REVISION’S SHEET
ERRRORS AND IMPROVEMENTS:
A) Qualitative food test:
ERRORS:
• Difficulty in judging colours especially if the
concentrations are low resulting in light colours
• If heating, then temperature is not constant, as there is
heat loss to the surrounding
• Different molecules in the same solution,may interfere
with the colours
Improvements:
• In case of heating,Thermostatic water bath to control
the temperature and avoid the heat loss to the
surrounding
• Colour standard to help judging the colour
• Use black card/ white card to help judging colours
B) Quantitative food test:
ERRORS:
• In case of determining unknown concentration: it can
be between 2 known concentrations
• Difficulty in comparing colours
• Temperature varies due to heat loss to the
surrounding
• In case of vitamin c :Drops fall on the walls of the test
tube
Improvements:
• Use more concentrations with narrower range(
smaller gaps between concentrations) or wider range(
wider gaps between the concentrations), you can
then plot a graph with the obtained results and find
your unknown by reading off the graph, this provides
more accurate estimate for the unknown
• Use colourimeter or colour standards to help judge
the colour better
• Use black card or white card to enable you from
observing the colour difference
• Use thermostatic water bath to avoid heat loss to the
surrounding
• In case of introducing drops , use wider test tube
• Use graduated pipette for more accurate volumes
than the syringe
• Repeat and take average
How to make Benedicts test more reliable
• Use the same volume of the substance under test.
• Use the same volume of the reagent.
• Leave the tubes in the same water bath for the same period
of time.
C) OSMOSIS
How to keep fair comparison in osmosis experiments?
1-When using petri dishes , use petri dishes of the sme
size.
2- When using sucrose or any other solution , put the
same volume in each dish.
3- When using onion epidermis or potato, must be of the
same size.
4- Leave the potato or onion epidermis in the solution for
the same period of time.
General precautions in osmosis experiments:
1- Cover the petri dishes during the experiment to void
evaporation of water which cn affect concentration of the
solution.
2- When using droppers or syringes , use separate
dropper for each solution or wash and dry it after each
step.
3- During preparing slide of onion epidermal cells ,lower
the cover-slip gently to avoid trapping of any air bubbles.
ERRORS:
• Time tissues left in solutions is not enough to
observe complete plasmolysis
• Evaporation of solution may take place and therefore
change the concentration
• Difficulty in judging degree of plasmolysis
• Difficulty in cutting the samples into correct
dimensions
• Parallex error may occur during reading the lengths
IMPROVEMENTS:
• Leave for longer time in the solutions
• Prepare more concentrations with narrower / wider
range (give examples and method of dilution)
• Cover the petri dish to avoid evaporation of solution
• Prepare more than one sample in each solution
• Ensure that the tissues are completely immersed in
the solutions
D) ENZYME REACTIONS:
ERRORS:
• Difficulty in judging the endpoint (in amylase, the
colour with iodine is not clear, in renin the
coagulation is not clear, colour of litmus paper with
urease is not clear)
• Temperature/PH is not controlled (if they are not the
factors under investigation)
• Bubbles of different sizes and are all counted as
one,bubbles may be too fast for counting, gas
expansion interfere with the results, small bubbles
may be missed as not seen (in case of counting
bubbles with catalyse (potato)or with yeast)
• Paper may stick to the walls and bottom of the test
tube ( in case of litmus paper with urease, or paper
dipped in catalyse then placed in hydrogen peroxide
or paper dipped in iodine then placed in amylase
using splint)
IMPROVEMENTS:
• Use colorimeter instead of judging colors or use
black or white card against the tube for better color
comparison
• Measure the volume of gas using gas syringe
instead of counting bubbles
• Use wider test tubes( in case of beads or paper)
• Control temperature using thermostatic water bath
• Control PH using buffer
Effect of heavy metals such as copper sulphate and lead
nitrate
➢ Can act as inhibitors.
➢ May cause protein to clot or coagulate.
➢ May denature proteins.
➢ May breakdown bonds so that can alter tertiary and quaternary
structures.
