pH Scale Visualiser
Slide from pH 0 to 14 to watch the universal-indicator colour change, see the balance of H⁺ and OH⁻ ions, test seven indicators and find where everyday substances like lemon juice, milk and soap sit on the scale.
pH measures how acidic or basic a solution is from the concentration of hydrogen ions: pH = −log₁₀[H⁺]. Below 7 is acidic, exactly 7 is neutral and above 7 is basic. If [H⁺] = 1 × 10⁻³ mol/L the pH is 3. The scale is logarithmic, so pH 3 holds ten times more H⁺ ions than pH 4.
Ion counts are an illustration on a log scale.
Neutral
Ion balance
At 25 °C, [H⁺] × [OH⁻] = 1 × 10⁻¹⁴, so when one goes up the other goes down.
pH calculator
Enter a concentration in mol/L (e.g. 0.001, 1e-3 or 2.5×10^-4).
For a strong acid like HCl, [H⁺] = its concentration; for H₂SO₄ it is about 2 × the concentration. For NaOH, [OH⁻] = its concentration.
Indicator tester
What each indicator looks like in a solution of pH 7.0.
Olfactory indicators (smell changes)
Everyday substances
Tap any substance to move the scale to its typical pH. Values are approximate.
Compare two substances
pH in everyday life NCERT Class 10
Why pH matters for our body, our teeth, farms and even insect stings.
Natural sources of acids
| Natural source | Acid it contains | Natural source | Acid it contains |
|---|---|---|---|
| Vinegar | Acetic (ethanoic) acid | Sour milk (curd) | Lactic acid |
| Orange, lemon | Citric acid | Ant sting | Methanoic (formic) acid |
| Tamarind | Tartaric acid | Nettle sting | Methanoic acid |
| Tomato | Oxalic acid | Stomach (gastric juice) | Hydrochloric acid |
What the pH scale really measures
pH tells us how acidic or basic a solution is by measuring the concentration of hydrogen ions (H⁺) in it. Lower pH means more H⁺ ions and a stronger acid.
Drag the slider
Or tap a number, use the ±0.1 buttons or type an exact pH.
Read the values
See pOH, [H⁺], [OH⁻] and how many times more acidic or basic than water it is.
Test indicators
Watch litmus, phenolphthalein, methyl orange, turmeric and more change colour.
Explore real substances
Tap lemon juice, milk or soap to jump straight to its pH, or compare two.
The pH formula
pH = −log₁₀[H⁺]
If [H⁺] = 1 × 10⁻³ mol/L, pH = 3. The “p” comes from the German word potenz (power). Pure water has [H⁺] = 1 × 10⁻⁷ mol/L, so its pH is 7.
Each step is 10 times
The scale is logarithmic: a solution of pH 3 has 10 times more H⁺ ions than pH 4 and 10,000 times more than pH 7.
pH + pOH = 14 (at 25 °C)
Indicators
- Litmus: acids turn blue litmus red; bases turn red litmus blue.
- Phenolphthalein: colourless in acids, pink in bases.
- Methyl orange: red in acids, yellow in bases.
- Universal indicator: a mixture that shows a different colour for each pH.
Strong vs weak
Strictly, an acid is strong if it ionises completely in water (HCl, H₂SO₄, HNO₃) and weak if it ionises only partly (acetic acid, citric acid). pH also depends on concentration — very dilute HCl can have a higher pH than concentrated vinegar.
pH Scale Visualiser — your questions
What is the pH scale?
The pH scale runs from 0 to 14 and tells you how acidic or basic a solution is. A pH below 7 means an acidic solution with more H⁺ ions than pure water, 7 means neutral, and above 7 means basic with more OH⁻ ions. The letters come from the German word potenz, meaning power.
How do you calculate pH from hydrogen ion concentration?
Use pH = −log₁₀[H⁺] with the concentration in mol/L. If [H⁺] = 1 × 10⁻⁵ mol/L, the pH is 5. Pure water has [H⁺] = 1 × 10⁻⁷ mol/L, so its pH is 7. At 25 °C, [H⁺] × [OH⁻] = 1 × 10⁻¹⁴, which is why pH + pOH always equals 14.
What is the pH of lemon juice, milk and soap?
Lemon juice is about 2.2 because it contains citric acid, milk is about 6.6 and becomes more acidic as it sours into curd, and soap solution is about 10. Other common values are gastric juice 1.5, pure water 7.0, human blood 7.4 and milk of magnesia 10.5.
How does litmus paper show whether something is an acid or a base?
Acids turn blue litmus red; bases turn red litmus blue. Litmus is a natural dye obtained from lichen. Phenolphthalein stays colourless in acids and turns pink in bases, while methyl orange is red in acids and yellow in bases. Universal indicator goes further and gives a different colour for every pH value.
Why does tooth decay start when the pH in the mouth falls below 5.5?
Bacteria in the mouth break down leftover sugar into acids. Once the pH drops below 5.5, the acid begins to corrode tooth enamel, which is made of calcium hydroxyapatite, the hardest substance in the body. Brushing with a mildly basic toothpaste neutralises the acid and stops the attack.
What is the pH of human blood?
Human blood is kept in a very narrow range of 7.35 to 7.45, slightly basic. The body as a whole works between about 7.0 and 7.8. Even small changes outside this range are dangerous, so the blood carries buffers that mop up extra acid or base before the pH can shift.
What is the difference between a strong acid and a concentrated acid?
A strong acid such as HCl, H₂SO₄ or HNO₃ ionises completely in water; a weak acid such as acetic or citric acid ionises only partly. Concentration is a separate idea — how much acid is dissolved. Very dilute hydrochloric acid can therefore have a higher pH than concentrated vinegar.