Week 2: Reaction Types & Conservation

Grade 8 Science | Rosche

MS-PS1-3 Matter and Its Interactions

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The Phenomenon: The Disappearing Solid Mystery

Anchoring Context & Focus Question

How can mixing two clear liquids make something solid appear?

Three beakers show the disappearing-solid mystery: clear silver nitrate plus clear sodium chloride turns cloudy as a white silver chloride precipitate settles. The same atoms remain, but they rearrange into a new compound.
Two clear liquids make a visible solid because ions swap partners and form an insoluble precipitate.
Animated explanation of the week's focus question. Watch then reason about why this happens.
Key Vocabulary (8 terms) — Practice Tool

Cognate Strategy: Many science words look similar in English and Spanish — use your Spanish to learn science!

Term Spanish Definition
synthesis síntesis Reaction combining two substances into one
decomposition descomposición Reaction breaking one substance into two or more
single replacement sustitución simple One element replaces another in a compound
double replacement sustitución doble Two compounds exchange elements
balanced equation ecuación balanceada Same number of atoms on both sides
coefficient coeficiente Number in front of a formula in an equation
combustion combustión Reaction where a substance burns in oxygen, producing carbon dioxide and water
endothermic endotérmico Reaction that absorbs heat energy
2x2 quadrant chart of the four reaction types: synthesis (combine), decomposition (split), single replacement (one swap), double replacement (partner swap). Coefficients and balanced-equation idea connect them all to conservation of mass.
Four reaction types in one quadrant - and how balanced equations tie them together.

Hook – The Disappearing Solid Mystery

Observe how mixing two clear liquids produces a visible solid.
A double-replacement precipitation diagram showing barium chloride and sodium sulfate mixing to form solid barium sulfate plus aqueous sodium chloride.
Two clear solutions combine; a solid precipitate appears when ions rearrange into an insoluble compound.

Focus Question: How can mixing two clear liquids make something solid appear?

Imagine pouring two perfectly clear liquids together. The instant they touch, a white solid appears and sinks to the bottom. Neither liquid contained a visible solid by itself.

What changed? The atoms did not disappear or get created. Ions from both liquids rearranged in a double replacement reaction, producing a new solid compound that does not dissolve well in water.

COMPLETE THE HOOK FORM

Observe and question what causes a solid to appear from clear liquids.

Complete Your Worksheet

Complete the Hook section on your worksheet:

  • Sketch what you observe before and after the two liquids mix.
  • Write one question about where the solid came from.
  • Circle the evidence that suggests a chemical reaction occurred.
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Worked Example – Classifying Reaction Types

Synthesis shows separate atoms bonding into one compound, while decomposition shows one compound splitting back into simpler substances.
Two reaction patterns are opposites: synthesis builds; decomposition breaks apart.

Worked Example: Classifying Reaction Types

Problem: A chemistry student drops a strip of magnesium ribbon into a test tube of hydrochloric acid. They observe rapid bubbling, the ribbon disappearing within seconds, the tube warming to the touch, and a colorless gas collecting at the top. The balanced equation is Mg(s) + 2 HCl(aq) → MgCl₂(aq) + H₂(g). What type of reaction is this, and how do we know mass is conserved?

Step 1: Compare reactants to products

On the LEFT we have 1 element (Mg) plus 1 compound (HCl). On the RIGHT we have 1 compound (MgCl₂) plus 1 element (H₂). The pattern A + BC → AC + B matches a single replacement reaction. Mg took H's spot in the chloride compound, and H came out as a free element.

Step 2: Check that the equation is balanced

Reactants: 1 Mg, 2 H, 2 Cl. Products: 1 Mg, 2 H, 2 Cl. Atoms match on both sides — the coefficient "2" in front of HCl is what makes Cl balance. A balanced equation is the symbolic version of conservation of mass: every atom that goes into a reaction has to come out.

Step 3: Confirm conservation with mass

Imagine the student weighed 24.3 g of Mg and 73.0 g of HCl (the exact stoichiometric ratio). Total reactant mass = 97.3 g. After the reaction the test tube + sealed gas balloon weighs 97.3 g — exactly. The 2.0 g of H₂ gas plus 95.3 g of MgCl₂(aq) account for every atom. If the student forgot the gas balloon, the open-tube reading would seem to "lose mass" — but no atoms were destroyed; the H₂ just escaped to the air. Conservation of mass holds in closed systems.

