Texas Grab-and-Go Substitute Packet

Conservation of Mass in Chemical Reactions

Course: Chemistry (Grades 9–12) Subject: Science Time: ~50 min standard · ~90 min block Science

Overview

This is a self-contained, no-technology substitute packet in which students investigate the law of conservation of mass in chemical reactions. Working alone with a pencil (a calculator is allowed), students read a short original passage on reactants and products, conservation of mass, closed vs. open systems, and balanced equations; study an original labeled particle diagram of a simple reaction; analyze an original table of reactant and product masses; check whether mass was conserved in a closed system; use conservation of mass to find a missing reactant mass; explain an apparent mass loss in an open system (gas escaping); count atoms to decide whether an equation is balanced; distinguish observation from inference; and write an evidence-based explanation (CER) of whether mass was conserved. It is a paper investigation with no lab materials and no hazards. Runs in a ~50-minute standard period; a ~90-minute block adds balancing a simple equation with coefficients.

At a glance

Course: Chemistry (Grades 9–12)

Subject: Science

Time: about 50 minutes standard, or a ~90-minute block (block adds balancing an equation)

Materials: printed packet and a pencil (a calculator is allowed; no computer or lab materials)

Work mode: independent

Product: a data analysis and a claim–evidence–reasoning explanation (CER)

Standards (provisional): 19 TAC Chapter 112 (Chemistry) — conservation of mass in chemical reactions (reactants → products; equal atoms on both sides) and balancing/interpreting simple equations with coefficients. Provisional — pending educator verification against the current official TAC source; the section number is intentionally left unassigned. Standards are paraphrased, not quoted. This packet does not claim formal alignment to the TEKS.

Accessible version of the student activity

The full student activity is reproduced below in plain, screen-reader-friendly HTML. It reflows on phones and at 200% zoom. Write your answers on the printed packet.

Start (5 minutes) — Notice & Wonder

When wood burns, only a small pile of ash is left and the wood looks like it "disappeared." Write one thing you notice and one thing you wonder about where the rest of the wood's material went.
If you sealed a fizzing tablet and water inside a capped bottle and weighed the whole bottle before and after it fizzed, would the total mass go up, go down, or stay the same? Give your best first idea and say why.

Build (8–12 minutes) — Read the science

The starting substances in a reaction are the reactants; the new substances that form are the products. We write reactants → products. The law of conservation of mass says matter is never created or destroyed in a reaction — the atoms are only rearranged — so the total mass of the reactants equals the total mass of the products. This is clearest in a closed system (nothing gets in or out). In an open system, a gas product can escape (mass seems to drop) or a gas can be pulled in from the air (mass seems to rise), but the atoms are still conserved. A coefficient is the big number in front of a formula (the 2 in 2 H₂O); a subscript is the small low number inside a formula (the 2 in H₂O). An equation is balanced when the same number of atoms of each element appears on both sides of the arrow — which is conservation of mass written in symbols.

Word bank

reactant: a starting substance in a chemical reaction (left of the arrow).
product: a new substance formed by a chemical reaction (right of the arrow).
coefficient: the number in front of a formula telling how many of that particle react (the 2 in 2 H₂O).
conserved: kept the same total; not created or destroyed (atoms and total mass are conserved).
closed system: a container that lets no matter in or out, so all products stay and can be weighed.

Figure 1 — full text alternative. A before-and-after particle diagram of the reaction 2 H₂ + O₂ → 2 H₂O. Before (reactants): two hydrogen molecules, each drawn as two small dotted circles bonded together, and one oxygen molecule drawn as two large hatched circles bonded together — a total of 4 hydrogen (H) atoms and 2 oxygen (O) atoms. After (products): two water molecules, each drawn as one large hatched oxygen atom bonded to two small dotted hydrogen atoms — still 4 H atoms and 2 O atoms. The atoms are only rearranged from H₂ and O₂ molecules into H₂O molecules; none are created or destroyed, so mass is conserved, and the printed atom tally reads "4 H, 2 O" on both sides. In the printed packet this is a labeled line drawing with real text labels and grayscale-safe patterns (dots for hydrogen, hatching for oxygen) rather than color.

