Summary

In this lesson students will use simple laboratory tests to characterize differences between solutions, colloids, and suspensions. They will then apply those tests to paints to classify them as specific types of mixtures.

Grade Level

High School

NGSS Alignment

This lesson 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.
• HS-PS2-6: Communicate scientific and technical information about why the molecular-level structure is important in the functioning of designed materials.
• Scientific and Engineering Practices:
  • Analyzing and Interpreting Data
  • Planning and Carrying Out Investigations
  • Engaging in Argument from Evidence

Objectives

By the end of this lesson, students will be able to

• Differentiate mixtures as solutions, colloids, or suspensions by particle size of the solute or dispersed phase.
• Correctly use the terms: dispersed phase, dispersing agent, colloid (or colloidal dispersion), and suspension.
• Identify solutions, colloids, and suspensions by results of simple laboratory tests.
• Explain how the properties of different mixtures can be used to design a product (paint).

Chemistry Topics

This lesson supports students’ understanding of

• Mixtures
• Solutions
• Colloids
• Suspensions
• Tyndall Effect
• Separating Mixtures
• Molecular Structure

Time

Teacher Preparation:

• 60 minutes to label and prepare all samples for Part I.
• 20 minutes to label and prepare all samples for Part II.

Lesson:

The following times are valid if the optimal amount of equipment is available and the time-saving tips in the teacher notes are followed:

• Engage: 20 minutes
• Explore: 45 minutes
• Explain: 30 minutes (this may be completed as homework)
• Elaborate: 30 minutes
• Evaluate: 60 minutes (final writing may be completed as homework)

Materials (Teacher Demo)

• 2, 1-L graduated cylinders or beakers
• Penlight
• Index card, any size
• Tap water
• ~1 g Salt
• ~1 mL Milk (any %)

Materials (Part I)

See “Explore” section for possible alternative equipment/supplies.

• 1 lab centrifuge (if you have more, this will reduce waiting time)

For each group of four students:

• 18 medium-size, 15 x 150mm, test tubes w/ stoppers in 2 racks (choose tubes that fit your centrifuge)
• 7 funnels (you can use less, but it will take longer)
• 7 small flasks, any size, to hold funnels (alternatively, you may use a funnel rack)
• 7 Qualitative filter papers
• 1 Spatula
• 1 plastic pipet
• 1 Test tube holder (tongs)
• 1 Penlight (cell phone light can work, it is just less focused, so the beam will be more spread out)
• 1 Napkin or porous paper towel
• 1 Candle and match
• Tap water
• 1 Small container of each solid:
  • Salt
  • Mud
  • Unsweetened drink mix powder
  • Flour
• 1 Small bottle of each liquid w/ dropper:
  • Tap water
  • Food dye
  • Milk, any %
  • Oil

Materials (Part II)

• 1 lab centrifuge (if you have more, this will reduce waiting time)
  For each group of four students:
  • 16 medium-size, 15 x 150mm, test tubes w/ stoppers in 2 racks (choose tubes that fit your centrifuge)
  • 7 funnels
  • 7 small flasks, any size (to hold funnels)
  • 2 spatulas
  • 1 mortar and pestle (to mix the solid binder w/ solid pigment)
  • 1 penlight
  • 1 paintbrush
  • 3 index cards
  • 1 watch glass (for paint mixing)
  • 1 glass stirring rod (for paint mixing)
  • 1 small container of pigment*
  • 1 small container of gum arabic
  • 1 small dropper bottle of linseed oil**
  • 1 small dropper bottle of citrus solvent***
  • Tap water
  *Pigments may be purchased or, if time is not a factor, you can have students use simple precipitation reactions to make their own pigments. Lead(II) iodide is a bright yellow pigment that is easy to make. There are many other possibilities, as well.
• **Raw linseed oil, though considered superior by many artists, will take about 2 days to harden, while the boiled linseed oil will harden overnight (~12 hours). Additionally, the raw linseed oil shows the Tyndall effect when pure, and so will confuse the classification of anything with which it is mixed. Boiled linseed oil works better for this lab
• ***Turpentine or paint thinner may be substituted for citrus solvent. The citrus solvent is NOT the same as citrus based cleansers. It is a pure extract from an orange peel and is less toxic and safer than the alternatives.

