Definition · Plain-language
Concentration
Concentration is a measure of how much solute is present in a given amount of solution or solvent.
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How much solute, in how much solution
Concentration answers a simple question: how much of a dissolved substance is packed into a given amount of liquid? A solution with a large amount of solute per unit volume is concentrated; one with little is dilute. The same solute can be made more concentrated by adding more of it, or more dilute by adding more solvent. Because chemistry depends so often on how much of a substance is present — how fast a reaction goes, what colour or pH a solution shows — concentration is one of the most-used quantities in the subject.
Common ways to express it
There are several standard measures. Molar concentration, or molarity, gives the number of moles of solute per litre of solution, with units of mol/L (sometimes written M); it is favoured because reactions occur in mole ratios. Mass concentration gives grams of solute per litre. Percentage concentration expresses the solute as a percentage of the whole, by mass or by volume. Very dilute solutions may be quoted in parts per million (ppm). The right choice depends on the context, but molarity is the workhorse of laboratory chemistry.
Concentration is not the same as strength
It is easy to confuse concentration with the strength of an acid or base, but they are different ideas. Concentration is about quantity — how much is dissolved. Strength is about how fully a substance ionises in water. A weak acid can be highly concentrated, and a strong acid can be very dilute. Both influence properties such as pH, but they answer different questions: concentration asks "how much?", strength asks "how completely does it ionise?". Keeping the two apart avoids a common source of error.
Key facts
At a glance
- Definition: the amount of solute in a given amount of solution or solvent
- Molarity: moles of solute per litre (mol/L)
- Other units: grams per litre, percentage, parts per million
- Concentrated: much solute per unit volume
- Dilute: little solute per unit volume
- Not the same as: strength (degree of ionisation)
Common misconceptions
What people often get wrong
Often heard: Concentration and strength mean the same thing.
Actually: They are different. Concentration is how much solute is dissolved; strength is how fully an acid or base ionises. A weak acid can be concentrated, and a strong acid can be dilute.
Often heard: Adding more solvent increases the concentration.
Actually: Adding solvent dilutes the solution, lowering its concentration, because the same amount of solute is spread through more liquid. Concentration rises by adding more solute or removing solvent.
Often heard: A more concentrated solution always reacts more dangerously.
Actually: Higher concentration often speeds reactions, but hazard depends on the substance, its strength and the situation too. Concentration is one factor among several, not a complete measure of risk.
Going deeper
