Evidence of Chemical Reactions - Study Guide

[!] STUDY GUIDE: Big Ideas Use to answer form questions

1.Mass is always conserved; atoms rearrange but aren't created or destroyed. Students often think mass is lost or gained in chemical reactions, but evidence from investigations shows otherwise. Use data from today's stations to identify which observations best support this understanding.

2.Chemical changes form new substances; physical changes alter form but not composition. Students often think difficulty distinguishing chemical from physical changes, but evidence from investigations shows otherwise. Use data from today's stations to identify which observations best support this understanding.

3.Atoms are conserved; they rearrange into new molecules but aren't destroyed. Students often think reactions destroy or create atoms, but evidence from investigations shows otherwise. Use data from today's stations to identify which observations best support this understanding.

DRAW KEY TERMS Google each → Images tab → sketch what you see

OTHER VOCAB: precipitate · indicator · exothermic

Chemical Reaction

Process where substances transform into new substances

Reactant

Starting substance in a chemical reaction

Product

New substance formed in a chemical reaction

Conservation of Mass

Mass is neither created nor destroyed in reactions

WRITE FOCUS QUESTION Your initial thinking

You snap a glow stick at a concert and it blazes neon green. Hours later it is dim; by morning it is dead. The chemicals inside mixed when you bent the tube, but something ran out -- even though the stick weighs exactly the same.

Why do glow sticks eventually stop glowing?

I think...glow sticks stop glowing because the chemical reaction inside them runs out of reactants. The two chemicals that react to produce light are mixed when you bend the stick, but once one of them is used up, the reaction stops and no more light is produced.

WRITE AFTER HOOK FORM Reflect before continuing

I learned that... that a chemical reaction produces evidence we can observe — color change, gas production, temperature change, or precipitate formation — and that these changes signal new substances have been created, not just the original materials rearranged physically.

Station 1 — Reaction Evidence Lab

I observed that... each reaction demo showed at least one observable piece of evidence — the rusting nail turned orange and the liquid turned green (color change), the baking soda and vinegar produced bubbles (gas), the hand warmer packet heated up (temperature change), and the mixing of two clear liquids produced a white cloudy solid (precipitate).

This shows that... these are all chemical changes, not physical changes, because new substances with different properties were formed — the original materials cannot simply be separated back out by physical means like filtering or evaporation.

The key concept is... a chemical reaction is a process where reactants are rearranged at the atomic level to form products with different properties, and we identify reactions by observable evidence: color change, gas production, temperature change, or precipitate formation.

CHECK DAY 1 CHECK-IN Before you leave

Q1: Based on today's investigation of evidence of chemical reactions, explain one key finding using evidence from your data. Connect your answer to this week's focus question: Why do glow sticks eventually stop glowing?

One key finding is that chemical reactions produce observable evidence — in Station 1, the rusting nail produced both a color change (orange rust) and a temperature change (the beaker warmed slightly), which means new substances with different properties formed. This connects to why glow sticks stop glowing: the chemiluminescence reaction produces light as evidence that reactants are being converted to products. When all the reactants are consumed, no more observable evidence (light) is produced — the reaction is complete.

Confidence:

Still building Getting clearer Ready to explain

Station 2 — Mass Conservation Investigation

The data shows... when we tracked mass before and after each reaction, the total mass of the closed system stayed the same — reactants and products weighed exactly the same combined, even though the substances looked completely different.

This connects to... the focus question because it proves that even though the glow stick dims and eventually goes dark, no mass is lost — the atoms that made up the reactants are still there, just rearranged into products that don't emit light.

The main idea is... chemical reactions conserve mass because atoms are neither created nor destroyed — they are rearranged from reactants into products, which is why a balanced chemical equation has the same number of each type of atom on both sides.

This connects to what we learned because... it explains why the glow stick stops glowing even though all the atoms are still present — once the specific reactant molecules needed for the chemiluminescence reaction are fully converted into products, no more light-producing reaction can occur, even though mass is conserved.