Safety

• When using centrifuge, samples must be in a balanced arrangement!
• Always wear safety goggles when handling chemicals in the lab.
• Do not consume lab solutions, even if they’re otherwise edible products.
• Open flames can cause burns. Liquid wax is hot and can burn the skin.
• When lighting the match, be cautious with the flame.
• 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.

• An operational fire extinguisher should be in the classroom.
• For Part II: Citrus solvent, and other solvents for oil paints, are organic. They often will interact with cheaper plastics, like a disposable plastic cup, and appear to melt or dissolve it. Do not substitute these plastics for the glassware when working with the oil paint.

Teacher Notes

• Mixture Cards for the Engage Activity, as well as sample answers and results for Student Activity Part 1 and 2 can be found in the Appendix document.

Teacher Background Information:

• There is some disagreement in whether to classify colloids as homogeneous or heterogeneous. Many definitions describe them as macroscopically homogeneous, but microscopically heterogeneous. Students may be told, if needed, that heterogeneous means that you can see different parts, while homogeneous means that you can only see one part. This can then be further explained after the testing.

Brief terminology:

• The more abundant component in a solution is called the solvent. The more abundant component in a colloid or suspension is called the dispersing medium.
• The less abundant component in a solution is called the solute. The less abundant component in a colloid or suspension is called the dispersed phase.
• The term, “colloid” may be used to describe a mixture or its dispersed phase. “Colloidal” means that the particles are in the right range to make a colloid. A mixture may also be called a colloidal dispersion.

• The difference between solutions, colloids, and suspensions is in particle size of the dispersed particle.

• The light-scattering effect described above is called the Tyndall effect and can be observed by shining a narrow beam of light through a sample. If the sample is transparent or translucent, a light beam will be visible inside a colloid sample and a suspension sample, but not inside a solution sample. If the mixture is opaque, the light will not make it to the inner part of the mixture, so the Tyndall effect is an invalid test for this type of mixture.
• Each possible combination of solid, liquid, and gas in a colloid has a subcategory. These subcategories are not addressed in this activity, but can be found in many textbooks.
• Paints are interesting mixtures. The job of the binder is to hold the pigments in place. The job of the solvent is to allow the paint to be spread. In some cases, the wet paint is a suspension, because it includes all three components, binder, pigment, and solvent, but must be mixed or shaken before using due to settling of the pigment. In other cases, the wet paint mixture does not need to be mixed because it does not settle over time, thus making it a colloid. Dried paint is typically considered to be a colloid, as the dispersed particles are indefinitely spread throughout the binder (dispersing medium). When paints are manufactured, the pigments are very finely ground, often with the binder, to create a particle size that is colloidal.

• Student pre-requisite knowledge:

• Mixture vs pure substance
• Homogeneous vs heterogeneous mixtures
• Parts of a solution: solute and solvent
• Particulate view of matter

