practical

Question 1

sources of error

Answer 1

systematic or random

Question 2

systematic errors

Answer 2

• don’t differ throughout investigation(predictable+acting in same direction every time)
  built in errors in measuring equipments
  due to limitations in reading scales
  due to lack of accuracy+precision of measuring instruments

may not affect trend in results

Question 3

random errors (uncontrolled variables, subjectivity in perception) affects trends

Answer 3

due to difficulties controlling standardised variables
due to difficulty measuring dependent variables
differ throughout investigation- unpredictable
e.g. maintaining steady water bath temperature, difficulty judging colour, difficulty counting bubbles

Question 4

how to reduce random error

Answer 4

taking multiple measurements+calculating mean value

Question 5

how to reduce systematic error

Answer 5

adjust equipment/calibrate

Question 6

examples of improvements

Answer 6

• thermostatic controlled water bath instead of beaker and bunsen burner
• repeat exp at least 3 times to improve reliability
• use graduated pipette instead of syringe
• colourimeter instead of judging end point with naked eyes
• buffer maintain pH
• use equal volumes
• materials from same sources
• slow motion camera used for bubble counting
• micropipette instead of syringe

Question 7

justify suggested improvements

Answer 7

• will increase accuracy of results so improve confidence in data

Question 8

size of uncertainty error

Answer 8

half the value of the smallest division on the measuring scale

Question 9

total error

Answer 9

sum of the errors for each reading
If your recorded result involves measuring two values – for example, if you have measured a starting temperature and then another temperature at the end, and have calculated the rise
in temperature – then this error could have occurred for both readings.
Your final value for the change of temperature you have measured would then be written: 18.0°C + 1.0°C.

Question 10

control measures
- pH
- temp
- light intensity
- humidity
- biological materials

Answer 10

• buffer
• use of thermostatically controlled water bath, heater/air conditioner/incubator
• same watts lamp at** same distance**
• hygrometer
• from same organism

Question 11

accuracy

Answer 11

closeness to true value- better instruments

Question 12

precision

Answer 12

closeness to repeated readings- control all variables

Question 13

reliability

Answer 13

confidence in results- repeat readings+**take mean **
improves accuracy of results

Question 14

validity

Answer 14

agreement between hypothesis and investigation- check relation between key+derived variables

Question 15

benedicts test (procedure)

incl if non-reducing

Answer 15

2cm^3 benedicts + 2cm^3 glucose -> heat to 90°C

2cm^3 HCL + 2cm^3 sucrose + heat at 90°C
neutralise with NaOH
2cm^3 benedicts + 2cm^3 glucose -> heat to 90°C

Question 16

explanation of benedicts

Answer 16

reducing sugar reduces Cu2+ to Cu+ (forms a precipitate)

Question 17

explanation of benedicts non-reducing sugar test

Answer 17

acid hydrolyses the non-reducing sugar into reducing sugar
which reduces Cu2+ to Cu+

Question 18

benedict’s results colours of intensity

Answer 18

blue -> green -> yellow -> orange -> brown -> brick red

Question 19

using benedicts to observe rate of enzyme reactions

Answer 19

Add 2cm^3 and 2cm^s solution and heat to 90°C, observe for colour change
Compare colour standards highest: brick red ->brown -> orange->yellow->green->blue
Temperature of products of the reaction between starch and amylase
highest amount of reducing sugar has the shortest time

Question 20

identify the concentration of a reducing sugar by use of
semi-quantitative benedict’s test

Answer 20

• make serial dilution of glucose
• test diff concentrations of glucose with Benedict’s solution
• make colour chart
• use colour chart to estimate concentration of reducing sugar in unknown solution
• use colourimeter to increase sensitivity of reducing sugar test

Question 21

Describe how the student could obtain a quantitative estimate of the concentration of reducing sugar in a solution

Answer 21

-at least five known concentrations of reducing sugar
-Benedict’s test and time for first colour change for known concentrations of reducing sugar
-test unknown sample of reducing sugar (with Benedict’s)
-compare time for unknown sample with times for known samples of reducing sugar
-draw a graph of known concentrations of reducing sugar against time (to first colour change) - read off unknown sample of reducing sugar

Question 22

iodine test + exp

Answer 22

add few drops iodine solution into starch solution
starch+iodine form a complex

Question 23

biuret test

Answer 23

2cm^3 biuret+2cm^3 albumin?

