Practical Flashcards
how to control temperature
- use a water bath
- use a thermostatically controlled water bath rather than a beaker of water
how to control pH
- use buffer solution
error
potential error in a measurement is half the value of the smallest division on the scale
error = smallest division / 2
% error
(measured value - actual value/ actual value) x 100
constructing a table
1) draw table with ruled columns
2) include headings with units
3) independent variable comes first
4) use the same no. of s.f./d.p. everywhere
constructing a line graph
1) independent variable → x-axis
2) dependent variable → y-axis
3) graph should be more than half of grid
4) use x or ◉ when plotting points
5) don’t extrapolate unless mentioned
systemic errors
errors caused by lack of accuracy and precision
- same in size, act in the same direction on all readings
random errors
result from difficulties in controlling standardised or dependent variables
- likely to be different for different stages of investigation
methods to increase reliability
1) increase number of samples
2) repeat and take the average
3) exclude anomalous results
4) wide range of temperature/pH/etc
improvements in measurement
measure volumes using a burette or a graduated pipette
improvements in recording colour change
use colorimeter
improvements in the syringe experiment
- use wide mouthed test tube
testing non-reducing sugars
1) add 2cm³ food sample to test tube
2) add 1cm³ HCl
3) boil for 1 min
4) allow tube to cool
5) add alkali to neutralise
6) add equal volumes of benedict’s reagent and heat in a water bath at 80°C until colour changes are observed
precautions in osmosis experiments
1) cover petri dish to prevent evaporation
2) separate droppers
3) avoid air bubbles
4) completely immerse tissue in solution
biuret test without biuret reagent
1) 1 cm³ sample + KOH
2) few drops of CuSO₄
Emulsion test for lipids
Shake sample with ethanol, add water. If lipids are present solution turns into a cloudy white suspension
Test for starch
add drops of iodine solution to the sample, if a blue black colour is quickly produced, starch is present, if not, iodine solution remains yellow-brown
test for reducing sugars
add equal volumes of sample and Benedict’s reagent, heat in a water bath at 80°C
colour changes from blue, green, yellow, orange, brick red ppt
how you would modify this procedure to investigate the effect of temperature on rate of enzyme activity
1) use thermostatically controlled water-baths at different temperatures;
2 state at least five different temperatures between 0 °C and 100 °C ;
Ways to increase reliability of study
1) use a wider/narrower range of concentrations (depending on questions);
2) use at least five more concentrations;
3) use a method to ensure controlled variables are maintained;
4) run experiments one at a time / stagger the start times;
5) repeat measurements for each concentration and calculate the mean ;
How to determine the level of risk of experiment? (Low, medium, high)
The risk level is higher if there are:
1) Harmful chemicals (harmful/ irritant)
2) Heat
3) sharp objects
The differences between normal plant and xerophytes (plants that live in very dry environment)
1) sunken stomata
2) Curled leaf
3) Trichomes (Hairy-like structure; Function is to trap a layer of water vapour and reduces the water potential gradient)
4) Thick waxy cuticle / thick layer of cell wall
Drawing a large plan diagram of plants
1) Use the 10x objective lens
2) uses most of the available space, at least two layers of tissue, no shading ;
3) draws part of plant organ and no individual cells ;
4) draws correct shape of plant organ ;
5) shows subdivision of the vascular tissue ;
6) label line and label to identify the tissue instructed in the question;
Serial dilution
1) correct percentage concentration (labels under correct sequence of beakers) and,% / percentage, once ;
2) shows transfer of 10 (cm3) from first beaker to second beaker and 1 (cm3) from second beaker to third beaker and 1 (cm3) from third beaker to fourth beaker ;
3) shows 10 (cm3) of water added to second, third, and fourth beakers
Drawing a few plant cells
1) Use the x40 objective lens.
2) Your drawings must be:
- appropriate size of cells to fill most of space available and lines thin and continuous ;
- draws only the number of cells instructed and each cell touching at least two of the other cells ;
- two lines drawn around each cell and three lines where cells touch ;
- correct shape of cells ;
- label line and label to identify the [structure as per -instructed] of one of the cells ;
1 mm = ___ um (micrometer)
1000 um
