Concentrations of solutions
3 marks · NormalA teacher claims that doubling the amount of solute added to a fixed volume of water will always double the concentration of the solution. Explain using calculations and an example whether this statement is correct.
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Also accept: Example used could be any salt; Clear use of concentration formula with numbers
Common mistakes: Ignoring the volume is fixed; Misapplying formula
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Marking points
B1 · 1 mark · States or explains concentration formula linking amount and volume (c = n / V)
M1 · 1 mark · Uses a numerical or symbolic example where solute amount is doubled, volume is kept fixed
A1 · 1 mark · Shows or explains that concentration doubles when amount doubles and volume remains fixed, or correctly argues the opposite if volume changes
Worked solution
The formula for concentration is: concentration = amount (mol) / volume (dm³).
For example, if originally 0.5 mol of a solute is dissolved in 1 dm³: concentration = 0.5/1 = 0.5 mol/dm³.
If the amount of solute is doubled (1.0 mol) and volume stays 1 dm³: concentration = 1.0/1 = 1.0 mol/dm³.
Therefore, the teacher's statement is correct if the volume of solution remains fixed.