Measurement and Laboratory Technique: Glassware Including Serial Dilutions

Overview

Recall questions on Glassware Including Serial Dilutions.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Why is the shoot cut under water and the potometer assembled under water?

   Model answer

   To stop air being drawn into the xylem, because an air bubble would break the continuous water column and prevent water uptake.

2. 2

   The capillary tube of a potometer has an internal radius of 0.50 mm and the bubble travels 60 mm in 5.0 minutes. Calculate the rate of water uptake.

   Model answer

   The volume is π × (0.5 mm)² × 60 mm = 47.1 mm³, so the rate is 9.4 mm³ min⁻¹.

3. 3

   What is the purpose of the reservoir and tap on a potometer?

   Model answer

   It allows water to be released to push the bubble back to the start of the scale so that repeat readings can be taken.

4. 4

   How would you prepare a tenfold serial dilution series from a stock culture?

   Model answer

   Transfer 1 cm³ of the stock into 9 cm³ of sterile diluent and mix thoroughly, then transfer 1 cm³ of that dilution into a further 9 cm³ of diluent, repeating for as many steps as required.

5. 5

   Why is a fresh sterile pipette used at every step of a serial dilution?

   Model answer

   Cells carried over on the outside of a used pipette would be added to the next tube, making later dilutions more concentrated than they should be.

6. 6

   A milk sample is diluted through five tenfold steps and 0.1 cm³ of the final dilution gives 63 colonies. Calculate the viable count of the original milk.

   Model answer

   The plated dilution contains 63 ÷ 0.1 = 630 colony-forming units per cm³, so the original milk contains 630 × 10⁵ = 6.3 × 10⁷ colony-forming units per cm³.

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