Theory behind this practical:

Although we know of 4 ways to affect the rate of a reaction, you are only required to practically investigate how rate is affected by concentration. In this experiment, you are going to vary the concentration of a reactant. We would expect that in a higher concentration, there are more particles within the same unit volume so there will be more collisions per second. You will also be able to observe the effect of changing the temperature.

During these practicals, don’t miss the opportunity to look at techniques, keeping the control variables constant and how to improve reliability/accuracy.


Disappearing cross

  • Pour 40cm3 or room temperature Sodium thiosulphate solution into the conical flask using the 100cm3 measuring cylinder.
  • Place the conical flask onto the cross provided
  • Add 5cm3 of hydrochloric acid to the 10cm3 measuring cylinder
  • Simultaneously, pour in the acid and start the timer
  • Record the time when you can no longer see the cross when looking down through the flask
  • Repeat the experiment using sodium thiosulphate solution from the water bath to observe the effect of raising the temperature on the rate of reaction.
  • Ensure that you do not mix up the measuring cylinders or the reaction will begin early.
  • All waste must go into the waste bucket, not down the sinks

Gas syringe CO2 collection

  • Using a weighing boat, weigh out 2g of calcium carbonate chips
  • Place these into the bottom of the conical flask (with side arm).
  • Connect the arm to the securely setup gas syringe
  • Carefully measure 50cm3 of 0.2 moldm-3 hydrochloric acid.
  • Simultaneously, pour the acid into the conical flask, insert the bung and start the timer.
  • Record the volume on the gas syringe every 5 seconds.
  • Repeat using 0.5 moldm-3 hydrochloric acid.
  • Empty waste into sieve/bucket provided

Technician notes:

  • Sodium thiosulphate solution – 40gdm-3.
  • Hydrochloric acid 1.0 moldm-3
  • Conical flasks
  • 10cm3 and 100cm3 measuring cylinders
  • Printed/laminated crosses
  • Timers
  • Water bath at 50°C with sodium thiosulphate solution
  • Waste bucket
  • Balances
  • Weighing boats
  • Spatulas
  • Calcium carbonate chips
  • Conical flasks, tubes and gas syringes
  • 100cm3 measuring cylinders
  • 0.2 and 0.5 moldm-3 hydrochloric acid
  • Timers
  • Bucket and sieve for waste


Usual lab rules must be adhered to:

  • Lose hair tied up
  • Goggles on throughout the lesson
  • Bags away and stools tucked under
  • Notify the teacher immediately if any spills or breakages occur
  • Better to have windows open when sulphur is being produced

For further/more detailed information, check all reagents and practices at CLEAPSS.