Lesson

• Engage: At this point in the lesson, it is recommended that the teacher avoid any specific terms related to mixtures, as this may inadvertently sway the students’ grouping of the cards. Rather, introduce the lesson by saying that students will be exploring the differences between different types of mixtures.
  • Distribute “mixture” cards to groups of 2-4 students.
  • Tell students that each card contains a mixture that is familiar from daily life. Prompt them to think about each mixture and to sort them into groups with similar types of overall properties. (Note: try to avoid having them simply sort by whether the components are solid, liquid or gas.)
  • Next, students should write a brief description to justify each of their groupings.
  • Ask a few (or all) groups to share just the description for each group.
  • You may then further probe students’ thinking by asking questions such as, but not limited to:
    • Did anyone put fog and mayonnaise into the same group?
    • Did anyone put salt water and milk into the same group?
    • Did anyone consider the phase of each substance of the mixture when determining grouping?
  • Ask students to record in their notebooks the descriptions and items in each grouping for review later in the lesson.
  Short demonstration – Tyndall Effect:
  • Darkening the room will make this effect a bit easier to see.
  • Before class, fill one large beaker or 1-L graduated cylinder with salt water and another one with any dilute colloid (water with a little milk is a good option). You should experiment with amounts for the colloid in order to ensure the light beam will be clearly visible to students. Place the two samples in an easily seen location and tell students that each contains a mixture.
  • Shine a penlight or laser light through each. This works best if the light source is placed directly on the side of the container. (A square container works better because the curved surface can distort the light, but if you are careful in your aim, the effect is just as clearly shown in cylinders or beakers.) Show students how the beam goes through the mixture by holding an index card on the other side so the light hits it.
  • Next, point out how the light beam is visible inside one sample, but not inside the other. Alternatively, you can ask the students what is different and elicit this response from them.
  • Ask students for reasons why they might be able to see the light beam in one, but not the other. You may need to prompt them to remember that both are mixtures. Perhaps ask them if they can tell that each is a mixture just by looking at it.
  • Eventually bring them to the understanding that the light is reflecting off the particles in one of the mixtures because they are bigger than those in the other mixture.
  • Next, reveal the identity of each mixture and lead a brief discussion of what they already know about each mixture.
    • Students should know that salt water is a solution and that means that the salt is dissolved, which means broken down to a particle-level.
    • If milk in water is used, they may recognize the term “homogenized” and could be questioned about what that might mean.
  • At this point, explain that the reason they see the light is that the particles in the milk/water (don’t say the word, colloid, yet!) are larger than individual atoms/molecules and therefore form interfaces with the liquid medium, creating tiny surfaces from which the light can be reflected. This could be simplified by simply saying that the particles are big enough to reflect the light.
  • Last, tell students they will explore a variety of common mixtures by observing, testing for the light beam, and testing whether they can easily separate the parts.

• Explore: Part I – Exploring Simple Mixtures on the student sheets
  • You will need to prepare ahead of time (assuming 6 lab groups)
    • 6-Individual containers or dishes, each, of salt, mud, unsweetened drink powder, flour
    • 6-Individual dropper bottles, each, of milk, oil
    • 6-Individual bottles of food coloring (Color does not matter, but blue and green can get pretty dark and the light may not go through enough to see the beam. If diluted, these colors work just as well as the others.)
    • 6-Candles
    • 6-matchbooks or boxes of matches
  • Ideally, all the following are available in your lab. If not, please see sub-bullets for alternate ways to manage the activity with less equipment.
    • Lab centrifuge
      • A mini-centrifuge (1.5 mL tubes) from the biology department can work, but it is difficult to see the light beam through the plastic tubes.
      • If no centrifuge is available, the samples could be left to settle overnight. Essentially, the centrifuge is just speeding up the settling, if it will happen.
    • Qualitative filter paper (42 papers)
      • Coffee filters could be used, but the mud sample will not be as well-separated. With the filter paper, only the colloidal portion goes through the filter paper and the particles large enough to settle stay in the paper.
    • 42 filtration set-ups (funnel + flask or graduated cylinder or test tube or funnel rack…)
      • You may need to consider having half of the class start with filtration while the other half starts with centrifugation.
      • It will take more time, but you can also give each group less funnels and just have them test the mixtures a few at a time.
      • You may have a funnel rack that holds several funnels. In this case you just need some kind of container under the funnel.
      • You could re-use test tubes after centrifugation to hold the funnels.
  • 108 test tubes with stoppers that fit your centrifuge (This procedure was written using a standard lab centrifuge borrowed from the physics department and 15x150mm tubes with #3 stoppers.)
    • 48 of the tubes could be replaced with beakers or cups, if needed (groups can pour the half of the sample to be filtered into a beaker instead of a tube).
    • Only 2 of the tubes truly need a stopper. If you need to reduce the number of stoppers, use them only on the “smoke+air” tubes.
    • One tube per group is simply the empty tube to balance the “smoke+air” tube. This can just be placed by the centrifuge and shared by the class.
    • Be careful to be consistent in using or not using a stopper when placing tubes in the centrifuge, as you want to ensure a balanced configuration.
  • 12 test tube rack.
    • Alternatively, beakers can be used to hold the test tubes.
  • 18 metal spatulas.
    • Each group can clean the spatula between samples, thus only needing 1 per group.
  • 6 penlights.
    • A narrow beam is best. I used cheap penlights that are used for testing pupil dilation.
    • You can use laser pointers, but must be careful that the color is not absorbed by the food dye (blue and green will absorb the red of most laser pointers). If the dye is dilute enough, some of the light will still make it through the solution (the beam should not be visible, but it may be visible in more concentrated solutions).
    • I’ve seen cheap laser pointers sold for playing with cats (apparently, they like to chase the light).

