Summary

In this lab, students will work in cooperative groups to conduct a chemical reaction by mixing vinegar with milk. This reaction will cause the curds to precipitate and separate from the liquid whey. The lesson can be extended by experimenting with different types of milk or by using different acids.

Grade Level

Middle School

Objectives

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

• Differentiate between chemical and physical change.
• Describe how evidence of chemical reactions indicates that new substances with different properties are formed.

Chemistry Topics

This lab supports students’ understanding of

• Chemical Reactions
• Chemical Change
• Physical Change
• Physical Properties

Time

Teacher Preparation: 30 minutes

Lesson: 40 minutes

Materials (per group)

• 30 ml of Milk
• 30 ml Vinegar
• 2 beakers (plastic containers will work also)
• 1 – 50 mL Graduated Cylinder
• 2 – 250 mL Glass Beakers
• Stirring Spoon
• Hot Plate per
• Hand Protector
• 1 Coffee Filter
• 1 Rubber Band

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.
• Exercise caution when using a heat source. Hot plates should be turned off and unplugged as soon as they are no longer needed.
• An operational fire extinguisher should be in the classroom.
• When working with acids, if any solution gets on students’ skin, they should immediately alert you and thoroughly flush their skin with water.
• Do not consume lab solutions, even if they’re otherwise edible products.
• Food in the lab should be considered a chemical not for consumption.

Teacher Notes

• This activity works well with 2 to 4 students per lab group.
• Advance Preparation includes:
  • Purchase milk, vinegar, stirring spoons, and coffee filters.
  • Prefill containers with milk and vinegar to have at each lab station.
  • Set out 1-50 mL graduated cylinder, 2-250 mL glass beakers, 1 stirring spoon, 1 coffee filter, 1 rubber band, 1 hot plate, and 1 hand protector per lab group.
  • Print and copy the Student Lab Report (1 per student).
  • Preview the video links to make sure they are accessible.
• Vocabulary – Students should define these words prior to the activity:
  • Chemical reaction – interaction of two or more chemicals that produces one or more new chemical compounds, or alters the properties of the mixed chemicals.
  • Precipitate – a substance deposited in solid form from a solution.
  • Colloid – a mixture of substances that do not settle out over time because the particles are extremely tiny.
  • Suspension – a mixture that settles into separate parts when left at rest.
  • Whey – liquid portion of milk containing water, sugar, minerals, and proteins.
  • Curd – solid custard like state of milk containing protein and fat.
• Engage Activity – Show the “Little Miss Muffet” Nursery Rhyme video and discuss to allow students to recall “curds and whey”.
• Pre-lab questions may be completed individually, as a whole group or as a “think-pair-share” activity.
• Begin the Little Miss Muffet Investigation, following the procedures on the student handout.
• Explanation of what is occurring:
  • Milk is a colloid of mostly water with small particles in it called milk solids or casein. To make cheese, the milk solids need to be precipitated out of the liquid. When you add an acid, such as vinegar, it causes the milk solids to precipitate. The vinegar contains hydrogen atoms with a positive charge. Since opposite charges attract, the positively charged hydrogen atoms in the vinegar are attracted to the negatively charged protein molecules. The attraction between these molecules causes them to clump together into a semi-solid mass, which is curd. Heat helps separate the curds from the liquid whey. Cheese-makers can use a variety of acids to precipitate out the curds including bacteria or an enzyme called rennet.
  • Looking at a glass of milk, you can’t see the particles that make it up, namely the curds (solid casein protein particles) and whey (liquid particles) because they are so small. Milk appears white even though the particles that make it up are mostly clear, because light is scattered by the tiny particles as it passes through the colloid. The milk was “curdled” when the acidic vinegar was added and lowered the pH of the milk, causing the casein particles to come out of the solution as solid chunks of curds floating in the liquid whey.
  • References:
    • Say Cheese! Experiment, from Planet Science
    • Cool Science Experiments: Make Curds and Whey
    • Curds and Whey: A Milky Experiment, from education.com
• Show the “Cheese: Not the Same Mold Story” video to investigate how cheese-maker Trent Hendricks makes a hybrid cheese he calls cheddar blue.
• The lesson can be extended by experimenting with different types of milk (skim, 1%, 2%, whole, soy) or by using different acids (vinegar, lemon juice, rennet).