Lab / experiment Grade 12 Health & Consumer Science
Boil-Down Lab: Why Boiling Concentrates Nitrate and Lead in Water
A grade 12 health lab where students boil a salt solution, track volume and dissolved solids with a TDS meter, and explain why boiling cannot remove nitrate or lead.
Objectives
- Students will measure how the volume and dissolved solids of a salt solution change when it is boiled down, compared with an unboiled control.
- Students will use volume and concentration ratios to explain where the dissolved material goes when water boils.
- Students will explain why boiling protects against germs but makes nitrate and lead more concentrated, and name safer choices for infant formula.
Vocabulary
- Total dissolved solids (TDS)
- Total dissolved solids is a measure of everything dissolved in water that you cannot see or filter out, like minerals and salts. Water with very high TDS can taste salty or bitter, even though it might look perfectly clear.
- Nitrate
- Nitrate is a form of nitrogen that plants use as food. It gets into water from fertilizer, animal waste, and septic systems, and too much of it is unhealthy for both people and streams.
- Boil water advisory
- A public notice telling people to boil tap water before drinking it because germs may have entered the pipes, often after a main break or a loss of pressure.
Also: concentration, control, significant figures, certified treatment
Teacher overview
Boiling is the right response to a boil-water advisory, because heat kills bacteria, viruses, and parasites. Many people extend that rule to every water problem, and the result can be harmful: when water with dissolved nitrate or lead is boiled, the water leaves as steam while the dissolved substance stays behind, so the water that remains is more concentrated than before. In this lab students test the idea directly. They boil a salt solution, which stands in for nitrate or lead because it dissolves the same way and is safe to handle, and track its volume and total dissolved solids with a pocket TDS meter against an unboiled control. The scenario is a fictional family in Brenlow County whose private well tested high for nitrate and who asked a public health nurse whether boiling would make the water safe for their baby's formula.
The misconception at the center of the lab is that boiling purifies water of everything. The data show that boiling concentrates what cannot evaporate, and the analysis asks students to turn that result into advice a caregiver could act on. Plan the first session for setup, boiling, cooling, and measurement, and the second for the analysis questions and a short class discussion of question 9. Check each TDS meter in distilled water before the lab; it should read at or near zero.
Procedure
Background. The Idowu family lives in fictional Brenlow County and gets its water from a private well. A lab test found nitrate above the drinking water limit. Nitrate is especially dangerous for young infants, because it reduces the ability of their blood to carry oxygen. The family's baby is a few weeks old and drinks formula. The parents asked the county nurse whether they could boil the well water before mixing formula. Nitrate and lead are dissolved in water, like salt. They do not boil away. In this lab, table salt stands in for them. A TDS meter estimates total dissolved solids by measuring how well the water conducts electricity, and it reports the result in parts per million (ppm).
Step 1 (safety): Put on goggles before you turn on the hot plate. Keep the hot plate in the center of the bench with its cord out of the way. Move a hot beaker only with gloves or tongs. Never taste or drink any liquid in this lab. Never put the TDS meter probe into hot water, because heat can damage the meter and the reading will not be accurate.
Step 2 (check the meter): Rinse the probe with distilled water. Dip it into a cup of distilled water and confirm that it reads close to zero. Rinse it again after every reading in this lab.
Step 3 (starting reading): Pour a small amount of the stock solution into a clean cup. Measure its TDS and record it as the starting TDS for both beakers, since both come from the same stock. Pour this cup out afterward rather than adding it to a beaker.
Step 4 (fill the beakers): Measure 200 mL of stock solution in the graduated cylinder. Read the bottom of the curved surface at eye level, record the exact volume you see, and pour it into Beaker A. Mark the liquid level on the outside of the beaker. Repeat for Beaker B.
Step 5 (set up the control): Cover Beaker B with foil or a paper towel and set it on the bench away from the hot plate. Beaker B will not be heated. It shows whether anything besides boiling changes the reading.
Step 6 (boil): Heat Beaker A on the hot plate until it boils gently. Let it boil until about half of the liquid is gone, which usually takes 20 to 25 minutes. Turn the heat down if it starts to splash.
Step 7 (cool): Turn off the hot plate. Use gloves or tongs to move Beaker A to a heat-safe spot. Let it cool until the glass feels only slightly warm, at least 15 minutes.
Step 8 (final readings): Pour Beaker A into the clean graduated cylinder and record its final volume. Pour it into a clean cup and measure its final TDS. Then do the same for Beaker B.
Step 9 (clean up): Pour all solutions down the sink with running water. Rinse the beakers, the cylinder, and the probe. Leave the hot plate to cool completely before putting it away.
