen ions, and to predict how dilution or acid strength changes the value. This guide sets up the scale clearly and shows how to read and reason about it.

What the pH scale is

The pH scale runs from 0 to 14 and measures how acidic or alkaline a solution is:

• pH below 7, acidic. The lower the number, the more acidic.
• pH 7, neutral. Pure water is the standard example.
• pH above 7, alkaline. The higher the number, the more alkaline.

The number reflects the concentration of hydrogen ions (H⁺) in the solution. A more acidic solution has a higher H⁺ concentration and a lower pH. Alkaline solutions instead have a high concentration of hydroxide ions (OH⁻).

Reading the scale with universal indicator

The everyday way to “read” pH in the lab is universal indicator, which shows a continuous range of colours. Add a few drops of universal indicator solution to the sample, or dip a strip of pH paper, then match the colour to the chart:

• Red, strongly acidic (about pH 1-2)
• Orange to yellow, weakly acidic (about pH 3-6)
• Green, neutral (pH 7)
• Blue, weakly alkaline (about pH 8-11)
• Purple / violet, strongly alkaline (about pH 12-14)

For a precise value you use a pH meter, which reads pH directly as a number on a screen, more accurate than matching colours by eye.

The point students miss: strong versus concentrated

This is the distinction examiners love. “Strong” and “concentrated” are not the same thing.

• Strength describes how completely an acid ionises in water. A strong acid (hydrochloric, sulfuric, nitric) ionises completely, releasing all its H⁺, so it has a low pH. A weak acid (ethanoic acid) ionises only partially, so at the same concentration it has a higher pH.
• Concentration describes how much acid is dissolved per unit volume, which you can quantify with molarity and concentration.

So at 0.1 mol dm⁻³, hydrochloric acid has a lower pH than ethanoic acid, even though the concentrations are equal, because the strong acid produces far more free H⁺.

How dilution changes the reading

Adding water to an acid lowers the H⁺ concentration, so the pH rises towards 7, but it can never cross 7 by dilution alone, because water itself is neutral. On the SPM scale, pH is a logarithmic measure of H⁺ concentration: each rise of one pH unit means the H⁺ concentration has fallen to one-tenth.

That gives a clean prediction. If 0.1 mol dm⁻³ hydrochloric acid has pH 1, diluting it ten times to 0.01 mol dm⁻³ gives pH 2, and diluting ten times again gives pH 3. You can watch this pattern in the effect of dilution and concentration on pH experiment. The reverse holds for an alkali: diluting it lowers the pH towards 7.

A worked reading

A student measures three solutions. Solution A turns universal indicator red; solution B turns it green; solution C turns it purple. Reading the scale: A is strongly acidic (low pH), B is neutral (pH 7), C is strongly alkaline (high pH). If the student then dilutes solution A tenfold, its colour shifts from red towards orange, because the pH has risen by about one unit as the H⁺ concentration dropped.

Why pH matters

pH is not just a textbook number, it is widely used and often appears in application questions. Soil that is too acidic can be treated with lime (calcium hydroxide or calcium oxide) to raise its pH into a range that suits crops. In the body, antacid medicines neutralise excess acid in the stomach to relieve heartburn. In every case the principle is the same: adding a base raises the pH, adding an acid lowers it, and complete neutralisation brings a solution to pH 7. Understanding these links makes pH questions meaningful rather than a matter of memorising colours.

Common mistakes to avoid

• Confusing strong with concentrated, a dilute strong acid can still have a lower pH than a concentrated weak acid.
• Thinking dilution can turn an acid alkaline, pH only moves towards 7, never past it.
• Reading the colour chart backwards, red is acidic, purple is alkaline, green is the neutral middle.
• Saying pH “measures acid” rather than the concentration of hydrogen ions.

The pH scale rewards understanding the H⁺ story behind the number, not just memorising colours. If the strong-versus-concentrated idea or the logarithmic pattern of dilution still feels slippery, that is a common point for a teacher to clarify one-to-one. Our online lessons run in English from RM50 an hour, with a paid one-hour trial, working through pH questions against real past papers until the reasoning is solid.