Testing Water for Hardness

This activity introduces students to the concept that there are substances dissolved in water. This awareness provides the basis for understanding the presence of heavy metals, nitrates, phosphates, and other water-soluble substances. Furthermore, the lab features many of the things that are desirable in a hands-on science experience, including measuring, observing, data collection, graphing, comparing, etc. To obtain a copy of the student worksheet, click on the HOT LINK below.

1. You and/or your students will need to obtain ten different water samples. Here are types I have used: Helena tapwater (surface water), East Helena tapwater (groundwater), distilled, 2-3 different types of "bottled mineral waters" from a grocery store, unsoftened water from students' wells, "limestone water" (soak a piece of limestone in a beaker of distilled water for a few days), Prickly Pear Creek, Trout Creek, Canyon Ferry Reservoir.

2. You will need to set up 10 water sample stations around the perimeter of the room. At each station there will be a graduated cylinder (25 ml or less), a beaker (for pouring), and a water sample (labeled).The photo below shows how to set up each of the 10 stations.

3. To prepare the soap solution you will need a bar of Ivory soap. Using a knife, scrape the bar to obtain about 7 grams of powdery shavings. This makes enough for over 50 groups to do the lab. Slowly stir these into 560 ml of distilled water. Prepare this 12-24 hours before it will be used for the lab. This should result in a thick, milky white liquid as shown in flask below.

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4. Each group will need one dropper bottle containing the soap solution and an eye dropper. They will also need five test tubes (10 ml). Set these in cups, which will serve as test tube holders (shown above on the right). After testing five samples of water, the students will need to rinse the tubes before doing the next five samples.

Getting the students ready . . .

Read the introduction. Explain that, in hard water, soap molecules combine with ions (such as calcium) to form curds. Remind them to look for this white flaky substance during the lab. Do the pre-lab questions together. The questions are intended to make sure the students have the background to understand what they will be doing in the lab.

1. An ion is an atom that has gained or lost one or more electrons. Positive ions are atoms that have lost, whereas negative ions have gained electrons.

2. calcium ions (2+), magnesium ions (2+), iron III ions (3+). . . calcium is the most common

3. These dissolve into the water as the water comes into contact with the rock. Some of the more common rocks and minerals that contribute to water hardness are limestone, magnetite, gypsum, and dolomite.

4. Distilled water is pure water. . . It has nothing dissolved in it.

5. Here are some of the problems caused by hard water.

A) It requires that more soap be used.

B) When soap is used, curds (a.k.a. soap scum) is formed, resulting in bathtub ring. The greasy deposit can also build up on clothes, hair, and skin.

C) When hard water evaporates, the dissolved minerals leave a white deposit (water spots)

D) When hard water is heated, a rock-like scale builds up inside pipes and hot water heaters. Unlike sugar and salt, calcium is more soluble in cold water than in hot water.