Testing Water Quality – Hardness and pH
GCSE Chemistry
Water may look clean, but its chemical properties vary widely depending on geology, treatment, and environmental factors. Two of the most important measures students learn at GCSE are water hardness and pH. These tests show how dissolved ions affect everyday life — from limescale in kettles to how soap lathers in hard or soft water.
What Is Hard Water?
Hard water contains dissolved calcium (Ca²⁺) and magnesium (Mg²⁺) ions.
These ions come from rocks such as limestone, chalk, and dolomite as rainwater slowly dissolves them.
Hardness affects:
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how well soap lathers
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the formation of limescale
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water taste
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efficiency of kettles, boilers, and washing machines
Testing hardness gives students a direct link between chemistry and household science.
The Soap Solution Test (GCSE Core Practical)
Equipment:
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Water samples (tap water, bottled water, distilled water, rainwater, river water, pond water, seawater)
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Standard soap solution
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Conical flasks
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Measuring cylinder
Method:
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Place 10 cm³ of water into the flask.
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Add soap solution a few cm³ at a time, shaking well.
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Measure how much soap is needed to form a stable lather for 10 seconds.
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Repeat for each water sample.
Interpretation:
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More soap needed → harder water
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Less soap needed → softer water
This test works because Ca²⁺ and Mg²⁺ ions react with soap to form scum, reducing lather.
Testing pH
pH tells us how acidic or alkaline water is. Most drinking water is pH 6.5–8.5, depending on treatment and natural minerals.
Methods:
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pH paper (quick, approximate)
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Universal indicator (colour scale)
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Digital pH sensor (accurate, ideal for A-level or more precise investigation)
Causes of variation:
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Dissolved carbon dioxide
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Natural mineral content
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Pollution or acid rain
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Water treatment chemicals (e.g. chlorine)
Students can compare pH values across water sources and relate differences to geology and human activity.
Typical Classroom Results
| Water Sample | Soap Added for Lather (cm³) | Hardness | pH |
|---|---|---|---|
| Distilled water | 1–2 | Very soft | ~7 |
| Local tap water | 4–6 | Moderately hard | 7.5 |
| Bottled spring water | 6–8 | Hard | 7 |
| Rainwater | 1–2 | Soft | 5.5–6 (slightly acidic) |
Students immediately see why some regions suffer from limescale — and why rainwater can be acidic despite looking clean.
Skills Highlight
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Performing fair comparative tests
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Measuring and recording pH values
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Interpreting data from qualitative and quantitative methods
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Understanding ions in solution and their effects on everyday life
Why It Works in Teaching
These tests connect GCSE Chemistry directly to real life. Students recognise the science behind household appliances, water treatment, soap use, and environmental issues — making the topic both relevant and memorable.
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