scientific skills

Question 1

why do we repeat experiments?

Answer 1

• reduce the impact of random erros by taking an average
• allows experimenter to check that the method is reliable/reproducable
• identify and exclude outliers

Question 2

which is a higher ratio: 1:4 or 1:2.5?

Answer 2

1:2.5

convert to fractions ie 0.25 vs 0.4 and pick the highest fraction

Question 3

fuel

assess validity of fuel experiments that repeat using different masses of water and fuel

experiment using combustion of fuel to heat water-> calc heat content

Answer 3

provided the experiments are completed under the same conditions, different m(H2O) and m(fuel) wont affect heat of combustion calculations therfore validity. Heat of combustion questions wont change as a result of using different masses ie an increase in fuel increases the resultant temp change

Question 4

fuel

how to improve efficiency of heating water using less fuel?

Answer 4

• decrease airflow around stove
• cover pot leaving small gap
• only heat water required
• decrease distance bw pot and flame
• use metal instead of glass/plastic

Question 5

common limitations of titrations

Answer 5

• haven’t repeated experiment -> repeat experiment 5 times to improve reliabiliy
• concentration calculation reliability is limited to a degree by the accuracy of identification of colour change at endpoint -> use digital pH meter to record endpoint
• inadequate mixing of volumetric flask contents -> variation in conc in different aliquots -> decrease accuracy and precision
• impropery rinsing (give specific eg) -> decrease accuracy and reliability of results
• not enough drops of indicator -> decrease likelihood of getting consistent endpoint colour -> decrease accuracy and reliability

Question 5

reasons for invalid/valid

Answer 5

if it changes more than 1 IV
- if you change more than one thing-> hard to determine what is actually causing the dependent variable to change

Question 6

how to improve precision

Answer 6

• repeat experiments and take an average to reduce the impact of random errors
• use digitalised measurement tool with error consideration (+/-)to remove error when reading apparatus

Question 7

accuracy

Answer 7

how close the value is to the true value of the quantity being measured

Question 8

measurement error

Answer 8

difference between a measured value and the true value

Question 9

how to improve accuracy

Answer 9

improve method or instruments to clearly reduce measurement error eg using a triggered electronic timer system rather than a stopwatch

Question 10

precision

Answer 10

how closely two or more measurement values agree with each other

Question 11

what makes a set of measurements precise

Answer 11

if they have very little spread about their mean value

Question 12

repeatability

Answer 12

the closeness of agreement between independent results obtained with the same method on identical test material, under the same conditions (same operator, same apparatus and/or same laboratory) or LOPI

Question 13

reproducibility

Answer 13

o the closeness of agreement between independent results obtained with the same method on identical test material but under different conditions (different operators, different apparatus and/or different laboratories).

Question 14

purpose of reproducing experiements

Answer 14

• checking of claimed precision
• uncovering of any systematic errors from one or other experiments/groups that may affect accuracy

Question 15

validity

Answer 15

if a measurement measures what it claims to be measuring

(no of IV)

Question 16

sources of error in calorimetry

Answer 16

• inaccurate voltage supply
• heat loss into surroundings
• insufficient oxygen -> incomplete combustion

Question 17

improving accuracy in a titration

Answer 17

• specific number of drops of indicator
• accurately measure mass of (s) being dissolved
• standardisation of standard solution (ie having consistent sig figs for all values ie if weight is 3sf, conc of soln is 3sf)