Measuring the rate where a precipitate forms

The apparatus needed depends on the equipment available.

Inspection

The rate can be measured by placing the reaction container on top of a black ‘X’ before adding the chemicals together (example shown in diagram below). If you time how long it takes until you can no longer see the X due to the formation of the precipitate, you can calculate the rate using the equation.

rate = 1 ÷ (time taken for X to be obscured)

A cross drawn on paper, with a conical flask above it containing sodium thiosulfate solution. When dilute acid is added, time how long it takes for the cross to disappear.

Using a light sensor

The amount of light passing through the reaction can be measured directly using a light sensor and a data logger. The rate can then be calculated by using the light sensor to work out how long it takes for the light level to stop decreasing.

Apparatus to measure the rate where a precipitate forms - showing a light sensor under a conical flask surrounded by a light shield.

Advantages for inspection/disadvantages for light sensor:

  • light sensors are more expensive and so may not be readily available
  • light from the surrounding area could possibly interfere with results from light sensor

Advantages for light sensor/disadvantages for inspection:

  • light sensors can provide data for the entire reaction, allowing the rate of reaction to be calculated for any point in the reaction
  • the rate of reaction can be determined more accurately
Graph showing the light intensity against time for a rate experiment using a four solutions and a light sensor. Solution A has the steepest slope, meaning it has the fastest rate of reaction.The graph above shows light intensity against time for a rate experiment using a light sensor. Solution A has the steepest slope, meaning it has the fastest rate of reaction.