Experimental techniques practice questions.

A potometer is set up with a leafy shoot. Which quantity does it measure most directly?

• The rate of water uptake by a shoot
• The rate of photosynthesis in the leaves
• The rate of aerobic respiration in roots
• The rate of mineral ion absorption

Answer: The rate of water uptake by a shoot

A potometer measures the rate at which the shoot takes up water, which is used as an estimate of transpiration; it does not measure photosynthesis or respiration directly.

A student wants to measure a cell in micrometres using the scale in the microscope eyepiece. Why must a stage micrometer be viewed first?

• To find what one eyepiece division is worth
• To check that the eyepiece scale is undamaged
• To bring the specimen into focus at high power
• To centre the specimen in the middle of the field

Answer: To find what one eyepiece division is worth

The eyepiece scale is arbitrary until it is compared against a slide bearing a scale of known length. Without that comparison the student can say a cell is fourteen divisions wide, but not how wide it is.

A coverslip is lowered onto a wet mount gradually from one edge rather than dropped flat. What does this achieve?

• Air is pushed ahead of it instead of being trapped
• The specimen is pressed flat against the slide
• The water spreads evenly to the edges of the slide
• The coverslip is less likely to crack under the pressure

Answer: Air is pushed ahead of it instead of being trapped

Trapped air appears as circles with thick dark outlines that are easily mistaken for cells, and they cannot be focused through. Flattening the specimen is a side effect rather than the reason for the technique.

A corrosive solution is splashed onto a student's forearm during a practical. What is the correct first action?

• Rinse the area with a large amount of running water
• Cover the area with a dry dressing and report it
• Neutralise the solution with a weak acid or alkali
• Wipe the area dry with paper towel before rinsing

Answer: Rinse the area with a large amount of running water

Dilution and removal are immediate and carry no further risk; the incident is reported once rinsing is under way, not before it. Attempting to neutralise on the skin adds a second reaction, and it releases heat where the damage already is.

A dichotomous key is being written for the trees of a woodland. What must be true of every pair of statements in it?

• They must be exclusive and cover all remaining specimens
• They must refer only to features visible in summer
• They must each lead directly to the name of a species
• They must use measurements rather than described features

Answer: They must be exclusive and cover all remaining specimens

If both statements can be true of one specimen, or if neither is, the user reaches a dead end. Later pairs may lead to a name, but the earlier ones lead to further pairs, which is what makes the key a branching structure.

In paper or thin-layer chromatography, the solvent in the tank must start below the pencil line on which the samples were placed. Why?

• Otherwise the samples dissolve into the solvent directly
• Otherwise the solvent evaporates before it reaches them
• Otherwise the paper floats instead of standing upright
• Otherwise the pencil line dissolves and marks the paper

Answer: Otherwise the samples dissolve into the solvent directly

The separation depends on the solvent rising through the paper and carrying the mixture with it; a sample sitting in the liquid simply washes off into the tank. This is also why the line is drawn in pencil, since ink would separate alongside the sample.