Lab / experiment Grade 6 Health & Consumer Science

Hard Water and Soap: The Nettlecrag Suds Test

A grade 6 consumer health lab comparing how many drops of soap it takes to make lasting suds in plain tap water and in water stirred with Epsom salt.

Grade: Grade 6
Subject: Health & Consumer Science
Material: Lab / experiment
Time: 50 min
Prep: Moderate prep
Materials: Clear jars with tight lids, about 500 mL each (2 per group), Epsom salt, about 2 teaspoons per group, Measuring teaspoon (1 per group), Liquid castile soap or another pure liquid soap (not dish detergent) in a dropper bottle (1 per group), Measuring cup marked in milliliters (1 per group), Ruler marked in centimeters (1 per group), Masking tape and marker for labels, Printed data sheet (1 per group), Paper towels for spills

Standards

6.RP.A.1 Common Core Math

Understand the concept of a ratio and use ratio language to describe a ratio relationship between two quantities. For example, 'The ratio of wings to beaks in the bird house at the zoo was 2:1, because for every 2 wings there was 1 beak.'

How this material aligns: Students state the drops of soap needed in hard water against the drops needed in tap water as a ratio, using the for-every phrasing this standard models.

All materials for 6.RP.A.1 →

6.RP.A.3 Common Core Math

Use ratio and rate reasoning to solve real-world and mathematical problems, e.g., by reasoning about tables of equivalent ratios, tape diagrams, double number line diagrams, or equations, including finding a percent of a quantity and converting measurement units.

How this material aligns: Students use their drop-count ratio to find how many times as much soap the hard sample needed and to scale that difference across a week of dishwashing.

All materials for 6.RP.A.3 →

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This material is in the public domain (CC0) and may be copied, edited, and shared freely, without attribution. Updated 2026-09-03.

Objectives

Students will count the drops of soap needed to make lasting suds in two water samples.
Students will write the hard-water and tap-water drop counts as a ratio and compare them.
Students will explain why hard water is a laundry and dish problem rather than a dirty-water problem.

Vocabulary

Key words: hard water, dissolved minerals, ratio, suds

Teacher overview

Water picks up calcium and magnesium as it moves through rock and soil, and water carrying a lot of those minerals is called hard. Hard water leaves spots on glasses, dulls laundry, and takes more soap to lather, which is why it shows up on a household budget rather than on a health warning. In this lab students add Epsom salt to one jar to make a hard sample, leave a second jar as plain tap water, and count the drops of soap each one needs before the suds last. The misconception to name out loud before students start is the common belief that hard water is dirty water. Hardness is dissolved mineral, not dirt, and a hard sample can look perfectly clear while still fighting the soap.

Hard Water and Soap: The Nettlecrag Suds Test — Grade 6 Health & Consumer Science

Name: ____________________________ Date: ______________

Procedure

Step 1 (safety, before you start): Do not drink or taste anything in this lab, including the plain tap water. Epsom salt is sold as a bath product and is not food. Keep lids tight while shaking, and wipe up spills right away so the floor stays dry.

Step 2 (label your jars): Put a tape label reading "tap" on one jar and "hard" on the other. Set them side by side where nobody will knock them over.

Step 3 (fill both jars the same way): Measure 300 mL of cold tap water into each jar. Using the same amount in both jars is what makes the test fair.

Step 4 (make the hard sample): Add 2 level teaspoons of Epsom salt to the jar labeled "hard." Put the lid on and shake for 30 seconds, then let it sit until the water looks clear again. Add nothing to the jar labeled "tap."

Step 5 (agree on what counts as suds): Decide as a group what a successful result looks like before you test. A workable rule is a layer of suds that still covers the whole surface 10 seconds after you stop shaking. Write your rule on the data sheet.

Step 6 (test the tap jar): Add 1 drop of soap, close the lid, and shake for 10 seconds. Check against your rule. If the suds fail, add 1 more drop and shake again. Keep going one drop at a time, and record the total number of drops it took.

Step 7 (test the hard jar): Repeat Step 6 exactly with the hard jar, adding one drop at a time and shaking the same 10 seconds each round. Record the total drops.

Step 8 (measure the suds): Stand the ruler against each jar and measure the height of the suds layer in centimeters right after the final shake. Record both heights.

Data & Analysis

Jar	What is in it	Drops of soap needed	Suds height (cm)
Tap	300 mL cold tap water
Hard	300 mL tap water plus 2 tsp Epsom salt

Write the ratio of hard-jar drops to tap-jar drops for your group, and say it out loud in ratio language starting with "for every."
Another group needed 4 drops in the tap jar and 10 drops in the hard jar. How many times as many drops did their hard jar need?
Using your own two counts, how many extra drops did your hard jar need compared with your tap jar?
Suppose a family in the town of Nettlecrag washes dishes 7 times a week and uses 5 extra drops of soap each time because the water is hard. How many extra drops is that in one week?
Compare the suds heights in your two jars. Which jar held the taller suds, and what does that tell you about how the soap behaved in each one?
A neighbor says the hard water at Nettlecrag must be dirty because it takes so much soap. Use your own results to explain why water can be hard and still be clean.

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 group's own two counts written as a ratio in the order hard to tap, such as 9 to 3. The spoken form should follow the pattern "for every 3 drops the tap water needed, the hard water needed 9."
Answer: 2.5 times as many. Divide 10 drops by 4 drops, which gives 2.5, so the hard jar needed two and a half times the soap.
Answer: Accept the difference between the group's own counts, found by subtracting the tap-jar drops from the hard-jar drops. Most groups land between 2 and 8 extra drops, so a much larger gap usually means the shaking or the suds rule was not applied the same way to both jars.
Answer: 35 extra drops. Seven washes at 5 extra drops each gives 7 times 5, which is 35 drops in one week.
Answer: The tap jar almost always holds the taller suds. Accept any answer matching the group's measurements that explains the minerals in the hard jar grabbed part of the soap first, so less of it was left to build a lather.
Answer: Because hardness is dissolved mineral, not dirt. Accept any answer noting that the hard jar looked clear after the salt dissolved yet still needed more soap, so the soap trouble came from what was dissolved in the water rather than from anything floating in it.

Differentiation

Support

Prepare the hard sample in advance for these groups so they begin at Step 5 with two ready jars. Give them a drop-count tally chart with numbered boxes to mark off, which keeps the count accurate while they concentrate on judging the suds.

Extension

Have students run a third jar with half a teaspoon of Epsom salt instead of two, then describe how the drop count changed and predict what a jar with four teaspoons would need.

ELL supports

Pre-teach "hard water," "dissolved," "suds," and "ratio" with a labeled picture of a spotted glass, a jar of clear water with dots drawn to stand for minerals, and a foam layer. Provide the sentence frame: "Our tap jar needed ___ drops and our hard jar needed ___ drops, so the ratio was ___ to ___."

Extensions

Ask students to look up how a home water softener works and write a short paragraph explaining what a softener would change about their two jars and what it would not change.