Question

part 2: using chemical formulas as evidence
for each change below:

• decide whether it is physical or chemical
• explain how the chemical formulas helped you decide

1. h₂o(l) → h₂o(g)

type of change: ______

1. zn(s) + hcl(aq) → zncl₂(aq) + h₂(g)

type of change: ______

1. co₂(s) → co₂(g)

type of change: ______

Explanation:

1. Analysis for $\boldsymbol{\ce{H2O(l) -> H2O(g)}}$

Step 1: Identify substance before/after

The reactant is $\ce{H2O(l)}$ (liquid water) and the product is $\ce{H2O(g)}$ (gaseous water).

Step 2: Check for new substances

The chemical formula is the same ($\ce{H2O}$), only the state (phase) changes. Physical changes involve state/phase changes without new substances.

Step 3: Determine change type

Since no new chemical substance is formed (same formula), it is a physical change (specifically, vaporization).

Step 1: Identify substances

Reactants: $\ce{Zn(s)}$ (zinc) and $\ce{HCl(aq)}$ (hydrochloric acid). Products: $\ce{ZnCl2(aq)}$ (zinc chloride) and $\ce{H2(g)}$ (hydrogen gas).

Step 2: Check for new substances

The products ($\ce{ZnCl2}$ and $\ce{H2}$) are different from the reactants ($\ce{Zn}$ and $\ce{HCl}$), so new chemical substances are formed.

Step 3: Determine change type

A change with new substances formed is a chemical change (this is a single - replacement reaction: $\ce{Zn}$ replaces $\ce{H}$ in $\ce{HCl}$).

Step 1: Identify substance before/after

Reactant: $\ce{CO2(s)}$ (solid carbon dioxide, dry ice). Product: $\ce{CO2(g)}$ (gaseous carbon dioxide).

Step 2: Check for new substances

The chemical formula is $\ce{CO2}$ for both, only the state (phase) changes from solid to gas.

Step 3: Determine change type

Since no new chemical substance is created (same formula, just phase change: sublimation), it is a physical change.

Answer:

Physical change (because the chemical formula remains $\ce{H2O}$, only the phase changes from liquid to gas, no new substance is formed).

2. Analysis for $\boldsymbol{\ce{Zn(s) + HCl(aq) -> ZnCl2(aq) + H2(g)}}$