Determining Hardness in Water (WJEC GCSE Science (Double Award)): Revision Note

Specified Practical: Hardness in Water

Introduction

• The amount of hardness in water can be determined using soap solution

• Hardness is caused by calcium and magnesium ions

• Hard water does not form a lather with soap as easily as soft water does due to the calcium and magnesium ions reacting with the soap to form scum

• More soap is therefore needed to form a lather

• Hard water can have temporary or permanent hardness

• Temporary hard water will become soft when it boils, whereas permanent water doesn't

Apparatus

• Conical flask and stopper 

• Dropping pipette 

• Measuring cylinder 

• Stopwatch 

• Soap solution 

• Water samples A, B, C and D, boiled A, B, C and D

Diagram

How to test for hard and soft water

Method

1. Measure 50 cm³ of water sample A into a conical flask 

2. Add 1 cm³ of soap solution using a dropping pipette 

3. Insert the stopper and shake vigorously for 5 seconds 

4. Repeat step 2 until a lather forms and lasts for 30 seconds 

5. Record the total volume of soap solution needed to create the lather 

6. Repeat these steps with 50 cm³ of each water sample 

Practical Tip

• Make sure the stopper is properly inserted into the conical flask and hold it in place using your hand when vigorously shaking to prevent any spillages.

Analysis of Results

• Results should be recorded in a table like this

• It is best to record the amount of soap solution as a tally each time 1cm³ is added. 

Evaluation

• Draw a bar chart of volume of soap solution against water sample 

• Use the results to identify which samples are hard water, temporary hard water, permanent hard water and soft water

Conclusion

• Hard water samples will require a high volume of soap solution to form a permanent lather 

• Soft water samples will require a low volume of soap solution to form a permanent lather 

• If the water has temporary hardness, the volume of soap solution to form a permanent lather will reduce after boiling

• If the water has permanent hardness, the volume of soap solution to form a permanent lather will not reduce after boiling