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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)

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.

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