November 2026 - Acids & Bases Study Guide for nursing students in
The pH scale runs from 0 to 14, with 7 neutral. Acids donate hydrogen ions and sit below 7; bases accept them and sit above. Each whole step is a tenfold change in concentration, which is why a pH of 4 is a hundred times more acidic than a pH of 6.
Try 4 short questions throughout this guide. Check each answer to learn from the explanation.
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The pH scale
pH
Nature
Hydrogen ions
Examples
0–6
Acidic
High
Stomach acid ~2, lemon juice ~2, coffee ~5
7
Neutral
Balanced
Pure water
8–14
Basic (alkaline)
Low
Blood 7.35–7.45, baking soda ~9, bleach ~13
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.
Human arterial blood is held between 7.35 and 7.45, which is slightly alkaline rather than neutral. That narrow range appears often, and so does the fact that moving outside it in either direction is life-threatening.
Question 1 of 4
A solution has a pH of 4. How does it compare with a solution at pH 6?
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Correct answer: C. It is 100 times more acidic.
Two whole steps on a logarithmic scale means 10², so a hundredfold difference in hydrogen ion concentration. "Two times" treats the scale as linear, which is the error the question is built to catch, and a lower pH is more acidic rather than more alkaline.
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.
Question 2 of 4
What is the normal pH range of human arterial blood?
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Correct answer: B. 7.35 to 7.45, slightly alkaline
Blood sits just above neutral, in a range narrow enough that departures in either direction are dangerous. It is neither exactly neutral nor acidic, and 7.8 would be well outside anything survivable.
Comparing two pH values
Worked example
A solution at pH 3 is how many times more acidic than one at pH 6?
Find the difference between the values: 6 - 3 = 3.
Each step is a factor of ten, so the difference is 10³.
10 × 10 × 10 = 1,000.
1,000 times more acidic. The intuitive answer of "three times" ignores that the scale is logarithmic, and it is always one of the choices.
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.
Which of these substances is acidic?
Anything with a pH below 7. Stomach acid is the standard example.
How many times more acidic is one pH than another?
Ten to the power of the difference between them.
What is the pH range of human blood?
7.35 to 7.45 — slightly alkaline, and tightly held.
What is produced when an acid and a base react?
A salt and water, with the mixture moving towards neutral.
Question 3 of 4
Which statement about a strong acid is correct?
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Correct answer: B. It dissociates completely, releasing all its hydrogen ions.
Strength describes how completely an acid dissociates, which is independent of concentration — a strong acid can be very dilute. A pH above 7 describes a base, and accepting hydrogen ions is what bases do.
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.
Question 4 of 4
An antacid tablet relieves excess stomach acid. What kind of reaction is taking place?
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Correct answer: A. Neutralisation, producing a salt and water
An antacid is a base, and a base reacting with an acid gives a salt and water while moving the pH towards neutral. Combustion requires oxygen and a fuel, nothing is decomposing, and lowering the pH would make the acidity worse rather than better.
TEAS · Acids and bases
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