6.1 Chemical vs Physical Change and Conservation of Matter

6.1 Chemical vs Physical Change and Conservation of Matter

Quick Answer: A physical change alters form (state, shape, mixture) but keeps the same substance identity. A chemical change rearranges atoms into new substances with new properties. In ordinary chemical reactions, matter is conserved: atoms are rearranged, not created or destroyed, so a balanced equation has equal numbers of each atom type on both sides. Balance by changing coefficients only—never chemical formulas (subscripts). Use a particle diagram or an atom-count table as a model; both encode the same math. Example count: 3 mol CO2 contains 6 mol O atoms. Worked balance: 2Na + MgSO4 → Mg + Na2SO4.

Physical Science is about 30% of Praxis Middle School Science (5442). Items that look like “Is melting ice chemical?” or “How many oxygen atoms are represented?” are really testing whether you (and your future students) can separate identity of matter from arrangement of matter, then apply conservation with a clear model.

Chemical Change vs Physical Change

Both kinds of change involve energy and observable effects. The exam distinction is about whether a new substance forms.

Exam traps to anticipate:

• Dissolving is usually taught as physical at middle grades when the solute can be recovered by evaporation (salt water). Do not call every solution “chemical” just because mixing happened.
• Boiling water produces a gas (steam), but the gas is still H2O—physical. Contrast with vinegar–baking soda, where the gas is new CO2.
• Phase changes (melting, freezing, vaporization, condensation, sublimation) are physical even though energy transfer is large.
• Cooking an egg or toasting bread are chemical: proteins denature and new compounds form; you cannot “un-cook” by cooling alone.

Teaching-scenario lens (~30% of 5442 items): If a stem shows a student claiming “bubbles mean chemical change,” the correct instructional move is to ask whether the gas is a new substance or the same substance in a new state—then test with evidence (identity of gas, mass in a closed system, or ability to reverse).

Conservation of Matter

The law of conservation of matter (mass) states that in a closed system, the total mass of reactants equals the total mass of products for ordinary chemical reactions. Atoms are rearranged; they are not created or destroyed.

Implications you must be ready to apply:

1. Closed vs open systems: If a gas escapes an open flask, the flask’s measured mass can drop even though atoms still exist—they left the system. Praxis items often ask why a burning candle’s open dish loses mass (CO2 and H2O vapor leave).
2. Balanced equations are the symbolic form of conservation: every atom counted on the left must appear on the right.
3. Coefficients scale whole formula units; subscripts define the formula. Changing H2O to H2O2 invents a different compound—illegal when “balancing.”

Conservation also connects to mole thinking at the middle-grades level: if a reaction consumes 2 mol of a reactant that contributes 3 mol of oxygen atoms per mole of formula, you can predict how many moles of O atoms moved into products without inventing atoms.

Anatomy of an Equation and Two Equivalent Models

A chemical equation has reactants (left), an arrow (yields / produces), and products (right). Coefficients sit in front of formulas.

Visual (particle) model: Draw each molecule or formula unit as a cluster of atom symbols. For conservation, the multiset of atom symbols left of the arrow must match the multiset on the right.

Mathematical model: Build an atom inventory:

Both models fail or succeed together. If students only “eye” the equation, push them to fill the table—the Praxis expects that SEP-style modeling habit.

Worked Example A — Mole and Atom Counting (Oxygen in CO2)

Prompt: How many moles of oxygen atoms are present in 3 mol of CO2?

Reasoning:

1. The formula CO2 means each formula unit contains 1 C and 2 O atoms.
2. Therefore 1 mol CO2 contains 2 mol O atoms (and 1 mol C atoms).
3. 3 mol CO2 × 2 mol O atoms / mol CO2 = 6 mol O atoms.

If the item asks for number of oxygen atoms (not moles), use Avogadro’s number conceptually: 6 mol O atoms × 6.02 × 10^23 atoms/mol ≈ 3.61 × 10^24 O atoms. Many 5442 items stay at the mole-of-atoms or atoms-per-formula level; either way, the coefficient 3 multiplies the entire formula, and the subscript 2 multiplies only oxygen.

Related check: In 2 mol H2O, there are 2 mol O atoms and 4 mol H atoms. Students who answer “2 mol H” forgot that the subscript multiplies hydrogen inside each water molecule.

Worked Example B — Balancing a Single-Replacement Equation

Unbalanced: Na + MgSO4 → Mg + Na2SO4

Step 1 — Inventory (unbalanced):

Sodium is unbalanced (1 vs 2). Magnesium, sulfur, and oxygen already match.

Step 2 — Adjust coefficients only: Place 2 in front of Na:

2Na + MgSO4 → Mg + Na2SO4

Step 3 — Re-check:

Balanced. Chemically, two sodium atoms displace magnesium from magnesium sulfate, forming sodium sulfate and free magnesium—a single-replacement pattern you will classify in the next section.

Particle-model reading of the same balance: Two Na atoms + one MgSO4 unit → one Mg atom + one Na2SO4 unit. Oxygen atoms stay grouped as SO4^2− in the ionic picture, but for atom counting you still tally four O on each side.

Worked Example C — A Second Quick Balance (Combustion Preview)

Unbalanced: CH4 + O2 → CO2 + H2O

Balance C (already 1), then H by putting 2 before H2O, then O by putting 2 before O2:

CH4 + 2O2 → CO2 + 2H2O

Left O: 4; right O: 2 (in CO2) + 2 (in 2H2O) = 4. Same conservation logic as the sodium example.

Classroom Mass Investigation You Should Be Ready to Coach

A classic closed-system demo: seal baking soda and vinegar in a bag, mix, and compare mass before and after. Mass stays essentially constant while the bag inflates—evidence that gas atoms were products, not “disappeared matter.” Open the bag and the scale reading falls; atoms left the system. Praxis teaching items often ask which student explanation is scientifically accurate.

Exam Strategy Snapshot

• First ask: new substance? → chemical vs physical.
• For equations: count atoms by element, change coefficients, never subscripts.
• For “how many atoms/moles of X in n mol of formula”: multiply n × subscript of X (and remember polyatomic units still contribute their atom counts).
• Watch open-system mass loss: conservation still holds for the universe of atoms; the system boundary explains the measurement.