3.4 Physical & Chemical Properties and Changes

3.4 Physical & Chemical Properties and Changes

GED Exam Core Concept: Distinguishing between physical and chemical attributes of matter is a foundational skill in physical science. Test questions will frequently require you to calculate density, classify changes, recognize indicators of chemical reactions, and categorize matter into pure substances or mixtures.

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Physical Properties vs. Chemical Properties

Every chemical substance possesses a unique set of characteristics that allow us to identify and classify it:

1. Physical Properties

• A physical property is a characteristic of a substance that can be observed or measured without changing the chemical identity or composition of the substance.
• Examples: Color, odor, state of matter, melting point, boiling point, density, solubility in water, electrical conductivity, hardness, and magnetism.

Sub-classification of Physical Properties:

• Intensive Physical Properties: Independent of the size or amount of the sample present. Used to identify unknown pure substances.
  • Examples: Density ($1.0\text{ g/cm}^3$ for water), boiling point ($100^\circ\text{C}$), melting point, refractive index.
• Extensive Physical Properties: Depend directly on the quantity of matter present in the sample.
  • Examples: Mass, volume, total length, total energy content.

2. Chemical Properties

• A chemical property describes the inherent ability (or inability) of a substance to combine with or change into one or more new chemical substances.
• Chemical properties can only be observed when a substance undergoes a chemical reaction that alters its molecular structure.
• Examples: Flammability (ability to burn in oxygen), toxicity, reactivity with acid, acidity/basicity (pH value), oxidation potential (ability to rust or tarnish).

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Deep Dive: Density & Buoyancy ($D = m/V$)

Density ($D$) is a fundamental intensive physical property defined as the mass ($m$) per unit volume ($V$) of a substance:

$\text{Density } (D) = \frac{\text{Mass } (m)}{\text{Volume } (V)}$

• Standard Units: $\text{g/cm}^3$ (for solids), $\text{g/mL}$ (for liquids), or $\text{kg/m}^3$.
• Note that $1\text{ cm}^3$ is exactly equal to $1\text{ mL}$.

Buoyancy & Floating/Sinking Rule:

• The density of pure liquid water at $4^\circ\text{C}$ is $1.00\text{ g/cm}^3$ (or $1.00\text{ g/mL}$).
• If an object's density is less than water ($D < 1.0\text{ g/cm}^3$), it will float in water.
• If an object's density is greater than water ($D > 1.0\text{ g/cm}^3$), it will sink in water.
• Water Displacement Method: To measure the volume of an irregularly shaped solid, submerge it in a graduated cylinder filled with water. The volume of the solid equals the volume of water displaced: $V_{\text{object}} = V_{\text{final}} - V_{\text{initial}}$

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Physical Changes vs. Chemical Changes

1. Physical Changes

• A physical change alters the form, shape, size, or physical state of a substance without breaking or forming chemical bonds.
• The chemical composition and chemical identity remain completely unchanged.
• Examples:
  • Phase changes (ice melting into liquid water, steam condensing).
  • Dissolving a solute in a solvent (dissolving sugar or salt in water).
  • Mechanical alterations (shredding paper, crushing an aluminum can, chopping wood).
• Reversibility: Most physical changes are readily reversible by physical separation techniques (e.g., evaporating water to recover dissolved salt).

2. Chemical Changes (Chemical Reactions)

• A chemical change occurs when one or more substances are transformed into entirely new chemical substances with different chemical formulas and different properties.
• Involves the breaking and making of chemical bonds.
• Examples: Rusting of iron ($4\text{Fe} + 3\text{O}_2 \rightarrow 2\text{Fe}_2\text{O}_3$), burning wood (combustion), digestion of food, baking a cake, souring of milk.

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Five Key Indicators of a Chemical Reaction

On the GED exam, experimental scenario questions often ask whether a chemical change took place. Look for these five primary empirical signs:

1. Unexpected Color Change: A distinct, unpredicted color transformation occurs (e.g., clear solutions mixing to turn bright yellow, or shiny copper turning green rust).
2. Gas Production (Effervescence / Bubbling): Bubbles form when two liquids or a solid and liquid react without external heating.
3. Precipitate Formation: An insoluble solid (called a precipitate) forms and settles out when two clear aqueous solutions are mixed.
4. Temperature / Energy Change: The reaction vessel becomes noticeably hotter (exothermic) or colder (endothermic) without being heated or cooled on a stove/refrigerator.
5. Emission of Light or Odor Change: Light or flames are produced (e.g., combustion or glow sticks), or a new distinct odor is released.

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Classification of Matter: Pure Substances vs. Mixtures

All matter can be classified according to its chemical composition:

                          MATTER
                            |
       +--------------------+--------------------+
       |                                         |
  PURE SUBSTANCES                            MIXTURES
  (Fixed Composition)                  (Variable Composition)
       |
   +---+---+                             +-------+-------+
   |       |                             |               |
ELEMENTS  COMPOUNDS                 HOMOGENEOUS     HETEROGENEOUS
 (Au, O2)  (H2O, NaCl)              (Solutions)     (Suspensions)

1. Pure Substances

• Have a constant, uniform chemical composition and distinct chemical properties throughout.
• Elements: Composed of only one type of atom (e.g., Gold $\text{Au}$, Oxygen gas $\text{O}_2$, Helium $\text{He}$). Cannot be broken down into simpler substances by chemical means.
• Compounds: Composed of two or more different elements chemically bonded in fixed, definite integer ratios (e.g., Pure Water $\text{H}_2\text{O}$, Carbon Dioxide $\text{CO}_2$). Can only be separated into elements through chemical reactions.

2. Mixtures

• Composed of two or more physical substances physically combined in variable proportions. Individual components retain their own original chemical identities.
• Homogeneous Mixtures (Solutions): The mixture has a completely uniform composition throughout; individual components are not visible (e.g., Saltwater, Brass alloy, Clean Air).
• Heterogeneous Mixtures: Composition is non-uniform; distinct individual phases or components are visible (e.g., Salad dressing, Granite rock, Muddy water).

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Physical Separation Methods for Mixtures

Because mixtures are held together by physical forces, they can be separated into pure components using physical properties:

• Filtration: Separates insoluble solids from liquids based on particle size.
• Distillation: Separates liquids based on differences in their boiling points.
• Evaporation: Removes a liquid solvent to leave behind a dissolved solid solute.
• Chromatography: Separates components based on relative solubility and molecular movement across a medium.

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Worked Examples

Worked Example 1: Density & Water Displacement

Problem: A student finds an unknown metallic rock. The mass of the rock is measured as $135.0\text{ g}$. To measure its volume, the student pours $50.0\text{ mL}$ of water into a graduated cylinder and lowers the rock inside. The water level rises to $65.0\text{ mL}$. Calculate the density of the rock and determine if it will float or sink in water.

Solution:

1. Calculate Volume ($V$) by Displacement: $V = V_{\text{final}} - V_{\text{initial}} = 65.0\text{ mL} - 50.0\text{ mL} = \mathbf{15.0\text{ mL}} = 15.0\text{ cm}^3$
2. Calculate Density ($D = m/V$): $D = \frac{135.0\text{ g}}{15.0\text{ cm}^3} = \mathbf{9.00\text{ g/cm}^3}$
3. Determine Buoyancy: Because $9.00\text{ g/cm}^3 > 1.00\text{ g/cm}^3$ (density of water), the rock will sink.