 Form a group of 3 students near where you are sitting.  Work with other members of the group toward an answer. Solitary, individual work is not permitted.  Convince each other of the correctness of the group’s response. Discuss, argue, and intellectually engage.  Record your answer with the names of your group members on your group answer sheet.

 Assign one person to be the scribe. This person is in charge of recording the group’s work on paper to be turned in.  Assign one person to be the spokesperson. This person is in charge of presenting the group’s work to the class.  Assign one person to be the runner. This person is in charge of obtaining and returning materials for the group.

The overall goal is to create a procedure to test for the presence of Ca 2+ and Mg 2+ ions and compare the relative hardness of two tap water samples.

Where does our school tap water come from? (Record your ideas on the worksheet.)

 MWRA's water comes from two protected reservoirs - the Quabbin and Wachusett Reservoirs.  Reservoirs are filled naturally by rain and snow that fall onto the watersheds and turn into streams that flow into the reservoirs.  As the water flows it comes into contact with soil, rock, and plants. This process cleans the water and dissolves some minerals into the water.

 The water is treated according to strict state and federal regulations.  The water is sent through the MetroWest Water Supply Tunnel and is stored in covered tanks.  Next it goes to distribution mains and then community pipes.  Local pipes serve each street and smaller pipes bring water to buildings.

 When water flows through minerals (such as limestone and dolamite), Ca 2+ and Mg 2+ ions may be dissolved into the water. These ions are not considered harmful to your health.  If household plumbing systems become corroded, then ions such as Cu 2+ or Pb 2+ can be dissolved into the water. The EPA maximum concentration is 1.3 mg/L for Cu 2+ and 0 mg/L for Pb 2+.

 Hardness depends on the concentration of “minerals,” such as Ca 2+ and Mg 2+ ions.  The lower the concentration, the softer the water.  It is easier to make soapy lather with soft water.  Hard water frequently leaves mineral deposits on water fixtures and other objects.  MWRA's tap water is soft.

Design a procedure to compare two samples of water for the presence of Ca 2+ and Mg 2+.

The procedure must provide a way to determine  if the sample contains Ca 2+ and/or Mg 2+  which sample is the “harder” water

 Samples of tap water to be tested  Microplate  All solutions from “Precipitation and Solubility” lab  All data & resources from “Precipitation and Solubility” lab  Test tubes  Liquid soap

(Use the worksheet for all written work.) 1. Write the procedure. 2. Get approval from the teacher. 3. Obtain the materials. 4. Perform the procedure. 5. Record results. 6. Report back to whole class.