Strong and Weak Acids and the pH Scale
An acid in water releases hydrogen ions. A strong acid ionises fully while a weak one only partly does, so strength and concentration are separate ideas, and pH shifts by one for every tenfold change in hydrogen ion concentration.
What a learner can do afterwards
- Separates strong from weak and concentrated from dilute, and gives an example of each pairing
- Predicts the pH change when an acid is diluted ten times
- Explains why a weak acid reacts more slowly with magnesium than a strong acid of the same concentration
- Writes what happens to hydrochloric acid and to ethanoic acid when each is added to water
1 · Read
An acid releases hydrogen ions in water. A strong acid, like hydrochloric acid, splits fully and lets go of every ion, so the splitting runs one way. A weak acid, like ethanoic acid, splits only partly: a few molecules release their ions while the rest hold on, and the mixture settles into a balance.
Strength and concentration are separate dials. A concentrated weak acid can hold a lower pH than a dilute strong acid, because pH counts the ions actually released, and a crowded bottle that partly splits can still release more than a nearly empty one. Crowded is not the same as split.
The pH scale shifts by one for every tenfold change in hydrogen ions. Dilute a strong acid ten times and its pH rises by one, while a weak acid shifts less. That is why a strong acid of the same concentration reacts faster with magnesium: it floods the metal with ions at once.
Write what each acid does in water. Hydrochloric acid gives all its hydrogen ions away, while ethanoic acid gives only a few and sets up a balance. Name one strong and one weak acid from memory.
Strong acids split fully, weak acids split partly, and each tenfold dilution lifts the pH of a strong acid by one.
The rewritten readings were written for this dataset.
More to read
Recognising acids and bases
Brønsted-Lowry acids give H+ to bases, which accept it. Hydrogen chloride ionises almost completely in water, making it strong. Acids can react with suitable metals, carbonates and bases. Identify acids by their ability to donate H+, not formulas alone.
Reading the pH scale
pH expresses H+ concentration logarithmically. The usual 0 to 14 range has exceptions. Each decrease of 1 pH unit means ten times the H+ concentration. For example, pH 2 has ten times that of pH 3. At 25°C, 7 is neutral: below is acidic, above alkaline. Indicators or meters measure pH. Dilution reduces ion concentrations, usually moving pH toward neutral.
Strength and concentration differ
Acid strength concerns the fraction that ionises, not the total amount of acid. Strong acids ionise almost completely; weak acids, such as ethanoic acid, ionise partly at equilibrium. Concentration measures acid per volume. Concentrated weak acid can contain more acid particles than dilute strong acid. Strength alone cannot tell you how fast an acid reacts.
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Where this leads
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