1. Using a microscope Key points:
Use a couple of drops of iodine helps cells show up on the slide.
The total magnification is found by:
Objective lens magnification x eye piece lens magnification
So if the eyepiece is x10 and the objective lens is x40 you will magnifying x400.

2. Culturing microbes – TRIPLE ONLY
The agar is sterilised by boiling, the petri dish with disinfectant and the inoculating loop can be passed through a flame.
The lid should be stuck on with tape to prevent accidental exposure and contamination but not sealed as the microbes will need oxygen.
The incubation temperature should be below 30oC to prevent growth of the more harmful microbes.
Use area = π r2
The bigger the area of inhibition the more effective the antiseptic or antibiotic.

3. Investigating photosynthesis
Independent variable = distance from light source
Dependent variable = number of bubbles produced per minute
Control variables = temperature of pond weed (hence the beaker of water to prevent it warming quickly), amount of sodium hydrogen carbonate in the water (affects the amount of carbon dioxide for the plant to use in photosynthesis).
This is not a very good way of measuring carbon dioxide produced as bubbles may vary in size. Collecting the gas in an inverted measuring cylinder would be more accurate.

4. Investigating osmosis
“Osmosis is the net movement of water from high water concentration to low water concentration across a semi-permeable membrane.”
Independent variable = concentration of salt or sugar solution.
Dependent variable = percentage mass change.
This will take some time to ensure osmosis has occurred.
Potential errors can occur if the concentrations are not measured correctly.
Percentage mass change is used as a better comparison between potato pieces because they will all have a different mass, even if they are the same size.
The potato must be dried with a tissue before weighing to remove any excess water that would add mass.
Remember to reset the balance so you do not have a zero error!

5. Watch out for curved lines of best fit if it’s an enzyme question!
Enzymes
Watch out for curved lines of best fit if it’s an enzyme question!
Amylase is added to starch solution.
Water baths at set temperatures can be used.

6. Food tests
Ethanol
Add the food to a test tube with some ethanol in it.
A cloudy / milky colour

7. Keywords Precision Sketch graph Error See also uncertainty. Resolution
A line graph, not necessarily on a grid, that shows the general shape of the relationship between two variables. It will not have any points plotted and although the axes should be labelled they may not be scaled.
Error See also uncertainty.
Measurement error
The difference between a measured value and the true value.
Anomalies
These are values in a set of results which are judged not to be part of the variation caused by random uncertainty.
Random error
These cause readings to be spread about the true value, due to results varying in an unpredictable way from one measurement to the next. Random errors are present when any measurement is made, and cannot be corrected. The effect of random errors can be reduced by making more measurements and calculating a new mean.
Systematic error
These cause readings to differ from the true value by a consistent amount each time a measurement is made. Sources of systematic error can include the environment, methods of observation or instruments used. Systematic errors cannot be dealt with by simple repeats. If a systematic error is suspected, the data collection should be repeated using a different technique or a different set of equipment, and the results compared.
Zero error
Any indication that a measuring system gives a false reading when the true value of a measured quantity is zero, eg the needle on an ammeter failing to return to zero when no current flows. A zero error may result in a systematic uncertainty.
Resolution
This is the smallest change in the quantity being measured (input) of a measuring instrument that gives a perceptible change in the reading.
Range
The maximum and minimum values of the independent or dependent variables; important in ensuring that any pattern is detected. For example a range of distances may be quoted as either: 'From 10 cm to 50 cm' or 'From 50 cm to 10 cm'.
Precision
Precise measurements are ones in which there is very little spread about the mean value. Precision depends only on the extent of random errors – it gives no indication of how close results are to the true value.
Accuracy
A measurement result is considered accurate if it is judged to be close to the true value.
Interval
The quantity between readings, eg a set of 11 readings equally spaced over a distance of 1 metre would give an interval of 10 centimetres.