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E2.8 · Investigate local drinking-water pollutant concentrations

Learn to investigate local drinking-water pollutant concentrations through clear examples and targeted practice.

Ontario Grade 11 Chemistry

Solutions and Solubility

A careful, evidence-based approach to finding and interpreting local water-quality information

Tap water usually looks clear, but appearance alone cannot show whether a pollutant is present or how much is in the water. A pollutant is a substance present in the environment that may cause harm, depending on the substance and its amount. In this lesson, you will learn how to investigate pollutant concentrations in local drinking water without treating an unverified result as fact. You can investigate by studying an appropriate public report or, where a suitable procedure and supervision are available, by collecting and testing a water sample.

What you will learn

Describe how a pollutant concentration reports the amount of a substance in a volume of water.
Plan a fair investigation using local drinking-water information or samples.
Use units carefully when recording and comparing reported concentrations.
Explain why a concentration result needs context, such as its source, date, and measurement method.

1. From an observation to a useful question

Start with an observable fact: water from two taps can look the same, even if a test would find different amounts of a pollutant. Clear water is not proof that every pollutant is absent. A colour change or other visible sign can be useful evidence, but it does not identify a substance or measure its concentration on its own.

Before investigating, review the idea of a measurement. A number needs a unit to say what was measured. For example, a report might state a concentration in milligrams per litre. The unit tells you that the reported mass of a substance is being compared with a volume of water.

A focused question helps keep an investigation manageable. For example: “What concentration of a named pollutant is reported for drinking water in my community, and when and where was it measured?” You may also compare results from different dates or locations if the source uses a consistent method.

Decide what counts as local drinking water for your question. It might be water supplied to a community or water from a particular private source. Do not treat one sample, one address, or one date as a result for every local water source.

Appearance alone cannot establish a pollutant concentration.
Name the pollutant, location, source, and time period in the question.
A measurement requires a number and a unit.

2. Plan the investigation and judge the evidence

For a first investigation, look for a recent drinking-water report from a responsible local or public organization. Check that the report identifies the water system or sampling location, the pollutant, the concentration, the unit, and the date or period covered. Also look for an explanation of how the sample was tested. If a detail is missing, record that limitation instead of guessing.

A sample is a portion of water chosen for testing. If you collect one, follow your teacher’s directions and any approved sampling procedure. Use the correct container, avoid touching the inside of its lid or container, label its source and collection time, and follow the instructions for storage and delivery. These steps help reduce contamination or changes that could affect a result. Do not drink a sample collected for testing.

An investigation should keep its comparison fair. When comparing two results, first check that they concern the same pollutant and use compatible units. Check whether the locations, collection dates, and testing methods are sufficiently similar for the comparison you want to make. A result from one location does not automatically describe another location.

Write down the evidence as reported, not as you think it should be. Record the value, unit, source, date, location, and any stated detection limit or method information. A detection limit is the smallest amount that a particular method can report. A statement that a pollutant was not detected does not necessarily mean its concentration is exactly zero.

If you compare a result with a health or regulatory value, obtain that value from a current, appropriate official source. Make sure it applies to the same pollutant and unit, and note its date and scope. This lesson does not supply a universal limit: requirements and guidance may depend on the substance and the water system.

Record where and when a result applies, along with its source and method.
Do not turn “not detected” into “zero.”
Use a current official source for any comparison value.

3. Understand the particle model and concentration units

At the particle level, a water sample contains water particles and may contain particles of other substances. A pollutant can be present in an amount too small to see. A concentration describes how much of a named substance is present in a stated amount of the mixture. It does not, by itself, explain where the substance came from or what caused its presence.

For dilute water samples, reports often use milligrams per litre. This means milligrams of the named substance in each litre of water. Another common unit is micrograms per litre. The prefix “milli” means one thousandth, and “micro” means one millionth. Therefore, one milligram equals one thousand micrograms. Keep the unit attached to the value when copying or comparing results.

Some reports use parts per million. In water, this is often treated as approximately equivalent to milligrams per litre for dilute samples, but do not assume units are interchangeable in every context. Use the unit and explanation given by the report, and convert only when the relationship and required information are clear.

If a result comes from a test, the instrument or test method produces evidence that must be recorded with its context. Follow the method’s instructions and safety directions. Do not claim that a proposed test has produced a result before it has been carried out. In a written investigation using public data, identify the report as the evidence instead.

c=\frac{m}{V}

Concentration links the amount of a named substance to an amount of water.
A pollutant can be present even when water looks clear.
Keep units and method information with every result.

4. Interpret and communicate the result

The concentration relationship uses

c

for concentration,

m

for the mass of the substance, and

V

for the volume of the sample. If mass is in milligrams and volume is in litres, the concentration unit is milligrams per litre. This relationship is useful when the mass and volume are both known. For a result taken from a report, use the reported concentration rather than inventing a mass or volume.

