Chemistry revision sheets

What Is pH? The pH Scale Explained Simply

You've seen "pH balanced" on shampoo bottles and "pH 7" in science class — but what does the number actually mean? It's one of the most useful measurements in all of chemistry, and the idea behind it is refreshingly simple.

The short answer: pH is a number from 0 to 14 that tells you how acidic or basic a solution is. It measures the concentration of hydrogen ions (H⁺) in the solution. A low pH means more acidic, 7 means neutral, and a high pH means more basic (alkaline).

What pH actually measures

pH is often described as the "power of hydrogen." It tracks how many hydrogen ions (H⁺) are floating around in a solution. The more H⁺ ions, the more acidic the solution — and the lower the pH number.

If you've met logarithms, the exact definition is pH = −log[H⁺]. If you haven't, don't worry — the key consequence is the part below.

The scale: 0 to 14

pH rangeMeaningExamples
0–6AcidicBattery acid (~0), lemon juice (~2), vinegar (~3), coffee (~5)
7NeutralPure water
8–14Basic / alkalineBaking soda (~9), soapy water (~10), bleach (~13)

The most important rule: each step is ×10

This is the part most people miss. The pH scale is logarithmic, which means each whole number is a tenfold change in acidity:

  • pH 3 is 10× more acidic than pH 4
  • pH 3 is 100× more acidic than pH 5
  • pH 3 is 1,000× more acidic than pH 6

So a small-looking change in pH is actually a big change in how acidic something is. That's why lemon juice (pH ~2) is dramatically more acidic than coffee (pH ~5), not just "a few points" more.

How pH is measured

  • Litmus paper gives a quick acid-or-base answer.
  • Universal indicator turns different colors you match to a pH chart for an approximate value.
  • A pH meter gives a precise digital reading.

Quick check

Which is more acidic, pH 2 or pH 5 — and by how much?
pH 2 is more acidic, and because each step is ×10, it's 1,000× more acidic (10 × 10 × 10).

Common mistakes to avoid

  • Thinking higher pH means more acidic. It's the opposite — higher pH means more basic.
  • Treating each step as a small change. Every unit is a 10× jump.
  • Assuming pH is always 0–14. That range covers everyday solutions, but very concentrated strong acids or bases can sit slightly below 0 or above 14.

(One technical note: pure water is exactly neutral at 7 at room temperature; the neutral point shifts slightly at other temperatures.)

FAQ

What does pH stand for?
It's commonly read as "power of hydrogen," reflecting that it measures hydrogen-ion concentration.

What is a neutral pH?
7 — the pH of pure water at room temperature. Below 7 is acidic, above 7 is basic.

Can pH be negative?
Yes, in extreme cases. Very concentrated strong acids can have a pH below 0, and very strong bases can exceed 14, but everyday solutions stay within 0–14.

How is pH related to acids and bases?
Acids have a pH below 7 (lots of H⁺ ions); bases have a pH above 7 (few H⁺ ions). pH simply puts a number on how acidic or basic something is.

The takeaway

pH is a 0-to-14 scale for acidity, based on the amount of H⁺ in a solution: lower is more acidic, 7 is neutral, higher is more basic. Just remember the golden rule — every single step on the scale is a tenfold change.

Read this alongside Acids vs Bases for the full picture, then test yourself on Ionic vs Covalent Bonds.

⏰ 5 Minutes in Chemistry — the study series from Chemistery

You just learned one topic the five-minute way. The series does it for your entire course — one printable page per topic: understand it, memorize it, test yourself. Five minutes. Next topic.

  • Vol 1 · Semester 1 — atoms, moles, stoichiometry, bonding & gases (22 sheets)
  • Vol 2 · Semester 2 — kinetics, equilibrium, acids & bases, electrochem (19 sheets)
  • Vol 3 · The Hard Stuff — cram charts & decision trees for the units worth the most points (15 sheets)

Built for advanced-level high school and first-year college chem. 56 sheets, printable, Letter + A4.

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