Week 2: Week 2 Content
Grade 7 Science | Rosche | Kairos Academies
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- Stuck? Look for the red "Stuck?" boxes with step-by-step help
NGSS Standards Covered This Week
MS-PS1-5 (Primary)
What it means: 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.
In student language: I can create models showing that atoms rearrange but are NEVER created or destroyed in reactions.
3-Dimensional Learning
| Dimension | What You'll Practice |
|---|---|
| SEP-2 Developing & Using Models | Track atoms before and after reactions |
| SEP-3 Planning & Carrying Out Investigations | Compare mass in sealed vs. open systems |
| DCI PS1.B Chemical Reactions | Learn that atoms rearrange but don't disappear |
| CCC-5 Energy and Matter | Matter is conserved because atoms are conserved |
Success Criteria - How You'll Know You've Got It
Target 1: Apply the law of conservation of mass to chemical reactions
Self-check: Can I explain why mass stays the same even when products are invisible?
Target 2: Balance chemical equations by tracking atoms
Self-check: Can I count atoms on both sides and make them equal?
Target 3: Explain why mass seems to disappear in open systems
Self-check: Can I identify invisible gas products (COβ, HβO vapor)?
Why This Matters to YOU:
Every breath you take is recycled atoms. Conservation of mass explains climate change, recycling, and why we can't "destroy" pollutionβatoms cycle through Earth forever, just rearranged into new forms.
The Phenomenon: The Candle Vanishing Act
Watch what happens when a candle burns:
- You weigh a candle: 100 grams
- You light it and let it burn for 30 minutes
- You weigh it again: 95 grams
- 5 grams of wax have disappeared!
But wait - last week you learned that matter isn't created or destroyed in chemical reactions. So where did those 5 grams go? Did the atoms just vanish into thin air?
The Mystery: The candle got lighter, but conservation of mass says atoms can't be destroyed. What happened to the missing atoms?
Focus Question: Do atoms disappear in chemical reactions?
Learning Targets
By the end of this week, you will be able to:
Connection to Week 1
Last week, you learned that gas production is one of the 4 types of evidence for chemical reactions.
This week, you'll discover that those invisible gases contain the "missing" atoms - proving that mass is conserved even when products are invisible!
Vocabulary
Cognate Strategy: Many science words look similar in English and Spanish β use your Spanish to learn science!
| Term | Spanish | Definition |
|---|---|---|
| conservation | conservaciΓ³n | Keeping the same amount; nothing created or destroyed |
| mass | masa | The amount of matter in something (measured in grams) |
| reactant | reactivo | Substance that goes INTO a chemical reaction |
| product | producto | Substance that comes OUT OF a chemical reaction |
| atom | Γ‘tomo | Smallest unit of an element; cannot be created or destroyed |
| coefficient | coeficiente | Number in front of formula showing how many molecules |
| balanced | equilibrado | Equal number of atoms on both sides of equation |
Worked Example
Step-by-Step Problem Solving
Problem Scenario
Review the problem scenario and work through each step below.
Hook: The Candle Vanishing Act
12 Points | ~10 Minutes
Your Mission
Watch a candle burn and predict what happens to the "missing" mass. Where do the atoms go?
What You'll Do:
- Observe candle burning (video or teacher demo)
- Notice the candle gets smaller and lighter
- Predict where the "missing" atoms went
- Connect to Week 1 evidence types
Need help? Click for hints
Hint 1: Remember Week 1 - gas production is evidence of a chemical reaction!
Hint 2: When things burn, they combine with oxygen to make new products.
Hint 3: Wax contains carbon and hydrogen. What gases might form?
Stuck? Step-by-step help
Step 1: Burning is a chemical reaction. What are the 4 evidence types? (Gas, color, temperature, precipitate)
Step 2: Which evidence type happens when a candle burns? (Gas production - you see smoke!)
Step 3: If the wax becomes gas, where are the atoms? (Still there, just in gas form!)
Complete this sentence: "The wax didn't disappear. It turned into invisible _____ (gases) that floated away."
Station 1: Sealed System Lab
20 Points | ~18 Minutes
Your Mission
Compare what happens to mass in OPEN systems (products escape) vs. SEALED systems (products trapped). This proves conservation of mass!
What You'll Do:
- Watch virtual lab: vinegar + baking soda in OPEN beaker
- Watch virtual lab: vinegar + baking soda in SEALED balloon
- Compare mass before and after in both setups
- Explain why results differ (gas escapes vs. gas trapped)
Key Observations
| Setup | Initial Mass | What Happens? | Final Mass |
|---|---|---|---|
| Open Beaker | 50g | Bubbles escape into air | 45g (gas escaped!) |
| Sealed Balloon | 50g | Balloon inflates (gas trapped) | 50g (gas still there!) |
Law of Conservation of Mass:
In a CLOSED/SEALED system, mass of reactants = mass of products. Atoms rearrange but are NEVER created or destroyed!
Interactive Simulation
Need help? Click for hints
Hint 1: The gas (COβ) has mass! It's not "massless."
Hint 2: Open system = products can escape. Sealed system = products stay inside.
Hint 3: The atoms didn't disappear in the open beaker - they floated away as gas!
Stuck? Step-by-step help
Step 1: What is the gas produced? (Carbon dioxide - COβ)
Step 2: Does COβ have mass? (Yes! All matter has mass, even gases)
Step 3: In open beaker: Where did the COβ go? (Escaped into the air)
Step 4: In sealed balloon: Where did the COβ go? (Trapped inside the balloon)
Complete this sentence: "Mass is conserved in the sealed system because the COβ gas _____ (stayed inside) instead of _____ (escaping)."
