Summary

In this lab, students will engineer simple hydrometers from straws and clay to learn about standard solutions, calibration, and instrument drift. They will use their hydrometers to test the salinity of samples of natural water.

Grade Level

High and Middle School

NGSS Alignment

This lab will help prepare your students to meet the performance expectations in the following standards:

• HS-PS1-3: Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles.   
• Scientific and Engineering Practices:
  • Asking Questions and Defining Problems
  • Using Mathematics and Computational Thinking
  • Developing and Using Models
  • Analyzing and Interpreting Data
  • Planning and Carrying Out Investigations

Objectives

By the end of this lab, students should be able to:

• Use percent composition to make standard solutions.
• Calibrate a simple instrument using standard solutions.
• Explain the role of standard solutions in calibration.
• Measure the concentration of solutions using their hydrometer.
• Sketch a solution at the particle level (optional).

Chemistry Topics

This lab supports students’ understanding of:

• Solutions
• Concentration
• Percent Composition
• Instruments
• Calibration

Time

Teacher Preparation: 30 minutes

Lesson: 90 minutes

Materials (for a class of 30)

• 20 Straws
• Clay: a baseball sized amount (note: playdough will not work)
• 15 Sharpies
• Digital Scales
• Salt
• 15 Containers for Standard Solutions (1 like 50mL tubes with lids)
• 15 index cards labelled: 1%, 2%, 3%, 4%, 5%, 6%, 8%, 9%, 10%, 12%, 14%, 15%, 16%, 18%, 20%
• 15 Containers for field collections or for “unknown” samples the students will measure in the lab.

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Students should wash their hands thoroughly before leaving the lab.
• When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.

Teacher Notes

Science Background:

• Most chemistry instruments that measure concentration of solutions do so indirectly. For example, an Ion Chromatograph (which you might use to find the concentration of sulfates or nitrates in a water sample) actually measures the electrical charge in carefully separated solutions. As a result, scientists must run standard solutions with known concentrations of a solute in order relate the electrical charge to the concentration. This is one way to calibrate an instrument. Instruments also are known to “drift” over time, but if scientists measure the drift by running standard solutions repeatedly over time they can mathematically correct their data. Salinity is often measured in percents and is important to measure for a variety of ecological reasons.

Procedure:

