Week 5, Session 15: Chemical Reactions and Conservation of Mass

Week 5, Session 15: Chemical Reactions and Conservation of Mass

Why this session matters

In a chemical reaction, substances rearrange into new ones — but the atoms themselves are never created or destroyed. That single rule, the conservation of mass, powers a whole set of GED® questions, including the classic "what total mass of product forms?" This session makes that rule automatic.

The big idea in plain words

A chemical reaction is like rearranging LEGO bricks. You take apart the starting molecules (reactants) and rebuild them into new ones (products), but you always end up with the same bricks (atoms). So in a sealed container, the total mass before equals the total mass after — nothing vanishes.

Picture it

Words you must own

• Chemical reaction: a process that turns reactants into new products.
• Reactant / product: starting materials / substances formed.
• Chemical equation: a shorthand like \(2H_2 + O_2 \rightarrow 2H_2O\).
• Coefficient: the big number in front (how many molecules); subscript: the small number (atoms in a molecule).
• Conservation of mass: in a closed system, total mass of reactants = total mass of products.
• Balanced equation: the same number of each atom on both sides.
• Signs of a chemical change: gas/bubbles, color change, temperature change, light, or a solid forming (precipitate).
• Catalyst: speeds up a reaction without being used up.

The science, step by step

1. Reactants become products, but every atom is accounted for.
2. Conservation of mass: if 4 g of one reactant fully reacts with 32 g of another in a sealed flask, the products weigh 4 + 32 = 36 g.
3. Balancing: adjust coefficients (not subscripts) until each element has the same count on both sides.
4. Open vs closed systems: if a gas escapes an open container, the leftover mass looks smaller — but no mass was truly lost; it left as gas.
5. Spotting chemical change: watch for a new gas, a color change, heat or light released, or a solid forming — these hint that new substances are appearing.

Exam-style passage

Passage. A student mixed baking soda and vinegar in two ways. In an open cup, the mixture fizzed and, when weighed afterward, the total mass was less than before. In a sealed plastic bag, the same amounts fizzed but the total mass stayed the same.

Watch me solve one

Question about the passage: Why did mass appear to decrease in the open cup but stay the same in the sealed bag?

• READ: Both fizzed (a gas, carbon dioxide, formed). Open cup lost mass; sealed bag did not.
• CHOOSE: The fizzing produces carbon dioxide gas. In the open cup, that gas escaped into the air, so the remaining mass looked smaller. In the sealed bag, the gas was trapped, so all the mass stayed and the reading was unchanged — mass was conserved the whole time.
• CHECK: Mass was never actually destroyed; conservation of mass holds. The open cup only seemed to lose mass because gas left the container.

10 Guided Examples (cover the answer, then check)

Examples 1–4 use the baking-soda passage.

1. Gas. What gas is produced by baking soda and vinegar?
   Answer: Carbon dioxide.
   Why: The fizzing bubbles are CO₂.
2. Open cup. Why did the open cup lose mass?
   Answer: The gas escaped into the air.
   Why: Nothing was destroyed; it just left the cup.
3. Sealed bag. Why did the sealed bag keep the same mass?
   Answer: The gas was trapped inside.
   Why: All products stayed, so mass was conserved.
4. Rule. Which law does the sealed bag demonstrate?
   Answer: Conservation of mass.
   Why: Mass of products equals mass of reactants in a closed system.
5. Closed reaction. In a sealed flask, 4 g hydrogen reacts with 32 g oxygen. What mass of water forms?
   Answer: 36 g.
   Why: 4 + 32 = 36 (mass is conserved).
6. Atoms. In \(2H_2 + O_2 \rightarrow 2H_2O\), how many hydrogen atoms are on each side?
   Answer: 4.
   Why: 2 H₂ = 4 H atoms; 2 H₂O = 4 H atoms.
7. Balancing. To balance an equation, do you change coefficients or subscripts?
   Answer: Coefficients.
   Why: Changing subscripts would change the substance.
8. Sign of change. Name one sign of a chemical change.
   Answer: Gas forms (or color/temperature change).
   Why: New substances often bubble, change color, or change temperature.
9. Physical vs chemical. Is dissolving sugar a chemical reaction?
   Answer: No, it is a physical change.
   Why: The sugar is still sugar; no new substance forms.
10. Conservation reasoning. In a sealed jar, reactants weigh 50 g. What do the products weigh?
    Answer: 50 g.
    Why: Closed system → mass is conserved.

The traps GED® loves

• "Mass disappeared." In an open container, escaped gas only makes mass look smaller.
• Balancing by changing subscripts: never — that changes the chemical itself. Change coefficients.
• Bubbles always mean chemistry? Usually a chemical change, but boiling also bubbles (physical) — read the context.
• Atoms are created in reactions? No — atoms are only rearranged.

Quick Check

Your practice — 10 questions

Answer from memory. For mass problems, remember: closed system means product mass = reactant mass.

Before you move on

• State the law of conservation of mass in your own words.
• Explain the open-cup vs sealed-bag result to an imaginary friend.
• Score at least 8 of 10 before Session 16.