Acidic
pH < 7More H+ than OH−
A lower pH means a higher hydrogen ion concentration in the dilute solutions studied here.
Blue litmus turns red.
ACIDS, BASES & SALTS / THE pH SCALE
A little number.
A lot of chemistry.
From the tang of a lemon to the chemistry inside a car battery, explore what pH tells us and where acids get to work.
01 / EXPLORE
Where does it sit on the scale?
Select a number, move the slider, or try an everyday example.
More acidic
NeutralpH 7
More alkaline
Everyday values are illustrative: formulation, concentration, and temperature affect pH. The colours represent universal indicator, rather than the liquid’s own colour; indicator charts vary.
Pure water
pH
NeutralEqual concentrations of hydrogen and hydroxide ions. Pure water is neutral at pH 7 at 25 °C.
Universal indicator: approximately green.
02 / UNDERSTAND
Read the number. Understand the ions.
Compare the hydrogen ions, H+, and hydroxide ions, OH−, in water. Their balance tells us whether a solution is acidic, neutral, or alkaline.
H+ Hydrogen ion
OH− Hydroxide ion
Neutral
pH = 7 at 25 °CEqual H+ and OH−
The two ions have equal concentrations. Neutral water still contains both kinds of ion.
Neither litmus colour changes.
Alkaline
pH > 7More OH− than H+
An alkali is a base that dissolves in water to produce hydroxide ions. A higher pH means greater alkalinity.
Red litmus turns blue.
Both ions are present in every beaker. Particle counts are schematic, not actual ratios; water molecules and other dissolved ions are omitted.
WHERE DOES H+ COME FROM?
An acid transfers H+ to water.
Watch a hydrogen ion leave HCl and join a water molecule.
- 1Start with HCl + water
- 2Transfer H⁺
- 3Form hydronium
The complete reaction, shown without motion.
HCl donates H+ to water, forming H3O+ and Cl−.
The H+(aq) labels in our beakers are shorthand for hydrated hydrogen ions, represented here as H3O+.
EACH STEP DOWN IN pH
One step. Ten times more H+.
A fall of 1 pH unit means about 10× the hydrogen ion concentration. From pH 4 to pH 2, that becomes 100×. The scale is logarithmic.
Dots show relative H+ concentration, not individual ions.
0–14 is the usual school range, rather than an absolute limit: some concentrated solutions lie outside it. The neutral pH also changes with temperature. This lesson uses 25 °C.
Strong and concentrated describe different things
TRY IT YOURSELF
Add acid. Add water. Watch the ions.
Switching acid keeps the amount and volume, so you can compare each acid under the same conditions.
0.06 mmol of acid
Each portion adds 0.01 mmol of the selected acid. Oppositely charged ions form together.
model solution volume
Water controls change the solvent only. The total amount of acid stays the same.
Try this: add more HCl, then add water. Does its strength change? Repeat with ethanoic acid.
YOUR SOLUTION100 mL
H+
Cl−
CH3COOH
ACID STRENGTHStrong acid
CONCENTRATION
ESTIMATED pH
Model key: red particles show H+, green particles show conjugate ions, and beige particles show intact acid molecules. Each label gives the full formula. Particle counts are rounded illustrations, so small populations may be omitted; use the calculated percentage for ionisation. Liquid height is schematic. Calculations use dilute solutions at 25 °C, with Ka = 1.8 × 10−5 for ethanoic acid; water auto-ionisation is negligible in the acid range shown. About acid strength.
Strength: how much of the acid ionises?
A strong acid, such as HCl, ionises essentially completely in water. A weak acid, such as ethanoic acid, CH3COOH, ionises only partially.
Concentration: how much acid per volume?
These beakers show the same strong acid at the same volume. Adding water dilutes an acid; it does not change it from strong to weak.
Model key: HA is an acid molecule; A− is the ion left after it donates H+. Particle counts and ionisation fractions are illustrative.
pH depends on both strength and concentration. A pH value alone cannot tell you whether an acid is strong or weak.
03 / CONNECT
Which acid belongs here?
Drag each acid onto its matching image. A correct match reveals the description.
Drag an acid
Use each acid onceDrop it onto an image
Drag an acid from the bank to begin.
Keyboard: press Space or Enter to pick up an acid, Tab to an image, then press Space or Enter to drop. Esc cancels.
Why measure pH?
A useful number for choosing and checking conditions.
Soil & growing
Soil pH affects nutrient availability. Testing helps growers understand whether conditions suit a particular crop.
Water & aquatic life
Fish and other aquatic organisms have different pH tolerances. Monitoring pH helps detect changes in water quality.
Food production
Producers measure pH to keep acidity consistent. Acidity is one factor in food preservation, alongside the full processing method.
Choose the right tool: litmus shows acidic or alkaline; universal indicator estimates a pH range; a calibrated pH meter gives a more precise numerical reading. Try the litmus experiment
04 / CHECK YOUR UNDERSTANDING
Ready to put a number to it?
Read a question, choose an answer, then check the chemistry behind it. All pH classifications here use 25 °C.
Question 1 of 100 correct