October 2026 - Acids & Bases Study Guide

The pH scale

The scale is logarithmic

Each whole number is a factor of ten. A solution at pH 3 has ten times the hydrogen ion concentration of one at pH 4, and a hundred times that of one at pH 5. Two solutions that look adjacent on the scale can differ enormously in strength.

This is the point most questions turn on. "How many times more acidic is pH 2 than pH 5?" is asking for 10³, which is a thousand — not three.

Properties worth recognising

• Acids donate hydrogen ions (H⁺), taste sour, react with metals, and turn blue litmus red.
• Bases accept hydrogen ions, feel slippery, and turn red litmus blue.
• Strong and weak refer to how completely a substance dissociates, not to how concentrated it is. A dilute strong acid and a concentrated weak acid are different things.
• An indicator changes colour at a particular pH. Litmus is the one the exam names most often.

Comparing two pH values

Neutralisation and buffers

An acid and a base react to produce a salt and water, moving the mixture towards pH 7. That is why an antacid — a base — relieves excess stomach acid, and why sodium bicarbonate from the pancreas neutralises acidic chyme entering the small intestine.

A buffer is a solution that resists pH change when small amounts of acid or base are added. The blood contains buffer systems, which is how it holds that narrow 7.35 to 7.45 range despite the constant production of acid by metabolism.

Terms to know

Acid

A substance that donates hydrogen ions; pH below 7.

Base

A substance that accepts hydrogen ions; pH above 7. Also called alkaline.

pH

A measure of hydrogen ion concentration, from 0 to 14.

Neutral

pH 7, where acidic and basic properties balance.

Logarithmic scale

A scale where each step is a tenfold change.

Neutralisation

The reaction of an acid and a base, producing salt and water.

Buffer

A solution that resists change in pH.

Indicator

A substance that changes colour at a particular pH.

Strong acid

One that dissociates completely. Separate from how concentrated it is.

What the TEAS asks most

The same core ideas appear in different wording. If you can answer these without stopping to think, you have what this section requires.

Key points

• pH runs 0 to 14; below 7 is acidic, above 7 is basic, 7 is neutral.
• Acids donate hydrogen ions; bases accept them.
• Each pH step is a tenfold change in hydrogen ion concentration.
• Blood is held between 7.35 and 7.45 — slightly alkaline.
• Neutralisation produces a salt and water; buffers resist pH change.