Conclusion: This is a single replacement reaction. We classify by pattern (one element swaps with one element of a compound), confirm balance (atom counts match), and confirm conservation (total mass before = total mass after, when nothing escapes). The same three checks work for any of the four reaction types.

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Station 1 – Reaction Type Classification

Classify chemical reactions by type using observable evidence and equations.
Single replacement shows one element bumping another out of a compound. Double replacement shows two compounds swapping partners.
Use the before-and-after pattern to classify replacement reactions.

Focus: Classify chemical reactions by comparing the reactant pattern to the product pattern.

Five Types of Chemical Reactions

TypePatternExampleEveryday Connection
SynthesisA + B → AB2H2 + O2 → 2H2OMaking water from hydrogen and oxygen
DecompositionAB → A + B2H2O → 2H2 + O2Electrolysis splits water
Single ReplacementA + BC → AC + BZn + CuSO4 → ZnSO4 + CuCoating objects with copper
Double ReplacementAB + CD → AD + CBNaCl + AgNO3 → NaNO3 + AgClPrecipitate reactions
CombustionFuel + O2 → CO2 + H2OCH4 + 2O2 → CO2 + 2H2OBurning natural gas for heat

Memory move: classify by what changes partners.

  • Synthesis puts pieces together.
  • Decomposition breaks one substance apart.
  • Replacement reactions swap partners.
  • Combustion burns fuel with oxygen and releases energy.
Interactive Simulator - Reaction Type & Equation Balancer

Use two modes: classify reaction patterns, then balance equations until the atom counters match.

COMPLETE THE STATION 1 FORM

Identify and classify different types of chemical reactions.

Complete Your Worksheet

Complete the Station 1 section on your worksheet:

  • Record one example of each reaction pattern.
  • Use the simulator to classify at least two reactions.
  • Explain how the product pattern helped you decide.
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Station 2 – Balancing Equations Lab

Practice balancing chemical equations to show conservation of atoms.
Two sealed flasks sit on balances before and after a baking soda and vinegar reaction. Both balances read 100.0 grams because the carbon dioxide gas stays trapped.
Closed system: the balance reads the same before and after. Atoms rearranged, but none were lost.

The Golden Rule: same atoms in = same atoms out. Balancing proves conservation of mass at the atomic level.

Coefficients vs. Subscripts

TermSymbolWhat It MeansCan You Change It?
Coefficient2H2OHow many moleculesYES - this is how you balance
SubscriptH2OAtoms per moleculeNO - this changes the substance

Common misconception: "Change subscripts to balance equations."

Correction: changing subscripts creates a different substance. Use coefficients only.

Balancing steps:

  1. Count atoms on each side.
  2. Adjust coefficients only.
  3. Re-count to verify equal atoms.
  4. Reduce to the smallest whole numbers.
COMPLETE THE STATION 2 FORM

Balance chemical equations and verify conservation of mass.

Complete Your Worksheet

Complete the Station 2 section on your worksheet:

  • Show atom counts before and after balancing.
  • Circle every coefficient you changed.
  • Write one sentence explaining why subscripts stay fixed.
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Station 3 – Design a Chemical Process

Apply understanding of reaction types to design a useful chemical process.
Water treatment sequence: dissolved lead ions in water, chloride added, solid lead chloride particles forming, then filtration removing the solid waste.
Design goal: turn dissolved lead ions into a filterable solid without losing track of the atoms.

Design Challenge: remove dissolved lead from contaminated water using a precipitation reaction.

The Problem

Lead ions (Pb2+) in drinking water can harm the nervous system, especially in children. A chemical treatment can make dissolved lead form a solid so it can be filtered out.

Chemical Process

  1. Add a safe source of chloride ions to react with dissolved lead.
  2. Lead ions combine with chloride ions: Pb2+ + 2Cl- → PbCl2(s).
  3. The precipitate forms solid particles that can be filtered out.
  4. Dispose of the filter waste carefully because the solid still contains lead atoms.

Real-world connection: water treatment plants use controlled precipitation and filtration to remove some dissolved metals. The chemistry helps only when the waste is captured and handled safely.

