Objective
Students will be able to distinguish between physical and chemical changes, identify the four signs of a chemical reaction (color change, gas production, temperature change, precipitate formation), balance simple chemical equations, and explain the law of conservation of mass. Students will correctly identify at least 4 out of 5 changes as physical or chemical and balance at least 3 out of 5 equations.
Standards
- NGSS MS-PS1-2 — Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.
- NGSS MS-PS1-5 — Develop and use a model to describe how the total number of atoms does not change in a chemical reaction and thus mass is conserved.
Materials
- Safety goggles (1 per student)
- Lab station supplies: baking soda, vinegar, steel wool, hydrogen peroxide, glow sticks, milk, food coloring, dish soap
- Digital scale (1 per station)
- Beakers, test tubes, and plastic cups
- Chemical reaction observation worksheet
- Balancing equations practice sheet
- Periodic table reference (1 per student)
- Anchor chart: "Physical vs. Chemical Change"
Warm-Up (5 minutes)
Hold up a piece of paper. Tear it in half. Ask: "Did I change the paper?" (Yes.) "Is it still paper?" (Yes.) Now hold up a match and light it (or show a video of paper burning). Ask: "Is the burnt paper still paper?" (No — it became ash, smoke, and gases.) Introduce the distinction: tearing is a physical change (the substance stays the same), but burning is a chemical change (a new substance is formed). Today we explore how to tell the difference and what happens during a chemical reaction.
Direct Instruction (12 minutes)
Build the anchor chart with two columns:
Physical changes: Change in size, shape, or state, but the substance stays the same. Examples: cutting, melting, freezing, dissolving, tearing. Reversible in most cases.
Chemical changes (chemical reactions): A new substance is formed with different properties. Usually irreversible. Evidence of a chemical reaction:
- Color change — not just mixing colors, but a fundamentally new color appears (e.g., iron turns orange when it rusts)
- Gas production — bubbles, fizzing, or odor (e.g., baking soda + vinegar produces carbon dioxide gas)
- Temperature change — the mixture gets noticeably hotter (exothermic) or colder (endothermic) without external heating
- Precipitate formation — a solid forms when two liquids are mixed
Introduce the law of conservation of mass: in a chemical reaction, matter is not created or destroyed — the atoms just rearrange. The total mass before the reaction equals the total mass after. Demonstrate by weighing baking soda + vinegar in a sealed container before and after reacting — the mass stays the same.
Then introduce chemical equations. A chemical equation is a shorthand way to describe a reaction. Show: reactants (left side) -> products (right side). Example: baking soda + vinegar -> carbon dioxide + water + sodium acetate. In chemical formulas: NaHCO3 + CH3COOH -> CO2 + H2O + NaC2H3O2.
Model balancing a simple equation: H2 + O2 -> H2O. Count atoms: 2 H on the left, 2 O on the left, 2 H on the right, but only 1 O on the right. Not balanced. Add a coefficient: 2H2 + O2 -> 2H2O. Now: 4 H left, 2 O left, 4 H right, 2 O right. Balanced. The number of each type of atom is the same on both sides — conservation of mass.
Guided Practice — Lab Stations (15 minutes)
Students rotate through 4 lab stations in groups of 4 (about 3–4 minutes per station). At each station, they observe and record on their worksheet:
Station 1 — Baking Soda + Vinegar: Add a spoonful of baking soda to vinegar in a cup. Observe: bubbling (gas production), feels slightly cool (endothermic). Chemical change.
Station 2 — Steel Wool + Vinegar: Place steel wool in vinegar and wait 2 minutes. Observe: color change (steel wool darkens), slight temperature increase. Chemical change (the vinegar dissolves the protective coating, accelerating oxidation).
Station 3 — Glow Stick: Bend the glow stick to activate it. Observe: light and color change. Chemical change (the chemicals inside react and produce light — chemiluminescence).
Station 4 — Milk + Food Coloring + Dish Soap: Pour milk in a plate, add food coloring drops, touch dish soap to the surface. Observe: colors swirl dramatically. Physical change (the soap breaks the surface tension of the fat in the milk, causing movement — no new substance is formed).
At each station, students answer: What happened? What evidence of change did you observe? Is this a physical or chemical change? Explain.
Independent Practice (8 minutes)
Students complete two activities at their desks:
- Classify 8 scenarios as physical or chemical change: dissolving sugar in water (physical), cooking an egg (chemical), bending a paperclip (physical), burning wood (chemical), mixing salt and pepper (physical), rusting iron (chemical), boiling water (physical), mixing baking soda and lemon juice (chemical).
- Balance 5 simple equations:
- Fe + O2 -> Fe2O3
- N2 + H2 -> NH3
- CH4 + O2 -> CO2 + H2O
- Na + Cl2 -> NaCl
- Mg + HCl -> MgCl2 + H2
Assessment
- Formative: Lab station observations — check that students correctly identify evidence of chemical vs. physical changes and use proper terminology.
- Summative: Collect worksheets. Score classification (4 points for 8 scenarios) and balancing equations (5 points for 5 equations, partial credit for correct attempt with minor errors). Total: 9 points.
Differentiation
- Struggling learners: Provide a physical vs. chemical change reference card with examples. For equation balancing, start with equations that only need one coefficient changed. Use colored counting chips to represent atoms when balancing. Work in a teacher-led group for the independent practice.
- ELL students: Pre-teach vocabulary (reaction, reactant, product, coefficient, balanced, conserve, precipitate) with visual diagrams. Provide lab station instruction cards with pictures alongside text. Allow verbal observations to a partner instead of written responses. Offer a bilingual periodic table.
- Advanced learners: Include more complex equations to balance (with 3+ reactants or products). Introduce types of reactions (synthesis, decomposition, single replacement, double replacement). Have them design their own experiment to test whether a change is physical or chemical and write a formal lab report.
- Students with IEPs: Provide a pre-formatted observation worksheet with sentence starters ("I observed ___. This is evidence of a ___ change because ___."). Pair with a supportive partner at lab stations. Reduce balancing equations to 3 instead of 5. Allow use of atom counting manipulatives.
Closure (5 minutes)
Return to the warm-up: "When I tore the paper, was that physical or chemical? When the paper burned, was that physical or chemical? How do you know?" Students should now articulate the difference using the four signs of chemical change. Then pose a challenge: "If I dissolve salt in water, is that a physical or chemical change? How could you prove it?" (Physical — you can evaporate the water and get the salt back. A chemical change produces a new substance you cannot easily reverse.) Preview tomorrow's lesson on types of chemical reactions and energy changes (exothermic vs. endothermic).