One in a Million: Using Serial Dilutions to Understand Concentration

Materials

Presweetened Powdered Drink Mix (Kool-Aid® or similar)*, 8 to 9 g
Plastic Spoon
Large Cup (10 oz or larger)
6 Small Cups (4 oz or larger)
Permanent Marker
Tap Water

Introduction

A serial dilution is a simple technique for reducing the concentration of a solution in a systematic way. In this activity, students prepare beverage solutions over a wide range of concentrations and determine the minimum concentration required to taste the sweetness of a solution.

Standards

This activity addresses the following AP® Biology concepts:

SYI-1

Living systems are organized in a hierarchy of structural levels that interact.

SYI-1.A

Explain how the properties of water that result from its polarity and hydrogen bonding affect its biological function.

SYI-1.A.3

The hydrogen bonds between water molecules result in cohesion, adhesion, and surface tension.

Procedure

Preparation

Use a permanent marker to label the 6 small cups with letters A through F, respectively.
Use a permanent marker to label the large cup “water.”
Fill the large cup with tap water.
To prepare solution A:
- Add 1 level spoonful of presweetened powdered drink mix to the small cup labeled “A.”
- Use the plastic spoon to transfer 9 spoonfuls of water from the “water” cup to the cup labeled “A.”
- Use the plastic spoon to stir the contents of the cup until they are completely dissolved.
To prepare solution B:
- Add 1 level spoonful of solution A to the cup labeled “B,” and then rinse off the spoon.
- Use the plastic spoon to transfer 9 spoonfuls of water from the “water” cup to the cup labeled “B.”
- Use the plastic spoon to stir the contents of the cup until they are completely dissolved.
Prepare solutions in the remaining cups by repeating step 5 using
- Solution B and the cup labeled “C.”
- Solution C and the cup labeled “D.”
- Solution D and the cup labeled “E.”
- Solution E and the cup labeled “F.”
Record observations about the intensity of the color in each cup.
Beginning with cup F, taste-test each solution by sipping a small amount of liquid from each cup. Note when you can first discern the sweet taste of the solution in the cup.

Extension Activities

Have students determine the concentration of each cup in terms of (volume of solute) / (total volume). Next, ask students to determine which cup represents 1 part per million (ppm), and how many more cups would be required to represent 1 part per billion (ppb).
Measure the mass of a level spoonful of sweetened powdered drink mix. Next, measure the mass of a spoonful of water. Have students determine the concentration of each cup in terms of (mass of solute) / (mass of solution).

Safety

This activity involves taste-testing a prepared solution. Note: Do not use laboratory materials or glassware. All materials should be purchased for consumer use and discarded at the conclusion of the activity. Consider performing the One in a Million activity in the school cafeteria or trading classrooms with a food science teacher for the day. Emphasize that students should not eat, drink, or chew gum during activities performed in their science classroom.

Buy the Kit

Explore the causes and effects of water pollution with this laboratory activity. Students first perform a serial dilution to grasp the concepts of ppm and ppb.