• Time-saving hints are built into the student activity and include:
  • Students split the work of preparing the mixtures.
  • Half of the group performs the filtration while the other half performs the centrifugation. All tubes are then analyzed again by entire group.
  • For the filtrations, the directions say to set up all funnels and pour all solutions in, so they are filtering at the same time. In my experience, careful lab students will still attempt one at a time. You may want to reinforce this direction to ensure timeliness.
  • Some of the filtrations will appear to be very slow. In these cases, it is likely that the liquid portion remaining in the filter paper has particles that are too large to go through. Prepare students for this eventuality so they are not wasting time waiting for something that will not go through the filter. They just need to collect enough liquid at the bottom to be able to analyze it using the light beam.
• Students will perform some laboratory tests to find differences between different kinds of mixtures. Depending on lab experience, the following should be addressed before beginning the lab:
  • Instruction on using the centrifuge. Show students how to achieve a balanced configuration.
    • When centrifuging the “smoke + air” tube (w/ stopper), students should balance this with either an empty tube w/ stopper or with another group’s “smoke + air” tube.
  • Show students how to hold the penlight against the tube to get the best results for the Tyndall effect.
  • You may need to show an example of using a spatula to obtain a sample “the size of a grain of rice”.
  • If individual lab sets are not created, be sure to explain how you’d like students to obtain their materials.
• Expected results are included at the end of the Teacher section.

• Explain:
  • This occurs during the Analysis and Conclusion questions of Part I. Using the properties discovered in the lab, along with some given information, students will match an appropriate scientific name to each type of mixture.
  • Teacher should discuss the answers to questions and the terminology before moving onto the next section.
• Elaborate: Part II – Create Your Own Paint, Part A – Investigation of Paint Components on the student sheets.
  • An introduction to the basic components of paints will set the tone for exploring a 3-component system and determining its mixture classification.
  • Students will make dilute mixtures of each possible pair of paint components and will test them to decide what type of mixture is formed by each combination. They will then make and test a dilute mixture including all components of a given paint type and decide how the overall mixture should be classified.
  • This part is designed as an open inquiry, where students design their own procedure. Depending on their experience with this, they may need some direction.
    • Guide them to use small amounts of the solids with larger amounts of the liquids, like what they did in Part I.
    • Remind them that they can use the same kinds of tests and observations as they did in Part I.
    • Coach them to write out the plan before enacting the plan.
• Evaluate: Part B – Design Your Own Paint, in Part II
  • Students will use what they have learned to determine how to mix the paint components to create a paint.
    • The goal is for students to consider how the substances interact to form solutions, suspensions, or colloids and to use the properties of each type of mixture to decide what proportions will be useful and how to put the components together. Prompt them to ask questions and think about whether amounts or order of mixing would affect the properties of the final paint mixture.
  • After creating and analyzing their designed paint, students will answer the prompt, “How do the components of paint interact to give it the desired properties that make it useful?”
    • The goal is for students to make connections between properties of solutions, suspensions, and colloids and their role in manufacturing and applying a paint.