Suspensions and Colloids

Active learning works for suspensions and colloids because students need hands-on experience to see how particle size affects mixture behavior. Direct observations at lab stations and timed tests let students connect theory to what they can feel, see, and measure.

• Grade 7
• Science
• 4 activities
• 30 min–45 min

Learning Objectives

1. Classify mixtures as solutions, suspensions, or colloids based on observable particle behavior and size.
2. Explain the process of particle settling in suspensions using concepts of gravity and particle mass.
3. Analyze the Tyndall effect to differentiate between colloids and true solutions.
4. Compare and contrast the properties of solutions, suspensions, and colloids.

Stations Rotation

⏱ 45 min·Small Groups

Lab Stations: Mixture Preparation

Set up stations for solution (dissolve salt in water), suspension (mix sand in water), and colloid (dilute milk). Students shake mixtures, observe initial appearance, then let stand to check settling. Test Tyndall effect with a laser pointer through each.

Prepare & details

Differentiate between a solution, a suspension, and a colloid.

Facilitation Tip: During Lab Stations: Mixture Preparation, set out clear labels and safety reminders so students focus on mixing and observing rather than setup.

Setup: Tables/desks arranged in 4-6 distinct stations around room

Materials: Station instruction cards, Different materials per station, Rotation timer

• Remember
• Understand
• Apply
• Analyze
• Self-Management
• Relationship Skills

Stations Rotation

⏱ 30 min·Pairs

Tyndall Effect Challenge: Light Scatter Test

Prepare samples of solution, suspension, and colloid in clear containers. Pairs shine flashlights through samples in a darkened room, sketch light paths, and classify based on beam visibility. Discuss results as a class.

Prepare & details

Explain why particles in a suspension eventually settle out.

Facilitation Tip: For the Tyndall Effect Challenge: Light Scatter Test, use a single flashlight beam in a darkened corner to maximize the scatter effect for all groups.

Setup: Tables/desks arranged in 4-6 distinct stations around room

Materials: Station instruction cards, Different materials per station, Rotation timer

• Remember
• Understand
• Apply
• Analyze
• Self-Management
• Relationship Skills

Stations Rotation

⏱ 35 min·Small Groups

Settling Race: Prediction and Timing

Students mix suspensions with varying particle sizes like flour, chalk dust, and soil in water. Predict and time settling rates in graduated cylinders. Graph results to identify patterns in particle behavior.

Prepare & details

Analyze how the Tyndall effect helps distinguish colloids from solutions.

Facilitation Tip: During Settling Race: Prediction and Timing, have students record predictions first to make the timing data meaningful and debatable.

Setup: Tables/desks arranged in 4-6 distinct stations around room

Materials: Station instruction cards, Different materials per station, Rotation timer

• Remember
• Understand
• Apply
• Analyze
• Self-Management
• Relationship Skills

Stations Rotation

⏱ 40 min·Individual

Colloid Maker: Everyday Examples

Individuals create colloids like gelatin or starch suspension, then test stability and Tyndall. Compare to homemade suspension and solution. Journal differences in appearance and behavior over 24 hours.

Prepare & details

Differentiate between a solution, a suspension, and a colloid.

Facilitation Tip: For Colloid Maker: Everyday Examples, provide labeled containers of common colloids so students can connect lab work to familiar items.

Setup: Tables/desks arranged in 4-6 distinct stations around room

Materials: Station instruction cards, Different materials per station, Rotation timer

• Remember
• Understand
• Apply
• Analyze
• Self-Management
• Relationship Skills

Teaching This Topic

Teach this topic by letting students experience the differences firsthand rather than explaining them upfront. Use guided inquiry so students notice patterns, then name the concepts. Avoid lecturing about particle sizes; instead, let observations drive the discussion. Research shows students retain concepts better when they test predictions and explain discrepancies.

What to Expect

Students will confidently classify mixtures by particle size, predict settling behavior, and explain observations using evidence from their experiments. They will use terms like Tyndall effect, Brownian motion, and particle size to justify their answers.

Watch Out for These Misconceptions

Common Misconception

During Lab Stations: Mixture Preparation, watch for students labeling all cloudy mixtures as solutions.

What to Teach Instead

Have students test each mixture with a flashlight in the Tyndall Effect Challenge afterward. Ask them to note which mixtures scatter light and which remain clear, using the scatter as evidence for particle size.

Common Misconception

During Settling Race: Prediction and Timing, watch for students assuming colloids settle like suspensions.

What to Teach Instead

After the Settling Race, review the data as a class. Ask students to explain why some mixtures settled quickly while others did not, pointing to particle size and Brownian motion in the colloid samples they made.

Common Misconception

During Tyndall Effect Challenge: Light Scatter Test, watch for students thinking particle size does not affect scatter.

What to Teach Instead

During the Tyndall test, have students compare the brightness and spread of the light beam across different mixtures. Ask them to link the intensity of scatter to particle size, using their observations to refine their understanding of stability.

Common Misconception

Assessment Ideas

Quick Check

Present students with three unlabeled beakers, each containing a different mixture (e.g., salt water, sand in water, milk). Ask them to observe each mixture, perform a flashlight test for the Tyndall effect, and then classify each mixture as a solution, suspension, or colloid, providing one reason for each classification.

Discussion Prompt

Pose the question: 'Imagine you are a scientist trying to determine if a new liquid is a solution, suspension, or colloid. What two tests would you perform, and what would you look for in the results of each test to make your determination?' Facilitate a class discussion where students share their proposed tests and expected observations.

Exit Ticket

Provide students with a scenario: 'A student mixes flour and water. After stirring, the mixture looks cloudy, and some bits of flour are visible. What type of mixture is this most likely, and what will happen if it sits for an hour?' Students write their answers, justifying their choice based on particle size and settling.

Extensions & Scaffolding

• Challenge students who finish early to create a new mixture and challenge peers to classify it using the Tyndall test and settling observations.
• For students who struggle, provide pre-mixed samples with visible particles to compare against clear solutions.
• Give extra time for groups to research and present one industrial or environmental example of a colloid or suspension, connecting their lab work to real-world uses.

Key Vocabulary

Solution	A homogeneous mixture where one substance dissolves completely into another, forming a uniform composition with particles too small to see.
Suspension	A heterogeneous mixture containing solid particles that are large enough to eventually settle out of the liquid or gas when left undisturbed.
Colloid	A heterogeneous mixture where particles are dispersed throughout another substance but are not heavy enough to settle out, remaining suspended indefinitely.
Tyndall Effect	The scattering of a beam of light as it passes through a colloid or suspension, making the light beam visible.