A good conclusion answers the investigation question and points to the evidence. State the pollutant, the reported concentration and unit, and the location and time period. Then explain what comparison you made, if any, and name the official source used for the comparison. If you cannot verify a detail, say so.

Avoid broad claims such as “all local water is safe” from a single result. A concentration result describes the sample or system covered by its source, not every tap at every time. Be clear about the limits of your evidence. Keep calculations readable, include units in each step, and round only to a precision supported by the reported data.

Conclude only what the evidence supports.
A result applies to the sample, place, and time described by its source.
Show units throughout calculations and avoid unsupported precision.

Worked example

Calculating a concentration from a hypothetical sample

A clearly labelled practice sample—not a real local measurement—contains

3.6\,\mathrm{mg}

of a named pollutant in

1.5\,\mathrm{L}

of water. Calculate its concentration in milligrams per litre.

Choose the relationship
Concentration is mass divided by volume. The given mass is in milligrams and the volume is in litres, so the result will be in milligrams per litre.
$c=\frac{m}{V}$
Substitute values with units
Use the stated mass and volume. Keeping the units in the substitution makes the result’s unit clear.
$c=\frac{3.6\,\mathrm{mg}}{1.5\,\mathrm{L}}$
Calculate and round
Divide the numbers. Both given values have two significant figures, so report the answer to two significant figures.
$c=2.4\,\mathrm{mg/L}$

Answer: The hypothetical sample has a concentration of

2.4\,\mathrm{mg/L}

Check: The units reduce to milligrams per litre, as required. The value is a practice calculation, not evidence about local drinking water.

Common mistakes and how to avoid them

Assuming clear-looking water has no pollutants.

Correction: A pollutant may be present in an amount that cannot be seen. Use a suitable test or a trustworthy report.

Copying a concentration without its unit, location, or date.

Correction: Record the value together with its unit, source, sampling location, and date or reporting period.

Treating a result from one sample as a result for an entire community.

Correction: State exactly what location and period the evidence covers. Do not generalize beyond that scope.

Reporting “not detected” as zero concentration.

Correction: Record the report’s wording and detection information. The method may not be able to detect amounts below its detection limit.

Comparing a result with a value for a different pollutant or using incompatible units.

Correction: Check the pollutant name, unit, source, date, and scope of both values before comparing them.

Lesson summary

A drinking-water pollutant concentration describes an amount of a named substance relative to an amount of water.
Investigate with a focused question and trustworthy, well-described evidence.
Record the source, location, date, method, value, and unit.
Use careful comparisons and conclusions that match the limits of the evidence.

Check your understanding

Question 1

A local report gives a pollutant result but does not identify the sampling date. What is the best next step?

Assume the result describes today’s water.
Find the date or reporting period from the source, or state that it is unknown.
Treat the concentration as zero until a date is found.
Use the number without a unit because the source is local.

Show answer and explanation

Find the date or reporting period from the source, or state that it is unknown.

A date or reporting period is needed to understand when the evidence applies. If it cannot be found, report that limitation rather than guessing.

Question 2

Which statement correctly interprets a result of

4.2\,\mathrm{mg/L}

The sample contains $4.2\,\mathrm{mg}$ of pollutant in every litre, as reported.
The sample contains $4.2\,\mathrm{L}$ of pollutant in every milligram.
The water has no pollutant because the number is small.
Every water source in the community has exactly this concentration.

Show answer and explanation

The sample contains

4.2\,\mathrm{mg}

of pollutant in every litre, as reported.

Milligrams per litre expresses pollutant mass per volume of water. The result applies only as far as the report’s location and time period allow.

Question 3

A method reports that a pollutant was not detected. Which conclusion is most careful?

Its concentration is proven to be exactly zero.
The pollutant is present at a concentration of one milligram per litre.
The method did not detect it; check the report for the method’s detection information.
The result proves that all local water is free of pollutants.

Show answer and explanation

The method did not detect it; check the report for the method’s detection information.

A method may not detect amounts below its detection limit. “Not detected” is not the same as proof of zero concentration.

Key terms

Pollutant: A substance present in the environment that may cause harm, depending on the substance and its amount.
Concentration: The amount of a named substance in a stated amount of a mixture.
Sample: A portion of water selected for testing or study.
Detection limit: The smallest amount a particular test method can report.
Significant figures: The digits in a measured value that show its reported precision.

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About this lesson and its review

Published by DoAssignment. This AI-assisted lesson follows Ontario Grade 11 Chemistry (SCH3U), expectation E2.8. It is a study resource, not an official curriculum publication.

Before publication, the draft is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability, and then requires administrator approval. Errors can still occur, so corrections are welcomed.

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