pH Scale of Common Substances — Chemistry Mnemonic
Target: mnemonic for ph values of common substances
Why is this hard to memorize?
The pH scale (0-14) chapter always includes a table of everyday substances, and board papers love asking you to arrange them in increasing or decreasing order of acidity. The substances themselves are easy to recognise, but their exact pH numbers blur together — is milk more acidic than lemon? Is blood acidic or basic? A single memorised sequence, from the strongly acidic gastric juice up to strongly basic caustic soda, removes all the guesswork.
Classic mnemonics you should know
"Gastric Juice(1.2) → Lemon(2.2) → Vinegar(3) → Tomato(4.2) → Black coffee(5) → Milk(6.4) → Pure water(7) → Blood(7.4) → Sea water(8) → Baking soda(9) → Soap(9.5) → Milk of magnesia(10) → Ammonia(11.5) → Bleach(12.5) → Caustic soda/NaOH(13-14)"
Reciting this single ladder from most acidic to most basic lets you place ANY substance from the question relative to the ones you already know, without memorising every number in isolation.
"Body fluids cluster near neutral: Blood = 7.4 (slightly basic), Saliva = 6.5-7.5 (near neutral), Gastric juice = 1.2-3 (strongly acidic, it's meant to digest food)"
Grouping body fluids separately fixes the common exam trap of assuming all body fluids are acidic — only gastric juice is strongly acidic; blood and saliva are close to neutral, tilted slightly basic.
"Pure water = 7 (the exact centre). Anything you can place ABOVE or BELOW water on the ladder is automatically basic or acidic."
Using pure water's pH of exactly 7 as a fixed anchor point makes it fast to judge whether an unfamiliar substance mentioned in a question is likely acidic (below 7) or basic (above 7).
The complete list
- Gastric juice (stomach acid) — pH 1.2 to 3
- Lemon juice — pH 2.2
- Vinegar — pH 3
- Tomato juice — pH 4.2
- Black coffee — pH 5
- Milk — pH 6.4 (slightly acidic, turns to lactic acid over time)
- Pure water — pH 7 (exactly neutral)
- Human blood — pH 7.4 (slightly basic)
- Sea water — pH 8
- Baking soda solution — pH 9
- Soap solutions — pH 9.5 to 10
- Milk of magnesia — pH 10
- Ammonia solution — pH 11.5
- Household bleach — pH 12.5
- Caustic soda (NaOH) — pH 13 to 14
Frequently asked questions
What is the pH of human blood?
Human blood has a pH of about 7.4, which is slightly basic (alkaline). The body tightly regulates this value — even a small deviation can be dangerous, which is why blood is not exactly neutral like pure water (pH 7).
Which common substance has the lowest (most acidic) pH?
Gastric juice in the human stomach, with a pH of about 1.2 to 3, is one of the most acidic common substances — it needs to be this acidic to digest food and kill harmful bacteria.
Is milk acidic or basic?
Fresh milk is very slightly acidic, with a pH of about 6.4 (just below the neutral value of 7). As milk turns sour, lactic acid forms and the pH drops further.
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