Design constraints:

  • Remove at least 95% of dissolved lead ions.
  • Use affordable treatment chemicals.
  • Work at room temperature.
  • Capture and dispose of the precipitate safely.
COMPLETE THE STATION 3 FORM

Design a chemical process using knowledge of reaction types.

Complete Your Worksheet

Complete the Station 3 design section on your worksheet:

  • Name the reaction type used in the water-treatment design.
  • Describe what gets filtered out and why it is still hazardous.
  • Use the design constraints to justify your process choice.

Strong designs connect chemistry, safety, and disposal.

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Exit Ticket – Reaction Patterns Integration

Synthesize understanding of reaction types and conservation of mass.
Classification flowchart: count reactants and products, check whether one reactant is a single element, and decide whether substances combine, split, or swap partners.
Use the flowchart to classify any reaction before you justify conservation of mass.

Question Structure

TypeCountFocus
NEW2Reaction types and balancing equations
SPIRAL2MS-PS2-3 magnetic forces and fields
INTEGRATION1Connect reaction patterns to conservation of mass
SEP-61Construct a scientific explanation

Before you submit:

  1. Classify the reaction from its pattern.
  2. Check whether atoms are balanced on both sides.
  3. Use evidence from the model or equation to justify conservation of mass.
COMPLETE THE EXIT TICKET

Demonstrate mastery of reaction types and equation balancing.

Complete Your Worksheet

Complete the Day 2 Exit Ticket and Science Circle sections on your worksheet:

  • Use complete sentences for the explanation question.
  • Show atom-count evidence when conservation of mass appears.
  • Bring one question or confidence rating to the Science Circle.
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Enrichment & Extension
Optional content if you finish early or want to go deeper.

Scientist Spotlight: Stephanie Kwolek

Stephanie Kwolek (1923–2014) was a Polish-American chemist at DuPont who invented Kevlar in 1965 — the incredibly strong polymer now used in bulletproof vests, cut-resistant gloves, and fiber-optic cables. She discovered it through a synthesis reaction: combining two monomers (small molecules) under specific conditions caused them to link into long polymer chains — a macromolecular synthesis reaction. Kwolek noticed that the resulting solution looked unusual and cloudy rather than clear, and instead of discarding it (as lab protocol suggested), she convinced a colleague to test it anyway. The fibers were five times stronger than steel by weight. Her willingness to question a result that didn't match expectations is exactly the scientific practice of analyzing and interpreting data — this week's Station 1 skill. Kwolek's synthesis reaction has saved an estimated 3,000 lives per year among law enforcement officers alone. DuPont's Kevlar plant is located in Richmond, Virginia, but the company has deep roots in the St. Louis chemical industry corridor.

Antoine Lavoisier's sealed-flask experiment: a balance scale with a closed flask of mercury, before and after heating. Both readings match — the founding experimental proof of conservation of mass.
Lavoisier's 1789 sealed-flask experiment — conservation of mass, demonstrated.

Environmental Justice: Chemical Precipitation & the Meramec River

This week's precipitate reaction — two clear liquids forming a solid — is the same chemistry behind one of Missouri's most persistent water-quality problems. For decades, industrial plants along the Meramec River near Fenton and Valley Park discharged wastewater containing dissolved heavy metals: lead, cadmium, zinc, and chromium. When these metal ions reached the river and mixed with naturally occurring sulfate and carbonate ions, double-replacement precipitation reactions deposited the metals as insoluble solids in river sediment. The sediment looked clean — no color, no obvious contamination — but the metals were locked in solid form along the riverbed. During floods (the Meramec floods dramatically — 1993, 2015, 2016), those sediments resuspend and spread. Communities downstream — predominantly Latino and low-income — face fish consumption advisories and well-water contamination. Chemists use the same precipitation reactions in reverse to clean water: adding a precipitating agent pulls dissolved lead back out of drinking water as a harmless solid that can be filtered out. This is the double-replacement reaction from Station 3's design challenge — applied at city scale.

Schematic showing power-plant emissions of SO₂ + NOₓ entering the atmosphere, reacting with water to form sulfuric and nitric acid, then falling as acid rain that damages forests, lakes, and statues — with downwind communities most affected.
Combustion → atmospheric reaction → acid rain. Same chemistry, environmental consequence.
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Week 2 Complete!

Next Week: Reaction Rates & Factors — why does food cook faster on a hot day?