Science · Grade 7 · Pure Substances and Mixtures · Term 3

Suspensions and Colloids

Differentiating between solutions, suspensions, and colloids based on particle size and stability.

• Ontario Curriculum Expectations
• MS-PS1-1

About This Topic

Suspensions and colloids are heterogeneous mixtures defined by particle size and stability. Solutions feature particles smaller than 1 nanometer, fully dissolved and uniform. Suspensions have particles larger than 1000 nanometers that settle out under gravity due to their size and weight. Colloids contain particles between 1 and 1000 nanometers, remaining dispersed without settling and scattering light in the Tyndall effect. Grade 7 students in Ontario differentiate these through observation, addressing curriculum expectations for pure substances and mixtures.

This topic strengthens classification skills and connects to physical properties of matter. Students explain settling in suspensions by considering particle mass and gravitational pull. They analyze the Tyndall effect as a tool to distinguish colloids from solutions, since only colloidal particles scatter visible light beams. These investigations promote evidence-based reasoning central to scientific literacy.

Active learning shines with this topic because particle behaviors are invisible yet testable. Students preparing mixtures, timing settlements, and using flashlights for Tyndall tests collect data firsthand. Such experiences make distinctions clear, build confidence in predictions, and turn abstract sizes into observable differences.

Key Questions

1. Differentiate between a solution, a suspension, and a colloid.
2. Explain why particles in a suspension eventually settle out.
3. Analyze how the Tyndall effect helps distinguish colloids from solutions.

Learning Objectives

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

Before You Start

Properties of Pure Substances and Mixtures

Why: Students need to understand the basic difference between a pure substance and a mixture before they can classify different types of mixtures.

Particle Theory of Matter

Why: Understanding that matter is made of tiny particles and that these particles have different sizes and behaviors is foundational for differentiating between solutions, suspensions, and colloids.

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.

Watch Out for These Misconceptions

Common Misconception

All cloudy mixtures are solutions.

What to Teach Instead

Cloudiness signals larger particles in suspensions or colloids, not dissolved solutes. Hands-on mixing and Tyndall tests let students see solutions stay clear while others scatter light, clarifying distinctions through direct evidence.

Common Misconception

Colloids settle like suspensions, just slower.

What to Teach Instead

Colloidal particles stay suspended due to Brownian motion. Observing no settling over days in student-made colloids, paired with suspension comparisons, helps revise this view via sustained inquiry.

Common Misconception

Particle size does not affect settling speed.

What to Teach Instead

Larger particles settle faster under gravity. Timing races with different suspensions reveals this pattern, as groups collaborate to measure and plot data, reinforcing size-stability links.

Active Learning Ideas

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.

45 min·Small Groups

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.

30 min·Pairs

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.

35 min·Small Groups

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.

40 min·Individual

Real-World Connections

• Milk is a colloid; the fat globules are dispersed throughout the water, giving it a cloudy appearance and preventing them from settling out, which is why it doesn't separate like a suspension.
• Pharmaceutical companies use their understanding of colloids and suspensions when formulating liquid medications, ensuring that active ingredients remain evenly distributed for consistent dosage.
• In food science, chefs and food technologists create emulsions, a type of colloid, in products like mayonnaise and salad dressings, where oil and water are stabilized to prevent separation.

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.

Frequently Asked Questions

How do you differentiate solutions, suspensions, and colloids in grade 7 science?

Use particle size, stability, and Tyndall effect. Solutions dissolve completely with tiny particles; suspensions settle quickly with large particles; colloids stay mixed and scatter light. Students test by mixing samples, waiting for settling, and shining lights through them to observe differences firsthand.

Why do particles in suspensions settle out?

Particles exceed 1000 nanometers, making them heavy enough for gravity to pull them down despite agitation. Smaller colloidal particles resist via constant motion. Classroom demos with timed observations help students connect size to settling rates.

What is the Tyndall effect and how does it identify colloids?

The Tyndall effect is light scattering by colloidal particles, visible as a beam. Solutions transmit light without scattering; suspensions block it. Flashlight tests in darkened rooms provide clear proof, helping students distinguish reliably.

How can active learning help teach suspensions and colloids?

Active methods like station labs and settling challenges engage students in creating, testing, and comparing mixtures. They time settlements, check Tyndall with lasers, and graph data, turning theory into evidence. This builds deeper understanding, reduces confusion, and sparks curiosity through collaboration and prediction.