Osmosis: use simple compound measures of rate of water uptake

The idea

Rate of water uptake is calculated as change in mass or volume divided by time taken, giving units such as g/min, g/h, cm³/min or cm³/h. For example, if a potato cylinder gains 1.8 g in 15 minutes, the rate is 1.8 g ÷ 15 min = 0.12 g/min. You should be able to read data from a table or graph, calculate the rate, compare rates between different solution concentrations, and explain that the rate depends on the concentration gradient across the partially permeable membrane.

What earns the marks

• Select the correct formula: rate of water uptake = change in mass or volume ÷ time taken.
• Calculate change in mass by subtracting the initial mass from the final mass, for example 5.2 g − 4.0 g = 1.2 g.
• Divide the change by the time in the stated unit, for example 1.2 g ÷ 10 min = 0.12 g/min, and include the compound unit in the answer.
• Compare rates at different concentrations and link a faster rate to a steeper concentration gradient between the external solution and the cell contents.
• Read values accurately from a graph or table, including choosing two points and finding the change in the y-value over the change in the x-value.
• Recognise that a negative rate indicates water loss by osmosis when the external solution is more concentrated than the cell contents.

Common mistakes and how to put them right

• Using final mass instead of change in mass in the calculation.

  Put it right: First work out change in mass = final mass − initial mass, then divide by time.

• Giving a rate with no unit or with a single unit such as g.

  Put it right: A rate is a compound measure, so write g/min, g/h, cm³/min or cm³/h.

• Assuming the rate is constant throughout the experiment.

  Put it right: The concentration gradient changes as water moves, so the rate may decrease over time; use the overall change and total time unless told otherwise.

Exam tips

• Write the formula, substitute the numbers with units, then give the answer with the correct compound unit.
• If a question asks you to compare rates, calculate both rates first and then state which is greater and by how much.
• Link any calculated rate back to osmosis by referring to the concentration gradient and the partially permeable membrane.