• Hook: Create a meaningful scenario for your students in which they need to measure salinity.
  • Here in Salt Lake City we talk about varying salinity in the Great Salt Lake and we perform this activity on a lakeside field trip where the students test different areas of the lake and produce an Illustrated Field Guide to Waters at the Great Salt Lake.
  • If you have an ocean nearby you could do bays, tidepools or estuaries. If you are not able to do this as a field trip you could make your own fake samples of “winter runoff” after roads have been salted for icy conditions. These are just examples. Come up with something that is important in your local ecosystem.
  • Explain to students that they will be making homemade hydrometers to measure the salinity of water samples found in the natural world.
• Step 1: Students make standard solutions.
  • This should only take about 20 minutes. Put students in pairs and hand each pair of students an index card with a percentage written on it. Explain to them that before making and using their hydrometers they will need to make saltwater samples with a known salinity. These are standard solutions. The standard solutions are used to “calibrate” the hydrometers which means teach them the correct levels. The more precisely you make and measure your standard solutions the more accurate your hydrometer will be.
  • Ask students to calculate the amount of water and the amount of salt needed to make 50mL of the percentage of saltwater solution listed on their card. They should use the units of grams but they can use a graduated cylinder to measure water since distilled water is 1g/1mL. Have them write the amount of salt and the amount of water on their index card. Check their answers as they go into the lab to make their solutions.
  • They should weigh the salt and measure the water and mix thoroughly in their container. They should also label their tube with the percent salinity they made. Have a refractometer or professional hydrometer on hand so that they can check their solutions to be sure it’s accurate.
  • Discuss and problem-solve with them if it is not. If you will be testing waters in the field on a field trip, I recommend making the standards in class before you go.
• Step 2: Make hydrometers.
  • Salt water is more dense than fresh water (that’s why it is easy to float in the Great Salt Lake!), and a hydrometer just works by measuring density. Show them that the hydrometer is just made by putting a small ball of clay on the bottom of the straw and then floating it in a solution. This DIY video
    might be helpful for background/understanding (note that my students don’t use a ruler, we just add lines based on the results of the standard solutions).
  • Students use the standard solutions to draw and label a line on the straw where the surface of the saltwater solution hits. They will have to write small, but they should label the line with the percent salinity of the standard solution. Student should draw at least 4 different lines (using at least 4 different standard solutions to calibrate their hydrometer). This means that they will also be using standard solutions from other groups.
  • I suggest that the teacher ask students several thinking questions throughout the calibration process, and let them discuss with their partners (but don’t necessarily tell them all the answers). Students will figure out the answers while they are making and calibrating the hydrometers. Also, you do not need to have the same discussion with each group, different groups may learn different things and that is just fine. They will all be getting powerful engineering experiences. Below are some thinking questions to discuss with either the whole group, or smaller individual groups (answers follow in parenthesis):
    • Will the hydrometer sit higher or lower in very salty water? (higher)
    • What if your hydrometer is resting on the bottom of the container? (there’s too much clay and it won’t work)
    • What if water leaks into the straw? (it will sink further and give inaccurate readings)
    • Which way will the numbers go on the straw? (numbers should increase as you move down toward the clay)
    • What if the clay dissolves a little in the water, how will that effect the next measurement you make with the hydrometer? (it will get lighter and give higher salinity readings)
    • What if your clay gets wetter and heavier each time you use it, how will that effect it’s measurements?  (it will give lower salinity readings)
    • What does it mean if the lines on your hydrometer are not in numeric order? (answers vary, but it could mean the standards were not accurate)
    • Is it better to pick standard solutions with salinities close together or spread out? (spread out, but also, the more standards the better)
    • Advanced question: does the relationship between salinity and density appear to be a linear relationship? Are your lines spaced evenly? (it does not seem to be linear)
• Step 3: Use the hydrometers to measure the natural water samples.
  • Walk around the class to check the hydrometers before you send students out to test natural waters. Make sure the numbers go in numeric order and that the numbers are opposite of the direction you would see on a thermometer. Treat inconsistencies as positive learning opportunities and opportunities for discussion!
  • Once students have a functional hydrometer, they can collect water samples in a small container and use their hydrometer to measure the salinity by just floating it in the sample and seeing where the surface of the water hits. They may have to estimate (read between the lines) or extrapolate (read outside of the lines).
  • After they have measured 3-5 natural samples, they can re-calibrate their hydrometer with the standard solutions to see if they had any instrument drift over time. I.e., is the 10% solution level still at the 10% you labelled?
• Create a product that shows the results of their measurements.
  • There are lots of different differentiation possibilities for this part of the experience. I encourage you to adapt the students handout for your students, and refer to some of my suggestions below.
  • As shown on the student handout, I have students draw a particle level sketch of their natural water samples and in the process I get to have a variety of discussions ranging from simple (which one has more NaCl particles?), to midlevel (are your drawings proportional? Do you show the Na and Cl ions dissociating when they dissolve?), to advanced (what is the orientation of the water molecules in relation to the Na ions as opposed to the Cl ions?).
  • The student handout could be enhanced with a map, for example of The Great Salt Lake, or any other body of water that is used to fit a scenario/version of this lab.
  • Another option is to have students report which areas freshwater fish might prefer, or which streets are more of a concern for salt pollution.
  • Be sure that students also have a chance to evaluate their level of confidence in their results. Students are likely to be critical of this low-tech tool, and overly trusting of more high tech tools, but really both of their accuracies can just be evaluated by how precisely they are calibrated and whether they change over time and use. Valuable lessons for future chemists!