Here’s the full method (this will get you full marks in the exam). Anything that is highlighted in yellow is parts of the experiment in which we made it reliable:
1) Place a black cross on a white tile using a marker pen and label all equipment to avoid cross contamination.
2) Measure out 15ml of 10g/l (concentration) sodium thiosulphate using a measuring cylinder and place into a clean labelled boiling tube.
3) Measure out 15ml of 1 Mol/l (concentration) hydrochloric acid using a 25ml measuring cylinder and transfer into a clean labelled boiling tube.
4) Measure the temperature of the acid using a thermometer.
5) Add both chemicals to a small conical flask placed upon the marked white tile and immediately start the timer.
6) Standing directly above the conical flask record the time taken until you can no longer see the cross.
7) Take the temperature of the liquid at the end of the reaction and record the average.
8) Empty the flask, rinsing it as well as the thermometer to avoid contamination.
9) Repeat a further two times.
10) Add 150ml of water to a 250ml beaker and heat to 30 degrees on a tripod and gauze using a gentle blue Bunsen flame.
11) Measure out the appropriate volumes of hydrochloric acid and sodium thiosulphate as in steps 2 and 3.
12) Place the boiling tubes into the warm water and leave for one minute to get to temperature.
13) Repeat as steps 5-9.
14) Repeat for temperatures of 35, 40, 45 and 50 degrees.
15) Ignore anomalous results and do extra repeats to compensate.
The method has been made valid by:
– Keeping concentration of the liquids the same so it doesn’t affect rate of reaction and successful collisions.
– Volume of liquids as volume affects depth so the amount of liquid the observer looks into will be more.
– Same conical flask as the same depth of liquid will be between cross and observer.
– Keeping the same observer as eye sight may differ between observers.
– Same volume of acid and sodium thiosulphate so there will be same amount of particles in conical flask so same chance of collision.
– Blackness/size of cross as cross may go out of sight easier if its smaller or thinner.
The method was made reliable by:
– Labelling all equipment to prevent contamination.
– Clean tubes when reusing them to prevent contamination.
– Immediately start the timer when the two liquids are in conical flask.
– Stand directly above when observing.
– Record average temperature.
– Repeat experiment.
– Ignore anomalous results.
Here are some other Science IAA experiments you may be interested in: