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Properties of acid and base solutions

The three properties

The standard restricts the properties of acid and base solutions to exactly three: conductivity, rate of reaction and pH.

All three follow from the same thing: how many ions the solution contains.

Comparing at the same concentration

Everything on this page assumes the two acids are at the same concentration — otherwise you are not comparing strength at all.

Property	Strong acid	Weak acid	Why
$[H^+]$	high	lower	complete vs partial ionisation
pH	low	higher	pH depends on $[H^+]$
Rate with Mg	fast	slower	more $H^+$ to collide
Conductivity	high	lower	more ions to carry charge

Conductivity

A solution conducts electricity if it contains ions that are free to move.
The more ions in the solution, the better it conducts.
A strong acid ionises completely, producing many ions, so it conducts well.
A weak acid ionises only partially, so most of it stays as neutral molecules and there are fewer ions — it conducts poorly.
Neutral molecules carry no charge, so they contribute nothing to conductivity.

Rate of reaction

Reactions of acids — with metals, carbonates, or in neutralisation — depend on the $H^+$ ions.
A higher $[H^+]$ means the hydrogen ions are closer together, so they collide with the other reactant more frequently, giving more successful collisions per second.
So a strong acid reacts faster than a weak acid of the same concentration.
But note carefully: the total amount of product is the same, because both contain the same total moles of acid. Only the speed differs — the weak acid's equilibrium keeps replacing the $H^+$ as it is used up.

pH

pH is a measure of $[H^+]$ — the lower the pH, the higher the hydrogen ion concentration.
A strong acid has a high $[H^+]$ and therefore a low pH.
A weak acid of the same concentration has a lower $[H^+]$ and therefore a higher pH.

Each pH unit is a tenfold change in $[H^+]$ : pH 2 has ten times the $[H^+]$ of pH 3, and a hundred times that of pH 4.

Bases work the same way

A strong base (NaOH) dissociates completely, giving a high $[OH^-]$ , a high pH and good conductivity.
A weak base ( $NH_3$ ) reacts only partially with water, giving a lower $[OH^-]$ , a lower pH (though still above 7) and poorer conductivity.

Using the properties as evidence

Given measurements, you can work backwards: two solutions of the same concentration where one has a lower pH, faster reaction and higher conductivity — that one is the strong acid.
All three properties point the same way, so they corroborate each other.

Tips and Tricks

Every explanation on this page should route through $[H^+]$ . Say the strong acid ionises completely, so it has a higher $[H^+]$ , and then say what that does to the pH, the rate or the conductivity. Jumping straight to "it conducts better because it is stronger" skips the step that carries the mark.

The trap that catches most students is the total amount of product. A weak acid reacts more slowly but produces the same volume of gas with excess metal, because it contains the same number of moles of acid overall and its equilibrium shifts to replace the $H^+$ that is consumed. Rate and yield are different questions.

And always check the question is comparing at the same concentration. If the concentrations differ, you cannot conclude anything about strength from the pH alone.

Worked ExampleIdentifying two acids from their properties

Two colourless solutions, X and Y, are both $0.10\ mol\ L^{-1}$ monoprotic acids. The following measurements are made:

	X	Y
pH	1.0	2.9
Conductivity	high	low
Time for 20 mL of $H_2$ with excess Mg	45 s	210 s

Identify which is the strong acid, explain each measurement, and predict the total volume of hydrogen each would eventually produce.

Step 1 — Use the pH to find $[H^+]$

X has pH 1.0. For a monoprotic acid at $0.10\ mol\ L^{-1}$ , complete ionisation would give $[H^+] = 0.10\ mol\ L^{-1}$ , which is exactly pH 1.0.

So X is fully ionised — it is the strong acid.

Y has pH 2.9, corresponding to $[H^+] \approx 0.0013\ mol\ L^{-1}$ — far below its acid concentration of $0.10\ mol\ L^{-1}$ .

So Y is only partially ionised — it is the weak acid.

Step 2 — Explain the conductivity

Conductivity depends on the number of ions free to move in the solution.

X ionises completely, so almost all of it is present as $H^+$ and its anion — many ions, so high conductivity.

Y ionises only partially, so most of it remains as neutral molecules, which carry no charge — few ions, so low conductivity.

Step 3 — Explain the reaction times

The reaction with magnesium depends on the $H^+$ concentration.

X has a much higher $[H^+]$ , so the hydrogen ions collide with the magnesium surface more frequently, giving more successful collisions per second. It reaches 20 mL of hydrogen in 45 s.

Y has a much lower $[H^+]$ , so collisions with the magnesium are less frequent and the reaction is slower — taking 210 s for the same volume.

Step 4 — Predict the total volume of hydrogen

Both would eventually produce the same total volume.

The magnesium is in excess, so the acid is limiting. Both solutions have the same concentration and volume, so they contain the same number of moles of acid, and each mole can donate one mole of $H^+$ .

For Y, as its $H^+$ is used up, the equilibrium

$HA \rightleftharpoons H^+ + A^-$

shifts right to replace the $H^+$ removed, so the un-ionised molecules act as a reserve and continue supplying protons until the acid is exhausted.

