IB MYP Integrated Science- Chemistry- pH scale and indicators-Study Notes - New Syllabus

pH Scale and Indicators

The pH scale is used to measure how acidic or alkaline (basic) a substance is. It is based on the concentration of hydrogen ions \( \mathrm{H^+} \) present in a solution.

The pH Scale

The pH scale ranges from 0 to 14:

• pH 0–6 → acidic
• pH 7 → neutral
• pH 8–14 → alkaline (basic)

The lower the pH, the higher the concentration of \( \mathrm{H^+} \) ions, and the more acidic the solution is.

Nature of the pH Scale

The pH scale is logarithmic, meaning each change of 1 pH unit represents a tenfold change in acidity.

\( \mathrm{pH = -\log[H^+]} \)

For example, a solution with pH 3 is 10 times more acidic than pH 4 and 100 times more acidic than pH 5.

Strong and Weak Acids/Bases (Link to pH)

• Strong acids → very low pH (0–3)
• Weak acids → higher pH (4–6)
• Weak bases → pH (8–10)
• Strong bases → high pH (11–14)

Indicators

Indicators are substances that change colour depending on the pH of a solution. They are used to identify whether a substance is acidic, neutral, or alkaline.

Common Indicators

Litmus

• Acid → red
• Base → blue

Universal Indicator

Universal indicator shows a range of colours depending on pH:

• Red → strong acid
• Orange/Yellow → weak acid
• Green → neutral
• Blue → weak base
• Purple → strong base

Other Indicators

• Phenolphthalein: colourless (acid) → pink (base)
• Methyl orange: red (acid) → yellow (base)

Using Indicators

Indicators can be used to:

• Identify acids and bases
• Estimate pH
• Detect neutralisation reactions

Neutralisation and pH

When an acid reacts with a base, the pH moves toward 7.

\( \mathrm{Acid + Base \rightarrow Salt + Water} \)

At the equivalence point, the solution is neutral (pH ≈ 7 for strong acid–strong base).

Real-World Applications

• pH of soil affects plant growth
• pH of blood must be maintained (~7.4)
• Water treatment uses pH control