Acid concentration and strength - Higher
A solutionMixture formed by a solute and a solvent. forms when a soluteThe dissolved substance in a solution. dissolveWhen a substance breaks up and mixes completely with a solvent to produce a solution. in a solventThe liquid in which the solute dissolves to form a solution.. The concentrationA measure of the mass or amount of solute dissolved in a given volume of solvent or solution. of a solution is a measure of how ‘crowded’ the solute particles are. The more concentrated the solution, the more solute particles it contains in a given volumeThe volume of a three-dimensional shape is a measure of the amount of space or capacity it occupies, eg an average can of fizzy drink has a volume of 330 ml..
Solutions are described as dilute or concentrated, depending on the ratioA ratio is a way to compare amounts of something. It is usually written in the form a:b. of dissolved solute to volume of solution:
- a dilute solution has a low ratio
- a concentrated solution has a high ratio
Take care to use the word ‘dilute’ correctly. It can be used to describe the concentration of a solution (as here), or to describe the process of adding more water to a solution to reduce its concentration.
Strong and weak acids
acidSubstance producing more hydrogen ions than hydroxide ions when dissolved in water. in solution are a source of hydrogen ionElectrically charged particle, formed when an atom or molecule gains or loses electrons., H+. The hydrogen ions are produced when the acid dissociationThe breaking up of a molecule into ions when dissolved in water. or breaks down to form ions.
Strong acids
Strong acids completely dissociate in solution. For example, hydrochloric acid is a strong acid. It completely dissociates to form hydrogen ions and chloride ions:
HCl(aq) → H+(aq) + Cl-(aq)
Nitric acid and sulfuric acid are also strong acids.
Weak acids
Weak acids only partially dissociate in solution. For example, ethanoic acid is a weak acid. It only partially dissociates to form hydrogen ions and ethanoate ions:
CH3COOH(aq) ⇌ H+(aq) + CH3COO-(aq)
The ⇌ symbol is used in the equation to show that the reaction is a reversible reaction, and does not go to completion.
pH and hydrogen ion concentration
The pH of a solution is a measure of its concentration of hydrogen ions. The higher the concentration of H+ ions in an acidic solution, the lower the pH.
If the hydrogen ion concentration in a solution increases by a factor of 10, the pH of the solution decreases by 1.
pH of alkaline solution
The higher the concentration of OH- ions in an alkaline solution, the higher the pH.