The pH Scale
Simple Explanation
The pH scale compresses the enormous range of possible [H⁺] concentrations into a simple, manageable 0–14 scale: pH below 7 is acidic, pH 7 is neutral, and pH above 7 is basic. Each whole pH unit represents a 10-fold change in [H⁺].
Why Do We Need It?
Without pH, describing acidity would mean writing out unwieldy numbers like [H⁺] = 0.0000001 mol/L every time — pH turns this into a single, easy-to-compare number (in this case, pH 7).
Formula
pH
pH = −log₁₀[H⁺]
A convenient logarithmic scale for expressing how acidic or basic a solution is, compressing the enormous range of possible [H⁺] values into a simple 0–14 scale.
- pH
- — the pH value (no units)
- [H⁺]
- — hydrogen ion concentration, in mol/L
When to use it: Whenever you need to convert a hydrogen ion concentration into the standard pH scale, or vice versa (using [H⁺] = 10⁻ᵖᴴ).
Worked Example
Calculate pH from [H⁺]
A solution has [H⁺] = 1.0 × 10⁻³ mol/L. Find its pH.
Why Does This Work?
Because pH uses a logarithm, each whole-number change in pH corresponds to a 10-fold change in [H⁺] — this lets one simple, small-range number (0–14) represent [H⁺] values that otherwise span 14 powers of 10.
Real-Life Example
Comparing stomach acid to blood
Stomach acid has a pH of about 1.5-2, while human blood has a pH of about 7.4.
The roughly 5-6 unit pH difference actually represents a difference of about 100,000 times in hydrogen ion concentration — the pH scale makes this dramatic difference easy to state and compare with just two small numbers.
Practice
A solution has [H⁺] = 1.0 × 10⁻¹⁰ mol/L. Find its pH.
MediumCommon mistake
Assuming pH changes are linear (i.e. pH 2 is only "twice as acidic" as pH 4) — because the scale is logarithmic, pH 2 actually represents a [H⁺] that is 100 times greater than pH 4, not just double.
Quick Review
- pH = −log₁₀[H⁺]. pH < 7 acidic, pH = 7 neutral, pH > 7 basic.
- Each whole pH unit is a 10-fold change in [H⁺].
- pH + pOH = 14 at 25°C.