Practical Skills
General Skills
Variables:
Independent variable: the factor whose values you manipulate. Always drawn on the X axis.
Dependent variable: the variable which is not under your control and is affected by the
independent variable.
Control: Used as a comparison to ensure that what you are testing for is actually occurring. To
make a control, remove the factor that you are investigating.
Use simple or serial dilution to change the concentration of the independent variable
Colorimeter: an instrument hat measures the amount of light that is absorbed by a tube
containing a colored liquid. The deeper the color, the more light is absorbed.
Standardized/controlled variables: all other variables that might affect the experiment but are
not being tested for. Variables that are kept constant in an experiment. Include temperature,
pH, concentration, light intensity, time.
o Temperature: Use a thermostatically controlled water bath.
o pH: use a buffer solution. This is a solution that have a particular pH and keep that pH
even if the reaction taking place produces an acidic or alkaline substance.
o Light intensity: Vary the distance of the light source. Problem with light is that it
introduces another variable – heat. Using a transparent piece of plastic in front of the
light can reduce heating effect of light source.
o Windspeed: Use a fan placed at different distances.
o Humidity: Measure of the water content of the air.
Can be increased by placing a container of water close to the plant being
investigated or by placing a plastic bag over the plant so that transpiration
eventually increases humidity.
Can be reduced by placing a container of calcium chloride by the plant which
absorbs water vapor.
o Biological molecule: Keep age, storage conditions, genotype, mass, volume, position, in
the organism constant.
Measurements:
Accuracy: how ‘true’ a measurement is.
Precision: the ability of the measuring instrument to give you the same reading every time it
measures the same thing.
Reliability: the degree of trust that you can have in the measuring tool. Use repetition to deal
with poor reliability.
You can always read to ½ the size of the smallest increment of a measuring instrument. There is
always +/- ½ the size of the smallest increment.
Tables:
Columns are headed with quantity and unit
Each measurement is taken to the same decimal place.
, The values calculated for the mean are given to the same number of decimal places.
Line graphs:
Independent variable on the X-axis and dependent variable on the Y-axis.
Each axis is fully labelled with units.
The scale on each axis goes up in equal intervals.
Use more than half of the graph paper.
A line of best fit is drawn, or points are joined with a straight line. Don’t extend the line beyond
the points plotted.
Bar charts/ histograms:
Bar chart: Used when you have discontinuous variables on the x axis and continuous variables
on the y axis. The bars are drawn with gaps between them.
Histogram or frequency diagram: continuous variables on the x axis.
Describing graphs:
Describe overall trend – increasing or decreasing
Look for changes in the gradient on the graph. Identify sharp changes.
Quote specific figures from the graph.
Calculating percentage change:
Find the difference between the first reading and the second reading.
Divide this value by the first reading and multiply by 100. State if is positive or negative.
Sources of error: unavoidable limitations of your apparats.
Uncertainty in measurements: results from lack of accuracy or precision in the measuring
instruments that you are using.
Difficulties in controlling standardized variables
Difficulties in measuring the dependent variable: such as inaccuracies with color.
Improvements:
Use measuring instruments that are more precise. Use a graduated pipette instead of a syringe.
Use techniques that are more accurate in measuring the dependent variable. Using a
colorimeter for color changes.
Use apparatus that standardize variables: thermostatically controlled water bath instead of
beaker.
Control important variables that were not controlled in the original experiment.
Do repeats to have several readings. Then calculate the mean.
Chapter 1: Cells and microscopy
Measurement
An eyepiece graticule is placed on the microscope eyepiece. This is used to find how many units
large the specimen is.
General Skills
Variables:
Independent variable: the factor whose values you manipulate. Always drawn on the X axis.
Dependent variable: the variable which is not under your control and is affected by the
independent variable.
Control: Used as a comparison to ensure that what you are testing for is actually occurring. To
make a control, remove the factor that you are investigating.
Use simple or serial dilution to change the concentration of the independent variable
Colorimeter: an instrument hat measures the amount of light that is absorbed by a tube
containing a colored liquid. The deeper the color, the more light is absorbed.
Standardized/controlled variables: all other variables that might affect the experiment but are
not being tested for. Variables that are kept constant in an experiment. Include temperature,
pH, concentration, light intensity, time.
o Temperature: Use a thermostatically controlled water bath.
o pH: use a buffer solution. This is a solution that have a particular pH and keep that pH
even if the reaction taking place produces an acidic or alkaline substance.
o Light intensity: Vary the distance of the light source. Problem with light is that it
introduces another variable – heat. Using a transparent piece of plastic in front of the
light can reduce heating effect of light source.
o Windspeed: Use a fan placed at different distances.
o Humidity: Measure of the water content of the air.
Can be increased by placing a container of water close to the plant being
investigated or by placing a plastic bag over the plant so that transpiration
eventually increases humidity.
Can be reduced by placing a container of calcium chloride by the plant which
absorbs water vapor.
o Biological molecule: Keep age, storage conditions, genotype, mass, volume, position, in
the organism constant.
Measurements:
Accuracy: how ‘true’ a measurement is.
Precision: the ability of the measuring instrument to give you the same reading every time it
measures the same thing.
Reliability: the degree of trust that you can have in the measuring tool. Use repetition to deal
with poor reliability.
You can always read to ½ the size of the smallest increment of a measuring instrument. There is
always +/- ½ the size of the smallest increment.
Tables:
Columns are headed with quantity and unit
Each measurement is taken to the same decimal place.
, The values calculated for the mean are given to the same number of decimal places.
Line graphs:
Independent variable on the X-axis and dependent variable on the Y-axis.
Each axis is fully labelled with units.
The scale on each axis goes up in equal intervals.
Use more than half of the graph paper.
A line of best fit is drawn, or points are joined with a straight line. Don’t extend the line beyond
the points plotted.
Bar charts/ histograms:
Bar chart: Used when you have discontinuous variables on the x axis and continuous variables
on the y axis. The bars are drawn with gaps between them.
Histogram or frequency diagram: continuous variables on the x axis.
Describing graphs:
Describe overall trend – increasing or decreasing
Look for changes in the gradient on the graph. Identify sharp changes.
Quote specific figures from the graph.
Calculating percentage change:
Find the difference between the first reading and the second reading.
Divide this value by the first reading and multiply by 100. State if is positive or negative.
Sources of error: unavoidable limitations of your apparats.
Uncertainty in measurements: results from lack of accuracy or precision in the measuring
instruments that you are using.
Difficulties in controlling standardized variables
Difficulties in measuring the dependent variable: such as inaccuracies with color.
Improvements:
Use measuring instruments that are more precise. Use a graduated pipette instead of a syringe.
Use techniques that are more accurate in measuring the dependent variable. Using a
colorimeter for color changes.
Use apparatus that standardize variables: thermostatically controlled water bath instead of
beaker.
Control important variables that were not controlled in the original experiment.
Do repeats to have several readings. Then calculate the mean.
Chapter 1: Cells and microscopy
Measurement
An eyepiece graticule is placed on the microscope eyepiece. This is used to find how many units
large the specimen is.