Properties of acid and base solutions

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The three properties

The standard restricts the properties of acid and base solutions to exactly three: conductivity, rate of reaction and pH.

All three follow from the same thing: how many ions the solution contains.

Comparing at the same concentration

Everything on this page assumes the two acids are at the same concentration — otherwise you are not comparing strength at all.

Property	Strong acid	Weak acid	Why
$[H^+]$	high	lower	complete vs partial ionisation
pH	low	higher	pH depends on $[H^+]$
Rate with Mg	fast	slower	more $H^+$ to collide
Conductivity	high	lower	more ions to carry charge

Conductivity

A solution conducts electricity if it contains ions that are free to move.
The more ions in the solution, the better it conducts.
A strong acid ionises completely, producing many ions, so it conducts well.
A weak acid ionises only partially, so most of it stays as neutral molecules and there are fewer ions — it conducts poorly.
Neutral molecules carry no charge, so they contribute nothing to conductivity.

Rate of reaction

Reactions of acids — with metals, carbonates, or in neutralisation — depend on the $H^+$ ions.
A higher $[H^+]$ means the hydrogen ions are closer together, so they collide with the other reactant more frequently, giving more successful collisions per second.
So a strong acid reacts faster than a weak acid of the same concentration.
But note carefully: the total amount of product is the same, because both contain the same total moles of acid. Only the speed differs — the weak acid's equilibrium keeps replacing the $H^+$ as it is used up.

pH

pH is a measure of $[H^+]$ — the lower the pH, the higher the hydrogen ion concentration.
A strong acid has a high $[H^+]$ and therefore a low pH.
A weak acid of the same concentration has a lower $[H^+]$ and therefore a higher pH.

Each pH unit is a tenfold change in $[H^+]$ : pH 2 has ten times the $[H^+]$ of pH 3, and a hundred times that of pH 4.

Bases work the same way

A strong base (NaOH) dissociates completely, giving a high $[OH^-]$ , a high pH and good conductivity.
A weak base ( $NH_3$ ) reacts only partially with water, giving a lower $[OH^-]$ , a lower pH (though still above 7) and poorer conductivity.

Using the properties as evidence

Given measurements, you can work backwards: two solutions of the same concentration where one has a lower pH, faster reaction and higher conductivity — that one is the strong acid.
All three properties point the same way, so they corroborate each other.

Tips and Tricks

And always check the question is comparing at the same concentration. If the concentrations differ, you cannot conclude anything about strength from the pH alone.

Worked ExampleIdentifying two acids from their properties

Two colourless solutions, X and Y, are both $0.10\ mol\ L^{-1}$ monoprotic acids. The following measurements are made:

	X	Y
pH	1.0	2.9
Conductivity	high	low
Time for 20 mL of $H_2$ with excess Mg	45 s	210 s

Identify which is the strong acid, explain each measurement, and predict the total volume of hydrogen each would eventually produce.

Step 1 — Use the pH to find $[H^+]$

X has pH 1.0. For a monoprotic acid at $0.10\ mol\ L^{-1}$ , complete ionisation would give $[H^+] = 0.10\ mol\ L^{-1}$ , which is exactly pH 1.0.

So X is fully ionised — it is the strong acid.

Y has pH 2.9, corresponding to $[H^+] \approx 0.0013\ mol\ L^{-1}$ — far below its acid concentration of $0.10\ mol\ L^{-1}$ .

So Y is only partially ionised — it is the weak acid.

Step 2 — Explain the conductivity

Conductivity depends on the number of ions free to move in the solution.

X ionises completely, so almost all of it is present as $H^+$ and its anion — many ions, so high conductivity.

Y ionises only partially, so most of it remains as neutral molecules, which carry no charge — few ions, so low conductivity.

Step 3 — Explain the reaction times

The reaction with magnesium depends on the $H^+$ concentration.

X has a much higher $[H^+]$ , so the hydrogen ions collide with the magnesium surface more frequently, giving more successful collisions per second. It reaches 20 mL of hydrogen in 45 s.

Y has a much lower $[H^+]$ , so collisions with the magnesium are less frequent and the reaction is slower — taking 210 s for the same volume.

Step 4 — Predict the total volume of hydrogen

Both would eventually produce the same total volume.

For Y, as its $H^+$ is used up, the equilibrium

$HA \rightleftharpoons H^+ + A^-$

shifts right to replace the $H^+$ removed, so the un-ionised molecules act as a reserve and continue supplying protons until the acid is exhausted.

Properties of acid and base solutions

The three properties

Comparing at the same concentration

Conductivity

Rate of reaction

pH

Bases work the same way

Using the properties as evidence

Step 1 — Use the pH to find [H+][H^+][H+]

Step 2 — Explain the conductivity

Step 3 — Explain the reaction times

Step 4 — Predict the total volume of hydrogen

Properties of acid and base solutions

The three properties

Comparing at the same concentration

Conductivity

Rate of reaction

pH

Bases work the same way

Using the properties as evidence

Step 1 — Use the pH to find [H+][H^+][H+]

Step 2 — Explain the conductivity

Step 3 — Explain the reaction times

Step 4 — Predict the total volume of hydrogen

Step 1 — Use the pH to find $[H^+]$

Step 1 — Use the pH to find $[H^+]$