Station 2: Atom Tracker
20 Points | ~15 Minutes
Your Mission
Use the PhET simulation to balance chemical equations by counting atoms. Prove that atoms are NEVER created or destroyed - just rearranged!
PhET Simulation Link:
What You'll Do (12 minutes in simulation):
- Open PhET simulation in new tab (link above)
- Start with "Introduction" mode
- Build 3 reactions: Combustion (CHβ + Oβ), Synthesis (Nβ + Hβ), Decomposition (HβO β Hβ + Oβ)
-
For EACH reaction:
- Count atoms on reactant side (left)
- Count atoms on product side (right)
- Adjust coefficients until balanced
- Screenshot balanced equation
- Answer form questions about atom conservation
Example: Balancing Water Formation
Reaction: Hβ + Oβ β HβO
| Step | Left Side (Reactants) | Right Side (Products) | Balanced? |
|---|---|---|---|
| 1. Count atoms | 2H, 2O | 2H, 1O | No (oxygen not equal) |
| 2. Add coefficient | 2Hβ + Oβ | 2HβO | Check again... |
| 3. Recount atoms | 4H, 2O | 4H, 2O | Yes! (all atoms equal) |
Balanced equation: 2Hβ + Oβ β 2HβO
Need help? Click for hints
Hint 1: Count each type of atom separately (H, O, C, N...).
Hint 2: The coefficient multiplies the WHOLE molecule. Example: 2HβO means 4 H atoms and 2 O atoms.
Hint 3: Change coefficients (the BIG numbers), NOT subscripts (the small numbers)!
Hint 4: If you're stuck, try the "Game" mode in PhET - it gives you hints!
Stuck? Step-by-step help
Step 1: Look at the LEFT side. Count each atom type. Example: CHβ has 1 C and 4 H.
Step 2: Look at the RIGHT side. Count each atom type. Example: COβ has 1 C and 2 O.
Step 3: Are the atom counts equal? If NO, you need to balance.
Step 4: Add coefficients (numbers in front). Try 2CHβ (now you have 2 C and 8 H).
Step 5: Keep adjusting until LEFT = RIGHT for ALL atom types.
Use this sentence frame: "The left side has ___ atoms. The right side has ___ atoms. I need to add a coefficient of ___ to make them equal."
Station 3: Candle Investigation
25 Points | ~20 Minutes
Your Mission
Use conservation of mass to calculate the mass of invisible products from a burning candle. This solves the Hook mystery!
What You'll Do:
- Review the candle phenomenon from the Hook
- Calculate the mass of COβ and HβO produced
- Explain where the "missing" atoms went
- Compare open vs. closed systems
The Candle Mystery - SOLVED!
Remember from the Hook:
- Candle mass BEFORE: 100g
- Candle mass AFTER: 95g
- Mass "lost": 5g
The chemical reaction: Wax (Cββ Hβ β) + Oβ β COβ + HβO
The question: If 5g of wax burned, what is the TOTAL mass of products (COβ + HβO)?
Conservation of Mass Formula:
Mass of Reactants = Mass of Products
This means: 5g wax + (oxygen from air) = (COβ + HβO produced)
Need help? Click for hints
Hint 1: The wax didn't disappear - it became COβ gas and HβO vapor!
Hint 2: These gases are INVISIBLE, so they seem to vanish, but they have mass.
Hint 3: The 5g of wax combined with oxygen from the air. The total mass of products equals the total mass of reactants.
Hint 4: If you weighed ALL the COβ and HβO, you'd find they weigh MORE than 5g because they include the oxygen atoms too!
Stuck? Step-by-step help
Step 1: What are the reactants? (Wax + Oxygen from air)
Step 2: What are the products? (COβ + HβO vapor)
Step 3: Are the products visible? (NO! Both are invisible gases)
Step 4: Did the atoms disappear? (NO! They're still there, just in gas form)
Step 5: Why did the candle get lighter? (The gas products floated away into the air)
Complete this sentence: "The candle lost 5g because the wax turned into invisible _____ (gases: COβ and HβO) that _____ (floated away). If we captured all the products, the total mass would be _____ (equal to wax + oxygen)."
Exit Ticket: Conservation of Mass Mastery
23 Points | ~15 Minutes
Show What You Learned
This tests whether you understand conservation of mass AND remember Week 1 evidence types.
Exit Ticket Structure (6 questions):
- 2 NEW questions: Conservation of mass, balancing equations
- 2 SPIRAL questions: Week 1 evidence types (review!)
- 1 INTEGRATION question: Combines Week 1 + Week 2 concepts
- 1 SEP-2 question: Develop a model of atom rearrangement
Before you start:
- Review your worksheet notes
- Remember: Atoms rearrange but are NEVER created or destroyed
- Remember: Open systems lose gas products; closed systems don't
- Remember: Week 1 evidence types (gas, color, temperature, precipitate)
Week 2 Complete!
You've mastered a fundamental law of chemistry:
Atoms are NEVER created or destroyed - they just rearrange into new molecules!
Next Week Preview: Now that you know atoms are conserved, you'll discover that ENERGY is stored in the bonds between those atoms. When bonds break and form, energy can be released (exothermic) or absorbed (endothermic)!
Week 3 Phenomenon: "Why do hot packs get hot but cold packs get cold?"
Enrichment & Extension
Optional deep dives for early finishers.
Optional content if you finish early or want to go deeper.
Scientist Spotlight
Research a scientist who contributed to this week's topic area and describe their key findings.
Environmental Justice Connection
Explore how this week's science concepts connect to environmental justice issues in our community.