The Tikkun Project
Lesson
6
Understanding Substances and Mixtures
Concept
Whether a harmful substance is part of a mechanical mixture, a solution, or bound into a living system determines what kind of solution is even scientifically possible separation, treatment, or prevention at the source.
Focus Question
If a harmful substance has gotten into something — water, food, air, our own bodies — is it possible to remove it, and how do we know?
Knowledge & Understanding Goals
Science and Technology, Matter and Energy Strand C (Pure Substances and Mixtures), Grade 7: Demonstrate an understanding of the nature of matter, including the properties of pure substances and mixtures, and describe these properties using particle theory; distinguish between mechanical mixtures and solutions.
Science and Technology, Matter and Energy Strand C, Grade 7: Explain various processes used to separate mixtures, including solutions, into their components, and identify some applications of these processes. (C2.7)
Thinking & Learning Skills
ATL — Thinking Skills: Applying particle-level reasoning to a real-world contamination problem; distinguishing a treatable problem from one that requires prevention at the source.
ATL — Research Skills: Investigating real remediation case studies.
Learner Profile: Knowledgeable, Thinkers.
Context
This lesson provides the science grounding many students will need to propose a scientifically credible response to their own product's harms, especially where that harm involves a substance mixed into water, air, soil, or living tissue. It doesn't carry a RINA-step tag itself it sits between RINA Step 2 (Lesson 5) and RINA Step 3 (Lesson 8), supplying background knowledge rather than advancing the RINA sequence directly. It also gives real substance to the unit's Central Idea that individual action is necessary but not sufficient by showing a case (mercury) where that's a matter of chemistry, not just values.
Lesson Outline
WHAT IS A MIXTURE, REALLY?
Review particle theory: all matter is made of particles in constant motion, held together by attractive forces. A pure substance (an element or compound) has only one type of particle; a mixture has more than one. Mixtures come in two kinds: mechanical mixtures, where the different components are still physically distinct (trail mix, muddy water), and solutions, where one substance is dissolved uniformly into another (salt water, air).
CASE STUDY: CAN MICROPLASTICS BE REMOVED FROM WATER?
Pose the question directly. Microplastics suspended in water are a mechanical mixture — the plastic particles remain physically distinct from the water, not dissolved into it. That means physical separation methods apply: filtration through progressively finer mesh, settling/sedimentation, density-based separation. Look at how real remediation projects (e.g., large-scale ocean cleanup efforts) apply these same principles at scale. Ask: why does the mechanical-mixture nature of this problem make removal at least physically possible, even if difficult and expensive?
CASE STUDY: CAN MERCURY BE REMOVED FROM FISH — OR ONLY PREVENTED?
Pose the contrasting case. Mercury in fish tissue isn't a simple mixture you could filter — it bioaccumulates, binding into the fish's muscle tissue biologically as it moves up the food chain. There's no practical way to "filter" mercury out of a fish about to be eaten. This means the realistic remediation strategy shifts from removal to prevention at the source — reducing mercury emissions from industrial sources before they ever enter the water system. Connect explicitly to the unit's Central Idea: this is a case where an individual's choices matter, but can't solve the underlying problem — that requires systemic, source-level change.
APPLYING THIS THINKING TO OUR OWN PRODUCT
Students return to the harms they've identified for their own chosen product and ask: is this harm more like the microplastics case (a mixture that might be physically separated or treated) or more like the mercury case (something bound into a system where prevention matters more than after-the-fact removal)? This distinction should start to shape what kind of action students consider building toward in the lessons ahead.
WRAP-UP
Exit ticket: using today's vocabulary (pure substance, mechanical mixture, solution, separation, bioaccumulation), students classify their own product's key harmful substance and explain which broad category of solution — removal/treatment or prevention at the source — makes more scientific sense for it.
Notes for Teaching and Evaluation
Pairing a genuinely treatable case (microplastics) with a genuinely non-treatable-after-the-fact case (mercury) is a deliberate design choice — presenting only removable examples would leave students with the false impression that any harm can be engineered away with enough effort.
Asking students to classify their own product's harm using this lesson's vocabulary, immediately after the two case studies, is what turns background science content into something that directly shapes their eventual action plan.