I can apply this by... predicting that if you cool a glow stick in ice, the reaction slows down (fewer collisions per second between reactant molecules), making it last longer — and warming it speeds the reaction up, making it brighter but shorter-lived. This is direct evidence of reaction rate tied to molecular collisions.

DRAW CONCEPT MODEL Show your understanding visually

Draw a model that shows the main concept. Include labels and arrows to explain relationships.

Draw a before-and-after diagram showing: LEFT SIDE (Reactants): label two molecule types A and B with specific atom counts (e.g., 6 atoms total). RIGHT SIDE (Products): show the same atoms rearranged into new molecule C, with the same total atom count (6 atoms). Add an arrow labeled 'chemical reaction' between sides. Below, draw a balance scale showing equal masses on both sides (reactants = products). In a corner, sketch the glow stick scenario: tube with two compartments → bend to mix → chemiluminescence glow → fading as reactants depleted. Label: 'Reactants consumed → reaction stops → no more light.' Teacher: draw a specific example like 2H₂ + O₂ → 2H₂O, counting H and O atoms on each side to show conservation.

Labels included

Relationships shown

Key terms used

CHECK TEST YOUR THINKING Rate each: FALSE ← 1-2-3-4-5 → TRUE

1. When something burns, mass is lost because the material disappears.

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2. Dissolving sugar in water is a chemical change because the sugar disappears.

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3. Chemical reactions destroy atoms and create new ones to form products.

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4. In a chemical reaction, atoms rearrange into new molecules but the total number of each type of atom stays the same.

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5. Evidence of a chemical reaction includes color change, gas production, temperature change, or precipitate formation.

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Pick any TWO statements -- especially any you rated 2, 3, or 4. Justify your rating with evidence:

Statement #___ -- I rated it ___ because:Statement #1 — I rated it 1 because: conservation of mass means atoms don't vanish when something burns. The carbon and hydrogen in wood combine with oxygen from the air to form CO₂ and H₂O gases. The gases escape into the air, making it look like mass was lost — but a sealed system experiment shows the total mass is unchanged.

Statement #___ -- I rated it ___ because:Statement #2 — I rated it 1 because: dissolving creates a mixture, not a new substance. The sugar molecules are still sugar — they're just spread out among water molecules. You can recover the sugar by evaporating the water, which proves no chemical change occurred. A true chemical change produces substances you can't simply separate back into the originals.

THINK SCIENTIST CIRCLE DEBRIEF Reflect after your Scientist Circle discussion

AHA! Moment

The one I changed my thinking most about is... that conservation of mass means the atoms in a glow stick don't disappear when it stops glowing — they're still there as product molecules, just no longer in a configuration that releases light energy. The reaction ends when reactants run out, not when atoms are destroyed.

Remaining Question

I still wonder... whether it's possible to 'recharge' a glow stick by converting the product molecules back into the original reactants — and what that would take in terms of energy input, and whether that would be considered a reverse reaction or a completely different reaction.

WRITE FOCUS QUESTION REVISIT Final written response

Why do glow sticks eventually stop glowing?

Word Bank: chemical reaction · reactant · product · conservation of mass · precipitate · indicator · exothermic

Glow sticks stop glowing because the chemical reaction inside them runs out of reactants. When you bend a glow stick, you break an inner glass capsule that mixes two chemicals — a dye compound and hydrogen peroxide with a phenyl oxalate ester. These reactants undergo a chemiluminescence reaction: the chemical energy released by their molecular rearrangement is emitted as visible light rather than heat.

The reaction continues as long as both reactants are present. Once one reactant (typically the phenyl oxalate ester) is fully converted into products, no more light-producing reactions can occur — even though all the atoms are still present in the tube, now as product molecules. This is conservation of mass: atoms are rearranged, not destroyed.

Temperature affects reaction rate: a cold glow stick slows the reaction (fewer molecular collisions per second), lasting longer but dimmer; a warm glow stick speeds the reaction, producing brighter light but running out of reactants faster. This predictable relationship between temperature and reaction rate is additional evidence that the glow is driven by a chemical reaction governed by collision theory.