Acids, Bases, and the pH Scale

Acid–base chemistry shows up throughout the TEAS Science section, and the same ideas underpin how the body keeps its fluids in balance. Master the handful of concepts on this page and you can answer most pH questions without hesitation — and you will recognise the same logic again in anatomy and physiology.

What acids and bases are

An acid donates hydrogen ions (H⁺) in solution. A base accepts hydrogen ions, or releases hydroxide ions (OH⁻). Acids taste sour and react with metals; bases feel slippery and taste bitter. In water, the two are always linked: as the concentration of H⁺ rises, the concentration of OH⁻ falls, and vice versa. A solution is never purely one or the other — it is the balance between them that pH actually measures.

The pH scale

pH measures how acidic or basic a solution is, on a scale from 0 to 14:

• 0–6 is acidic
• 7 is neutral (pure water)
• 8–14 is basic

The scale is logarithmic — each whole number is a tenfold change. A solution at pH 4 is ten times more acidic than one at pH 5, and one hundred times more acidic than one at pH 6. This is the single most tested idea on this topic, so make it automatic: to compare two pH values, count the gap between them and raise ten to that power. pH 3 versus pH 6 is a difference of three units, so 10³ — one thousand times more acidic.

Strong versus weak

A strong acid or base dissociates completely in water (hydrochloric acid, sodium hydroxide); a weak one only partly dissociates (acetic acid, the acid in vinegar). "Strong" describes how completely a substance dissociates — not how concentrated it is. A dilute solution of a strong acid and a concentrated solution of a weak acid can sit at the same pH. The TEAS likes to test exactly this distinction, so do not let the words "strong" and "concentrated" blur together.

Neutralization

When an acid and a base react, they form water and a salt. This is why an antacid — a base — relieves excess stomach acid: the base neutralises the hydrochloric acid the stomach produces, raising the pH toward a comfortable range. Neutralisation is also the basis of titration, the lab technique used to find an unknown concentration by adding a known one until the solution reaches neutral.

Buffers and acid–base balance in the body

A buffer is a solution that resists changes in pH when small amounts of acid or base are added. Blood is buffered — chiefly by the bicarbonate system — which is why healthy blood pH stays in the narrow range of 7.35 to 7.45. Stray outside it and the consequences are serious: a blood pH below 7.35 is acidosis, above 7.45 is alkalosis. The lungs (by adjusting how much CO₂ is exhaled) and the kidneys (by adjusting how much bicarbonate and H⁺ are excreted) work together to hold that balance. You will meet this again in physiology — the chemistry here is the foundation.

Common question traps

• Confusing logarithmic steps with linear ones — each pH unit is ×10, not +1 worth of acidity.
• Reading "strong" as "concentrated." They are independent properties.
• Forgetting that pH 7 is neutral only for pure water at room temperature.

Key takeaways

• pH 0–6 is acidic, 7 is neutral, 8–14 is basic.
• The pH scale is logarithmic: one unit equals a tenfold change.
• "Strong" and "weak" describe dissociation, not concentration.
• Neutralisation produces water and a salt.
• Blood is buffered to pH 7.35–7.45; the lungs and kidneys defend that range.