GCSEChemistryWJECRevision Notes1. Chemical Substances, Reactions & Essential Resources1.3 WaterDetermining Hardness in Water

Determining Hardness in Water (WJEC GCSE Chemistry)

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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

investigating-hard-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. 
Water sample Number of 1 cm3 volumes of soap solution added Total volume of soap solution added (cm3)
A    
B    
C    
D    
A- boiled    
B- boiled    
C- boiled    
D- boiled    

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

Worked example

A student obtained the following results which she then plotted on a bar chart . 

Identify the water samples as permanent hard, temporary hard or soft. 

Water sample Number of 1 cm3 volumes of soap solution added Total volume of soap solution added (cm3)
A up diagonal strike vertical line vertical line vertical line vertical line end strike up diagonal strike vertical line vertical line vertical line vertical line end strike vertical line vertical line vertical line vertical line 14
B up diagonal strike vertical line vertical line vertical line vertical line end strike up diagonal strike vertical line vertical line vertical line vertical line end strike vertical line vertical line vertical line 13
C up diagonal strike vertical line vertical line vertical line vertical line end strike up diagonal strike vertical line vertical line vertical line vertical line end strike ||||{"language":"en","fontFamily":"Times New Roman","fontSize":"18","autoformat":true} |{"language":"en","fontFamily":"Times New Roman","fontSize":"18","autoformat":true} 16
D ||{"language":"en","fontFamily":"Times New Roman","fontSize":"18","autoformat":true} 2
A- boiled ||||{"language":"en","fontFamily":"Times New Roman","fontSize":"18","autoformat":true}||||{"language":"en","fontFamily":"Times New Roman","fontSize":"18","autoformat":true} ||||{"language":"en","fontFamily":"Times New Roman","fontSize":"18","autoformat":true} 14
B- boiled ||{"language":"en","fontFamily":"Times New Roman","fontSize":"18","autoformat":true} 2
C- boiled ||||{"language":"en","fontFamily":"Times New Roman","fontSize":"18","autoformat":true} ||||{"language":"en","fontFamily":"Times New Roman","fontSize":"18","autoformat":true} ||||{"language":"en","fontFamily":"Times New Roman","fontSize":"18","autoformat":true} 15
D- boiled ||{"language":"en","fontFamily":"Times New Roman","fontSize":"18","autoformat":true} 2

bar-chart

Answer:

Either the table or the bar chart can be used to identify the water samples.

  • Sample A is permanent hard water
    • It requires a large volume of soap solution to form a lather before boiling
    • The volume of soap solution needed to form a lather remains the same after boiling 
  • Sample B is temporary hard water 
    • It requires a large volume of soap solution to form a lather before boiling
    • The volume of soap solution needed to form a lather reduces to the same levels as D
  • Sample C is permanent hard water
    • It requires a large volume of soap solution to form a lather before boiling
    • The volume of soap solution needed to form a lather remains very similar after boiling
  • Sample D is soft water 
    • It requires only 2 cm3 of soap solution to form a lather
    • This is the same after boiling

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    Alexandra

    Author: Alexandra

    Expertise: Chemistry

    Alex studied Biochemistry at Newcastle University before embarking upon a career in teaching. With nearly 10 years of teaching experience, Alex has had several roles including Chemistry/Science Teacher, Head of Science and Examiner for AQA and Edexcel. Alex’s passion for creating engaging content that enables students to succeed in exams drove her to pursue a career outside of the classroom at SME.