Rates of Reaction

• A hypothesis is a proposal that could explain a fact or observation, and in science a hypothesis must be testable
• Disappearing cross reaction

  1. Use a measuring cylinder to put 10 cm³ of sodium thiosulfate solution into a conical flask

  2. Place the conical flask on a printed black cross

  3. Add 10 cm³ of hydrochloric acid into the conical flask

  4. Swirl the solution and start a stopwatch

  5. Look down through the top of the flask until the solution turns cloudy and you can no longer see the cross

  6. Stop the clock

  7. Repeat the experiment using lower concentrations of sodium thiosulfate solution

  8. Calculate mean values for each concentration of sodium thiosulfate solution
• Reproducibility

  A measurement is reproducible if it can be repeated by another person or using a different technique or equipment and still get the same result
• Measuring the volume of gas produced


  1. Use a measuring cylinder to place 50 cm³ of hydrochloric acid into a conical flask

  2. Attach the conical flask to a bung and a delivery tube

  3. Place the delivery tube into a container filled with water

  4. Place an upturned measuring cylinder also filled with water over the delivery tube

  5. Add a 3 cm strip of magnesium to the hydrochloric acid and start a stopwatch

  6. Measure the volume of hydrogen gas in the measuring cylinder every 10 seconds until no more hydrogen is given off

  7. Repeat the experiment using different concentrations of hydrochloric acid
• The greater the concentration of a chemical in a reaction, the faster the reaction takes place
• This finding is reproducible because it is shown by two different experiments