Using Figure 1, count the hydrogen (H) and oxygen (O) atoms on the before side and on the after side, and state in one sentence whether the atoms were conserved and how the diagram shows it.

Apply (20–25 minutes) — Use the data

Masses of reactants and products for three reactions, in grams. A dash (—) means the value was not recorded (original data):

Table 1. Masses of reactants and products for three reactions (grams). Reactions 1 and 2 are closed systems (sealed flask); Reaction 3 is an open beaker where a gas escapes.
Reaction	System	Reactant A (g)	Reactant B (g)	Product(s) recovered (g)
1	Closed (sealed flask)	12.0	8.0	20.0
2	Closed (sealed flask)	15.0	—	40.0
3	Open (beaker, gas escapes)	50.0	0.0	34.0 (solid left in beaker)

Check conservation (closed system): for Reaction 1, add the two reactant masses (12.0 g + 8.0 g) and compare with the 20.0 g of product. Was mass conserved, and how can you tell?
Find the missing mass: in Reaction 2 (closed), Reactant B was not recorded. 5a) write mass of A + mass of B = total mass of products; 5b) substitute (15.0 g + B = 40.0 g) and solve for B by subtraction.
Explain the open system: in Reaction 3, 50.0 g of solid was heated in an open beaker and 34.0 g of solid was left. 6a) calculate how many grams went missing (50.0 − 34.0); 6b) explain what really happened to that mass and why the law was not broken (use the word gas).
Count atoms to check a balance for CH₄ + 2 O₂ → CO₂ + 2 H₂O: count carbon, hydrogen, and oxygen atoms on the left and right, then decide whether the equation is balanced (same number of each element on both sides).
Observation (O, read directly from Table 1) vs. inference (I, reasoned out):
- a) In Reaction 3, 34.0 g of solid was left in the beaker.
- b) In Reaction 3, 16.0 g of gas escaped into the air.
- c) In Reaction 1, the recovered product mass was 20.0 g.
- d) Reaction 2 must have taken in 25.0 g of Reactant B for mass to be conserved.
Closed vs. open — which is fair? Explain why a closed system fairly tests conservation of mass and why an open system can fool you, using the words reactant, product, and gas.

Explain (7–10 minutes) — Claim, Evidence, Reasoning

Question 10. Using Table 1, write a claim about whether mass was conserved in Reaction 1, support it with two pieces of numeric evidence (specific masses), and explain your reasoning using the law of conservation of mass and the idea of atoms being rearranged.

Sentence stems you may use: "Mass was / was not conserved in Reaction 1 because…"; "One piece of evidence is… (from Table 1)."; "A second piece of evidence is…"; "This shows mass was conserved because the atoms were…".

Close (5 minutes) — ACE

Articulate: explain the law of conservation of mass in your own words.
Connect: point to one row of Table 1 that shows mass being conserved (or seeming to change); name the numbers.
Extend: give a new everyday example where a gas escaping or being taken in makes mass seem to change, and say what really happens to the atoms.

Continue (optional) — Early finisher & block extension

Early finisher: design your own sealed-flask conservation problem — choose reactant and product masses that obey reactants = products, leave one mass blank, and show how the law solves for it.

Block extension (~+30 min): balance __ H₂ + __ O₂ → __ H₂O by filling in whole-number coefficients so H and O each have equal atoms on both sides, prove it with an atom-count table, and explain how a balanced equation shows the law of conservation of mass.

Turn in

Hand in the whole packet with your name, class period, and date, with questions 1–10 and the ACE box answered. Include the optional problem and block balancing task if you did them.

HS_CHEM_ConservationMass_01 — Conservation of Mass in Chemical Reactions Accessible landing page