Data & Analysis
| Measurement | Beaker A (boiled) | Beaker B (not boiled) |
|---|---|---|
| Starting volume (mL) | ||
| Starting TDS (ppm) | ||
| Final volume (mL) | ||
| Final TDS (ppm) |
- How many milliliters of water left Beaker A while it boiled? Subtract the final volume from the starting volume.
- Divide Beaker A's starting volume by its final volume. This number is the volume ratio.
- Divide Beaker A's final TDS by its starting TDS. This number is the concentration ratio.
- Compare the volume ratio from question 2 with the concentration ratio from question 3. What does their closeness, or the gap between them, tell you about where the dissolved salt went during boiling?
- Suppose a fictional well sample of 1,000 mL is boiled until 800 mL remain, and none of its nitrate leaves with the steam. By what factor does the nitrate concentration increase?
- Look at the smallest step your TDS meter can display and the smallest marking on your graduated cylinder. How many significant figures can you justify for the concentration ratio in question 3? Explain your choice.
- What happened to Beaker B's volume and TDS? Explain why the lab needed Beaker B.
- A boil-water advisory tells people to boil their tap water when bacteria might be present. Using your results, explain why boiling is the right step for bacteria but the wrong step for nitrate or lead.
- The Idowu family asked whether they can boil their well water to make formula. Based on the background and your results, what should the nurse tell them to do instead?
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Answer Key
- Answer: Accept the student's starting volume minus final volume, usually around 100 mL.
The result should match the student's own table. If a group boiled longer or more gently than others, a larger or smaller loss is still correct for their data. - Answer: Accept the starting volume divided by the final volume, usually close to 2.
For example, a starting volume of 200 mL and a final volume of 98 mL give a volume ratio of about 2.0. Check the division against the table. - Answer: Accept the final TDS divided by the starting TDS, usually close to the volume ratio.
For example, a starting TDS of 510 ppm and a final TDS of 1,020 ppm give a concentration ratio of 2.0. Check the division against the table. - Answer: The two ratios should be close, which shows that the salt stayed in the beaker while only water left as steam.
If half the water leaves and all the salt stays, the same amount of salt sits in half as much water, so the concentration doubles. Accept small gaps explained by measurement error, a little splashing, or a sample that was still warm when measured. A concentration ratio much lower than the volume ratio usually means liquid splashed out of the beaker, which carries salt away along with the water. - Answer: 1.25 times
The concentration rises by the ratio of the volumes: 1,000 mL divided by 800 mL equals 1.25. Check: 800 times 1.25 equals 1,000. A student who answers 0.8 has divided the volumes in the wrong order. - Answer: Usually two significant figures, and accept three when the student's meter and cylinder readings support it.
A meter that displays in steps of 10 ppm, or a cylinder read to the nearest 2 mL, limits how precise the ratio can be, so a calculator result such as 2.0408 overstates what the data show. Accept any answer that ties the number of significant figures to the precision of the two instruments. - Answer: Beaker B's volume and TDS should stay about the same as at the start.
The control shows that the change in Beaker A came from boiling rather than from standing on the bench, from the meter drifting, or from the stock solution itself. Accept any answer that explains the control as a comparison that isolates boiling as the cause. - Answer: Heat kills bacteria, so boiling makes water safe from germs, while nitrate and lead are dissolved and stay behind as water boils away, so boiling makes them more concentrated.
Strong answers cite the student's own concentration ratio as evidence that dissolved material was concentrated rather than removed. - Answer: Do not boil the well water; use bottled water or water from a tested safe source for formula, and ask about a treatment system certified to remove nitrate.
Accept answers that name any of these safe choices and that warn against boiling. Strong answers add that the family should retest the well and ask the county health department for help choosing a certified treatment method.
Differentiation
Support
Give students a partly completed data table with the starting volume already written as 200 mL, and provide a worked model of questions 2 and 3 using fictional numbers different from the class results. Pair students so that one reads the cylinder and the other checks the reading at eye level.
Extension
Ask students to collect a few milliliters of condensed steam by holding a cold, dry watch glass over the boiling beaker with tongs, let the drops run into a clean cup, and test them with the TDS meter if the probe can reach. They should explain how this reading supports or challenges their answer to question 4, and why distillation, which collects and cools the steam, is a real treatment method while boiling in an open pot is not.
ELL supports
Pre-teach "dissolved," "concentrate," "boil away," "control," and "formula" with a two-panel sketch showing salt grains in a full beaker and the same grains in a half-full beaker. Provide sentence frames for questions 8 and 9: "Boiling kills ___, but it does not remove ___ because ___." Allow students to answer question 4 with a labeled diagram and one sentence.
Extensions
Have students find the page on nitrate or lead in drinking water from their state health department and check whether it gives the same advice about boiling. Students can then write a short text message, in plain language, that a nurse could send to a family in the Idowus' situation.