Question 24

biuret exp

Answer 24

N atoms in peptide bonds+Cu2+ form peptide complex

Question 25

emulsion test

Answer 25

1 drop of oil + 5cm^3 ethanol -> shake then fill test tube with distilled water

Question 26

emulsion test exp

Answer 26

lipid insoluble in water but soluble in alcohol

Question 27

why do we repeat experimental procedure

Answer 27

improve ACCURACY of results reduce the impact of anomalies

Question 28

reasons for diffs in results

Answer 28

- diff optimum pH - diff optimum temperature

Question 29

With reference to your estimate in (a)(vii), suggest how you would modify this procedure to obtain a MORE ACCURATE value for the concentration of protease in fruit extract U

Answer 29

- use more concentrations with narrower intervals - states concentrations both sides of the estimate

Question 30

State one feature visible in Fig. 2.1 that adapts the plant to live in water

Answer 30

presence of air spaces and makes leaf buoyant

Question 31

adaptation of xerophytic plants

Answer 31

trichomes -> traps water vapour inside the leaf ; curled leaf -> increases humidity to reduce the water potential gradient ; sunken stomata -> exposed to the humid atmosphere with a low water potential gradient ; thick cuticle -> reduces water loss by transpiration ;

Question 32

how to accurately estimate the rate of reaction for the intermediate concentration for ...

Answer 32

more concentrations ; 2 colorimeter ; 3 plot a graph and read off the concentration of C ; 4 repeat and calculate the mean ; 5 carry out each concentration separately

Question 33

initial exp: measuring sugar conc Suggest how the student could extend this investigation to estimate the concentration of starch in a vegetable extract.

Answer 33

5 CONCENTRATIONS OF STARCH, stated same procedure but with iodine test + observe colour change

Question 34

Suggest modifications to this procedure that would allow you to obtain a more accurate estimate of the concentrations of reducing sugars in G1 and H1

Answer 34

smaller intervals between standard concentrations ; 2 plot graph of standard concentrations against time and interpolate

Question 35

Source of error when adjusting temp of yeast cell suspension

Answer 35

using same indicator each time measure volume of gas instead of counting bubbles temperature of yeast not measured directly ; (improvement) **place thermometer in test-tube** difficult to identify colour of indicator ; (improvement) use a colorimeter

Question 36

One way to improve the accuracy of the estimate of the total number of stomata on a leaf is to use a photomicrograph with a larger area. State one other way to improve the accuracy of the estimate of the total number of stomata on a leaf

Answer 36

repeat / sample more areas

Question 37

modify the procedure to investigate the **effect of pH on the time** taken for amylase to hydrolyse starch

Answer 37

Control amylase concentration 5 different pHs

Question 38

suggest how student could extend investigation to estimate concentration of starch in a vegetable extract

Answer 38

5 CONCS, determining **colour intensity**

Question 39

modify the experiment to **determine the concentration of reducing sugars** in a sample of milk

Answer 39

use benedict's reagent HEAT TO at least 80C time to first colour change or compare the result to standard or known concentrations more time means higher conc of sugars

Question 40

How procedure modified to **improve accuracy** of estimate of concentration of reducing sugars in C since they talk about improving accuracy, we need to i.e add more cons, carry each conc out separately

Answer 40

stated concentrations both sides of estimate, more concs, colorimeter plot graph+read off concentration of C repeat+calculate mean carry out each conc separately