11.1 Physical vs. Chemical Changes, Reaction Types & Conservation of Mass

Physical vs. Chemical Changes, Reaction Types & Conservation of Mass

Quick Answer: A physical change alters the form or appearance of a substance without altering its chemical identity (e.g., melting ice, dissolving sugar). A chemical change breaks and forms chemical bonds to produce new substances with unique properties (e.g., combustion, rusting). Evidence of chemical reactions includes precipitate formation, unexpected color changes, gas evolution, and temperature shifts. Under the Law of Conservation of Mass, matter is neither created nor destroyed; chemical equations must be balanced exclusively using stoichiometric coefficients without changing subscripts.

On the HiSET Science subtest, chemistry questions evaluate your ability to distinguish physical processes from chemical reactions, classify reaction types, and balance equations using mass conservation.

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Physical vs. Chemical Changes: Molecular Distinctions

The distinction between physical and chemical changes lies at the molecular level:

• Physical Changes: Alter physical properties such as shape, size, phase, or density while leaving chemical composition and intramolecular bonds intact. When water ($H_2O$) freezes, molecules form a crystalline lattice via intermolecular hydrogen bonds, but covalent $H\text{–}O$ bonds remain unchanged. Similarly, dissolving salt ($NaCl$) in water disperses ions throughout the solvent without creating new substances; evaporating water recovers the salt.
• Chemical Changes (Reactions): Break existing bonds in reactants and form new bonds to create chemically distinct products with new properties (altered melting points, reactivities, and densities). Electrolysis of water breaks covalent bonds to yield hydrogen ($H_2$) and oxygen ($O_2$) gases. Iron rusting occurs when metallic iron ($Fe$) reacts with oxygen and water to produce iron(III) oxide ($Fe_2O_3$).

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Four Observable Hallmarks of Chemical Reactions

Researchers rely on macroscopic experimental evidence to verify chemical reactions:

1. Precipitate Formation: Mixing two clear aqueous solutions produces an insoluble solid that separates as a precipitate. For example, aqueous potassium iodide ($KI$) and lead(II) nitrate ($Pb(NO_3)_2$) yield yellow solid lead(II) iodide ($PbI_2(s)$).
2. Unexpected Color Change: A permanent color change indicating new chemical species, such as shiny iron forming reddish-brown rust, or starch turning dark blue with iodine.
3. Gas Evolution (Effervescence): Vigorous bubbling without external boiling signals gas synthesis. Dropping zinc into hydrochloric acid ($HCl$) liberates hydrogen gas ($Zn + 2HCl \rightarrow ZnCl_2 + H_2 \uparrow$). Similarly, baking soda and vinegar yield carbon dioxide ($CO_2$).
4. Thermal Energy Shifts & Light: Reactions either release heat (exothermic, container warms, e.g., combustion) or absorb heat (endothermic, container cools, e.g., ammonium nitrate cold packs). Flames or chemiluminescence further confirm reactions.

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Five Fundamental Types of Chemical Reactions

Chemical reactions are systematically categorized by how atoms rearrange:

1. Synthesis (Combination): Multiple reactants unite into one product ($A + B \rightarrow AB$). Example: $2Mg + O_2 \rightarrow 2MgO$.
2. Decomposition: A single reactant breaks down into simpler substances ($AB \rightarrow A + B$). Example: $2H_2O_2 \rightarrow 2H_2O + O_2$.
3. Single Replacement (Displacement): A more reactive free element displaces a less reactive element from an aqueous compound ($A + BC \rightarrow AC + B$). Example: $Zn + CuSO_4 \rightarrow ZnSO_4 + Cu$.
4. Double Replacement (Metathesis): Cations and anions of two compounds exchange partners ($AB + CD \rightarrow AD + CB$), forming a precipitate, gas, or water. Example: $AgNO_3 + NaCl \rightarrow AgCl(s) + NaNO_3$.
5. Combustion: A hydrocarbon burns in oxygen gas ($O_2$), releasing heat, light, $CO_2$, and water vapor ($H_2O$): $C_xH_y + O_2 \rightarrow CO_2 + H_2O$. Example: $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$.

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The Law of Conservation of Mass & Balancing Chemical Equations

Formulated by Antoine Lavoisier, the Law of Conservation of Mass states that mass is neither created nor destroyed in a closed chemical system. Total reactant mass must equal total product mass: $\text{Total Mass of Reactants} = \text{Total Mass of Products}$

Reactions rearrange existing atoms without creating or destroying them. If an open-beaker reaction displays apparent mass loss, gaseous products ($CO_2, H_2$) escaped into the air. In a sealed container, mass remains strictly constant.

Chemical Coefficients vs. Subscripts

• Subscripts: Numbers indicating the fixed atom ratio in a molecule (e.g., the 2 in $H_2O$). Subscripts can NEVER be changed when balancing, as doing so alters chemical identity (e.g., changing $H_2O$ to $H_2O_2$ creates toxic hydrogen peroxide).
• Stoichiometric Coefficients: Integers placed before formulas indicating relative molecular quantities (e.g., $2H_2O$). Balancing is achieved exclusively by adjusting coefficients.

Step-by-Step Balancing Walkthrough: Propane Combustion

Balance the combustion of propane ($C_3H_8$): $C_3H_8(g) + O_2(g) \rightarrow CO_2(g) + H_2O(g)$

1. Inventory atoms: Reactants: 3 C, 8 H, 2 O. Products: 1 C, 2 H, 3 O (2 in $CO_2$ + 1 in $H_2O$).
2. Balance carbon: Place coefficient 3 before $CO_2$: $C_3H_8 + O_2 \rightarrow 3CO_2 + H_2O \quad (\text{Products: 3 C, 2 H, 7 O})$
3. Balance hydrogen: Place coefficient 4 before $H_2O$ ($4 \times 2 = 8$): $C_3H_8 + O_2 \rightarrow 3CO_2 + 4H_2O \quad (\text{Products: 3 C, 8 H, 10 O})$
4. Balance oxygen: Products contain 10 oxygen atoms ($(3 \times 2) + 4 = 10$). Divide 10 by 2 to place coefficient 5 before $O_2$: $C_3H_8 + 5O_2 \rightarrow 3CO_2 + 4H_2O$
5. Verify atom conservation: Both sides contain 3 C, 8 H, and 10 O. Mass verification: 44.1 g $C_3H_8$ + 160.0 g $O_2$ = 204.1 g reactants; 132.0 g $CO_2$ + 72.1 g $H_2O$ = 204.1 g products. Mass is conserved.