Skill Guide : P

Example planning table entries

variable	values or range	how measured
name or describe the variable	Can the variable have different values or is it pretty much fixed for this experiment? If the variable can change what are the absolute maximum and minimum values for the variable? What are the practical maximum and minimum values you can handle with a typical experimental set-up?	Explain how the variable can be measured. What equipment is needed? Is there a choice of equipment? What is the most appropriate piece of equipment, and why? What is the accuracy of the equipment?
Below are some typical examples. You wouldn't necessarily write all this down, but you should at least think about it and note down the important details. Leave a bit of space so you can add extra notes as and when you think of them. Remember, these are just examples picked at random. If you were to have the same variable in an experiment you might decide that a different piece of equipment might be the best solution for that particular experiment.
Example1: volume of gas variable type: continuous	For typical amounts of reactants the maximum amount of gas produced looks like it would be a few hundred cm³ in a few minutes. If the reaction goes slowly then only a small volume of gas may be produced within a time of, say, 10 minutes.	Eqpt: Gas jar plus beehive shelf and trough of water. Disadvantage: Gas jar doesn't have any scale markings. Eqpt: Measuring cylinder, plus trough of water. Advantage: Different sized measuring cylinders can be used to measure small or large gas volumes up to 1000cm³. Eqpt: Gas syringe. Advantage: Specially designed for measuring gas volumes. Easier to use than measuring cylinder and probably more accurate. Disadvantage: Only 100cm³ size available, graduated in 1cm³ intervals. Quite expensive, so have to be careful using it. Best choice: Gas syringe. Reasons: Covers required gas volume range, better accuracy.
Example2: Resistance variable type: continuous	Resistance could, in theory, be very large. However with the materials being used the resistance is quite small, typically below 10 ohms.	Eqpt: Digital resistance meter. Advantage: Simple and quick to use, has different ranges for small and large resistances. Safe to use, only puts a small current through the sample. Disadvantage: Accuracy is not good for small resistance values and the resistance of any connecting leads must be taken into consideration. Eqpt: Using ammeter and voltmeter and then calculating resistance. Advantage: Much more accurate for small resistances. Disadvantage: Takes longer to set up the circuit and take the measurements. Best choice: Ammeter and voltmeter. Reasons: Small resistances need to be measured, this method gives the best accuracy.
Example3: temperature of water variable type: continuous	Temperature could be varied, over the range 0 to 100 degrees Celsius.	Eqpt: Ordinary glass thermometer. Available options are ½ and 1 degree thermometers -10 to 110°C, plus tenth degree thermometer over a limited range. Eqpt: Temperature probe and data logging equipment. Best choice: small 1°C thermometer. Reasons: The temperature is going to be a control variable and is not expected to change. It also only has a minor effect on the outcome. It will only be measured at the start and end of the experiment. A temperature to the nearest degree will be accurate enough. A small rather than large thermometer will make it easier to take the measurement as there is not much room.

variable

values or range

how measured

name or describe the variable

Can the variable have different values or is it pretty much fixed for this experiment?

If the variable can change what are the absolute maximum and minimum values for the variable?

What are the practical maximum and minimum values you can handle with a typical experimental set-up?

Explain how the variable can be measured.

What equipment is needed?

Is there a choice of equipment?

What is the most appropriate piece of equipment, and why?

What is the accuracy of the equipment?

Below are some typical examples. You wouldn't necessarily write all this down, but you should at least think about it and note down the important details. Leave a bit of space so you can add extra notes as and when you think of them. Remember, these are just examples picked at random. If you were to have the same variable in an experiment you might decide that a different piece of equipment might be the best solution for that particular experiment.

Example1:
volume of gas

variable type:
continuous

For typical amounts of reactants the maximum amount of gas produced looks like it would be a few hundred cm³ in a few minutes.

If the reaction goes slowly then only a small volume of gas may be produced within a time of, say, 10 minutes.

Eqpt: Gas jar plus beehive shelf and trough of water.
Disadvantage: Gas jar doesn't have any scale markings.

Eqpt: Measuring cylinder, plus trough of water.
Advantage: Different sized measuring cylinders can be used to measure small or large gas volumes up to 1000cm³.

Eqpt: Gas syringe.
Advantage: Specially designed for measuring gas volumes. Easier to use than measuring cylinder and probably more accurate.
Disadvantage: Only 100cm³ size available, graduated in 1cm³ intervals. Quite expensive, so have to be careful using it.

Best choice: Gas syringe.
Reasons: Covers required gas volume range, better accuracy.

Example2:
Resistance

variable type:
continuous

Resistance could, in theory, be very large. However with the materials being used the resistance is quite small, typically below 10 ohms.

Eqpt: Digital resistance meter.
Advantage: Simple and quick to use, has different ranges for small and large resistances. Safe to use, only puts a small current through the sample.
Disadvantage: Accuracy is not good for small resistance values and the resistance of any connecting leads must be taken into consideration.

Eqpt: Using ammeter and voltmeter and then calculating resistance.
Advantage: Much more accurate for small resistances. Disadvantage: Takes longer to set up the circuit and take the measurements.

Best choice: Ammeter and voltmeter.
Reasons: Small resistances need to be measured, this method gives the best accuracy.

Example3:
temperature of water

variable type:
continuous

Temperature could be varied, over the range 0 to 100 degrees Celsius.

Eqpt: Ordinary glass thermometer. Available options are ½ and 1 degree thermometers -10 to 110°C, plus tenth degree thermometer over a limited range.

Eqpt: Temperature probe and data logging equipment.

Best choice: small 1°C thermometer.
Reasons: The temperature is going to be a control variable and is not expected to change. It also only has a minor effect on the outcome. It will only be measured at the start and end of the experiment. A temperature to the nearest degree will be accurate enough. A small rather than large thermometer will make it easier to take the measurement as there is not much room.