pH
7.0
Lesson
Proton donors and acceptors
An acid donates a proton (H⁺); a base accepts one. Pure water sits in the middle, lightly self-ionized. The pH scale is just a logarithmic tag for the proton concentration — small numbers mean acid, big numbers mean base.
Three ways to define an acid
Arrhenius: produces H⁺ in water. Brønsted–Lowry: donates a proton. Lewis: accepts an electron pair. The Brønsted view is the workhorse — it includes ammonia and bicarbonate.
The pH scale
pH = −log[H⁺]. Each unit is a factor of 10. Pure water at 25 °C has [H⁺] = 1×10⁻⁷, so pH = 7. pOH = 14 − pH because Kw = 1×10⁻¹⁴.
Strong vs weak
Strong acids (HCl, HNO₃, H₂SO₄) ionize 100% in water. Weak acids (CH₃COOH, HF) only partially. Same idea for bases. Strength is about how completely it dissociates, not how concentrated it is.
Indicators & titration
An indicator is a weak acid (or base) that changes colour over a narrow pH range. In a titration you slowly add a base of known concentration to an unknown acid until the indicator flips — the equivalence point reveals the unknown.
Quick formulas
pH
−log₁₀[H⁺]
[H⁺]
10⁻ᵖᴴ
pOH
−log₁₀[OH⁻]
pH + pOH
14 (at 25 °C)
Kw
[H⁺][OH⁻] = 1×10⁻¹⁴
Ka
[H⁺][A⁻]/[HA]
pKa
−log₁₀(Ka)
Henderson-Hasselbalch
pH = pKa + log([A⁻]/[HA])
Hands-on tools
Compute, mix and titrate.
pH ↔ concentration
Set pH directly, or enter [H⁺] / [OH⁻] and the other values follow. Strong-acid / strong-base assumption.
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
pOH
7.0
[H⁺] (M)
1.0×10⁻⁷
[OH⁻] (M)
1.0×10⁻⁷
Character
Neutral
Indicator simulator
Drag the slider to set the pH. Watch the flask and check which indicators have changed.
Titration curve simulator
Strong acid + strong base with phenolphthalein. Drag the slider to add titrant.
Strength chart
Strong species are essentially fully ionized. Weak ones only partly — listed by Ka or Kb.
Strong acids
HClhydrochloric
HBrhydrobromic
HIhydroiodic
HNO₃nitric
H₂SO₄sulfuric (1st H)
HClO₄perchloric
Strong bases
NaOHsodium hydroxide
KOHpotassium hydroxide
LiOHlithium hydroxide
Ca(OH)₂calcium hydroxide
Sr(OH)₂strontium hydroxide
Ba(OH)₂barium hydroxide
Weak acids (pKa)
HF3.17
CH₃COOH4.76
H₂CO₃6.35 / 10.33
H₃PO₄2.15 / 7.20 / 12.35
HCN9.21
NH₄⁺9.25
Weak bases (pKb)
NH₃4.75
CH₃NH₂3.36
C₅H₅N (pyridine)8.77
CO₃²⁻3.67
HCO₃⁻7.65
Quiz
Flashcards
Tap a card to flip. ← / → keys to navigate.
Acid
AcidA proton donor (Brønsted–Lowry).
1 / 22
Daily challenge
Five questions, one shot. Same set worldwide, refreshes at midnight UTC.
For teachers
Print-ready worksheet, answer key, teaching tips and standards alignment.
Teaching tips
Standards alignment
Reference
pH of common substances
Glossary
Photo gallery
Images sourced from Wikipedia.