Investigate the relationship between the concentration of calcium ions and the ‘hardness’ of water

This project should take approximately four hours to complete in full. The activities were initially created for 14–16 year-old learners but can be adapted for other age groups.

  • Previews of the Hard water challenge PowerPoint presentation slides, student workbook, teacher and technician notes

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    Get the student workbook, method sheets, teacher notes and technician notes as MS Word and pdf. Plus, editable MS PowerPoint and pdf slides.

Use the resource in a sequence of timetabled lessons, science clubs or during an activity day.

Learning objectives

  • Explain the construction and use of a look-up chart.
  • Follow instructions for carrying out a procedure to test hard water.
  • Compare the effectiveness of different water filters.
  • Describe the domestic and health issues associated with hard water.

Guidance notes

In this series of activities, learners will work in pairs or groups of three to investigate the relationship between the concentration of calcium ions and the ‘hardness’ of samples of water.

Use slides 3–7 of the PowerPoint to introduce what ’hardness of water’ means and how this can be tested.

An image of three school children looking in different directions surrounded by aspects of chemistry

This resource was developed as part of the Chemistry for All project. The project was set up to explore and address barriers to participation in UK chemistry undergraduate study through a longitudinal project. Read the findings relevant to teachers, outreach providers, education policymakers and parents in the summary report, or download the full research report.

Activity 1: measuring calcium concentrations

In Activity 1, learners will measure the concentration of calcium ions, in parts per million (ppm), of several solutions and assess their ‘hardness’ by adding a coloured indicator and a reagent called ethylenediaminetetraacetic acid (EDTA). They will then use their results to create a look-up chart. Find the method and questions in the student workbook.

Activity 2: investigating ion-exchange filters

Activity 2 requires the learners to investigate how effectively various water filters remove calcium ions from a sample of calcium chloride with a concentration of 500 ppm. They will do this by determining the calcium ion concentration after the samples have been filtered using their look-up charts. Use slides 10 and 11 of the PowerPoint to introduce the use of water filters and show some career links.

More resources

Activity 3: using dipsticks to measure the hardness of water

In Activity 3, the learners will confirm their results using commercially available dipsticks which can show the hardness of water samples in a very short time. The instructions for dipsticks tend to vary, so follow the instructions provided on the packaging.

Find the answers to all the activities in the teacher notes and on the slides. 


The Chemistry for All project found that recognising the value and importance of chemistry, and appreciating how it can lead to interesting and well-paid jobs strongly related to learners’ future aspirations.

Use the job profiles embedded in the PowerPoint to introduce learners to scientists who use their chemistry skills, knowledge and qualifications in their careers. Share our careers website, A Future in Chemistry, with learners to discover the different study options available to them, find more videos of chemists making a difference and try our career options game. 

Health and safety

Read our health and safety guidance and carry out a risk assessment before running any live practical. Specific safety advice for the chemicals used in this project can be found in the technician notes.

The safety equipment suggested is in line with CLEAPSS requirements. For non-hazardous substances, wearing lab coats can help to protect clothes. The safety rules might be different where you live so it is worth checking local and school guidance. 

Find more projects like this, plus shorter activities suitable for single science lessons or clubs on our Outreach resources hub.


Chemistry for All project resources