E) AGAR:
ERRORS
• Difficulty in cutting the agar squares into equal
dimensions
• Difficulty in judging the color change
• Agar is not of equal depth
• Pigmentation of the agar is not even
• Agar may be damaged during cutting
Improvements
• Use moulds for preparing agar squares with equal
dimension
• Use different indicator with clearer end point
• Use wider or narrower range of concentrations (In
case of determining unknown)
• Use black card or white card below the beakers for
better judgment of colors
F) IMMOBILIZTION OF ENZYME (BEADS FORMATION)
ERRORS
• Beads are not of equal sizes
• Beads stuck to the sides of the tube and to each
others
• Forceps causes damage the the beads
• Difficulty to introduce drops using syringe
• Test tube is not vertical and also test tubes are not of
equal sizes (in case of catalyse beads and hydrogen
peroxide)
• Temperature/PH is not controlled ( if they are not the
factors under investigation)
IMPROVEMENTS
• Use sieve with equal size holes to produce beads
with equal diameter
• Use wider test tubes
• Stain the beads for clearer movement
• Use resort stand to make the test tube vertical, use
test tubes with equal sizes (in case of catalyse beads
and hydrogen peroxide)
• Use spatula or spoon instead of forceps
• Control temperature using thermostatic water bath/
control PH using buffer
G)COLOR DIFFUSION
Diffusion of pigment from inside the tissue
(potato/beetroot) to the solution by the influence of salt or
temperature( due to the damge of cell membrane/cell wall)
ERRORS
• Difficulty in judging intensity of diffused color
• Some pigments remain on the surface of the tissue
despite washing
• Temperature is not controlled
• Time is not enough for the diffusion to take place
• There is narrow range of concentrations
IMPROVEMENTS
• Use color standards with degree of color and its name
for better colour describtion , or numbered colour
scale with images showing the color
• Use white/black card against the test tubes for
comparing colors
• Control the temperature using thermostatic water bath
• Dry the samples with paper towel before placing them
in the solutions
• Leave the samples for longer time in solutions
• Use wider range of concentration so the colors
produced show difference
H) Density of solution
(a method for determining the concentration)
In this method you pigment the unknown concentration by
adding 2 drops of methylene blue to 5 cm3 of the
unknown in a petri dish, u then take a drop of the
pigmented solution and introduce it to test tubes of
prepared known concentration, the drop produced will
move according to its concentration:
• If the drop has higher concentration than the
solution, it will move down
• If the drop has lower concentration than the solution,
it will move up
• If the drop has same concentration as the solution , it
stays
ERRORS
• Difficulty in introducing drops by syringe
• The pigmentation is too dark/ or too light
• The drop ruptures (it releases a flow which may be
not clear)
• Too wide/narrow range of concentrations
• Test tube is not vertical
IMPROVEMENTS
• Use dropper instead of syringe
• Increase or decrease the drops of the stain ( to make
the pigment lighter or darker) or use different stain
• Prepare wider/narrower range of concentration
• Use resort stand to make the test tube vertical
I) VISKING TUBES
Take care to open it using water
ERRORS
• Difficulty in opening and tying the visking tube
• Visking tubes is too short /or too long
• Difficulty in rinsing the visking tube ( water may enter
inside it)
• Mixing is not constant
• Drops may not of equal volume
• Difficulty in judging color
IMPROVEMENTS
• Make the visking tube longer or shorter
• Dry the visking tube with paper towel before placing in
the beaker
• Mix using electronic mixer
• Use syringe instead of dropper to introduce equal
volume of solution and indicator
• Use color standard/ or colorimeter
TYPES OF VARIABLES:
First independent variable
- The variable that affects the results by the experiment for
example
▪ In an experiment to investigate the effect of light intensity on
the rate of photosynthesis, light intensity is the independent
variable.
▪ In an experiment to investigate the effect of change in pH on
the activity of amylase, pH is the independent variable.
- It is represented on the X- axis of the graph.
Second Dependent variable
- It is the results obtained in the investigation, for example
▪ In an experiment to investigate the effect of light intensity on
the rate of production of oxygen, the volume of oxygen
produced per unit time is dependent variable.
▪ In an experiment to investigate the effect of change in pH on
the activity of amylase by measuring the time needed for
complete breakdown of a certain volume of starch, time is
the dependent variable.
- It is represented on the Y- axis of the graph.
TYPES OF CHARTS AND GRAPHS
A) Smoth curve
• You will always join all your points either by ruler or
hand
• If there is trials and means , you will always plot the
mean only, except if you were asked to plot trial 1
and trial 2 you will use a key and produce the
following figure
B) Histogram:
in the histogram all the bars will be attached to each
others, and will be of same thickness, your x axis will be
inervals (e.g 12-14, 14-16) rather than categories or
numbers
C) Bar chart
If he need a bar or histogram , he will ask you to plot a
chart, if he needs a smooth curve or line graph he willask
you to plot a graph
• In the bar chart, your x axis will be letters or
categories rather than numbers
• All the bars should be of equal thickness
• You will leave equal spacing between the bars , this
includes the space before the first bar as the below
figure (b):
N.B: the bar chart may for 2 classes e.g animal a and
animal b , in that case you will make the bars of the 2
classes (a and b) attached to each others but leave even
spacing between the intervals as the below figure shows:
INDICATORS:
1-Effects of litmus.
➢ In acidic medium its colour is red.
➢ In an alkaline medium its colour is blue.
2-Universal indicator
It is a pH indicator.
pH
0-3
3-6
Descripti colour
on
Strong
Red
acid
Acid
Orange/yel
7
Neutral
8-11 Alkaline
11-14 Strong
alkali
low
Green
Blue
Violet /
purple
3-Cobalt chloride paper
➢ It is used as an indicator for water.
➢ The dry cobalt chloride paper (anhydrous )has a blue
colour.
➢ In presence of water (when hydrated) its colour changes to
pink or mauve.
4-Hydrogen carbonate indicator.
➢ Its original colour is red .
➢ Increase in concentration of carbon dioxide makes it yellow (
increase in concentration of carbon dioxide makes the
medium acidic as carbon dioxide is acidic gas) .
➢ Decrease in concentration of carbon dioxide makes it purple
.
➢ ( removal carbon dioxide from the medium makes it
alkaline).
5-Methylene Blue: Oxygen Indicator
➢ methylene blue indicator is blue when oxygen is present
➢ If oxygen is removed from the solution, the blue color
disappears.
6-Bromothymol Blue pH Indicator
➢ Bromthymol blue changes color over a pH range from 6.0
(yellow) to 7.6 (blue)
➢ It is a good indicator of dissolved carbon dioxide (CO2) and
other weakly acidic solutions
7-Potassium permanganate
➢ It is a strong oxidizing agent. It dissolves in water to give
intensely pink or purple solutions
➢ Concentrated sulfuric acid reacts with potassium
permengnate forming acidic solution
➢ Acidic solutions of permanganate are reduced to the faintly
pink manganese(II)