6.1 Physical Properties of and Changes in Matter
Key Takeaways
- Density (d = m/v), boiling point, melting point, solubility, conductivity, luster, malleability, and ductility are physical properties because they can be measured without changing the substance's identity.
- Solids, liquids, and gases differ in particle arrangement, spacing, and motion; plasma is a fourth state where gas particles are ionized and respond to electric fields.
- A material's molecular or atomic structure explains its bulk properties — metals conduct because delocalized electrons move freely, crystals are rigid because of ordered lattice bonding, and polymers are flexible because long chains can slide.
- The periodic table organizes elements so that position predicts properties: metals on the left and bottom, nonmetals on the upper right, metalloids along the stair-step boundary.
- Physical changes (melting, freezing, dissolving) alter form or state without producing new substances; chemical changes (burning, rusting) do.
Physical Properties of Substances
A physical property is any characteristic that can be observed or measured without changing the substance's identity. The TExES competency lists the core set a 4-8 teacher must own.
| Property | Definition | Example |
|---|---|---|
| Density | Mass per unit volume, d = m/v | Gold ≈ 19.3 g/cm³; water = 1.0 g/cm³ |
| Boiling point | Temperature at which liquid vapor pressure equals atmospheric pressure | Water = 100 °C at 1 atm |
| Melting point | Temperature at which solid and liquid coexist in equilibrium | Ice = 0 °C at 1 atm |
| Solubility | Maximum amount of solute that dissolves in a given solvent at a temperature | 36 g NaCl per 100 g water at 20 °C |
| Thermal conductivity | Rate of heat flow through a material | Silver > copper > aluminum |
| Electrical conductivity | Ability to carry electric current | Copper high; rubber very low |
| Luster | How a surface reflects light | Metallic luster of polished silver |
| Malleability | Ability to be hammered into thin sheets | Aluminum foil; gold leaf |
| Ductility | Ability to be drawn into a wire | Copper wire |
Worked Example: Density
A graduated cylinder holds 30.0 mL of water. After a metal cube is submerged, the water level reads 37.5 mL. The cube's mass is 58.5 g. What is the metal's density and identity?
- Volume by displacement: V = 37.5 − 30.0 = 7.5 mL = 7.5 cm³.
- Density: d = m/V = 58.5 / 7.5 = 7.8 g/cm³.
That value matches iron/steel (≈7.8 g/cm³), so the cube is likely iron.
States of Matter and Molecular Structure
| State | Particle arrangement | Particle motion | Shape / volume |
|---|---|---|---|
| Solid | Packed, fixed positions in a lattice | Vibrate about fixed points | Definite shape and volume |
| Liquid | Close but can slide past one another | Flow; take container shape | Indefinite shape, definite volume |
| Gas | Far apart, mostly empty space | Rapid, random, straight-line motion | Indefinite shape and volume |
| Plasma | Ionized gas of free electrons and positive ions | High-energy, responds to electric and magnetic fields | Indefinite shape and volume; conducts electricity |
Adding energy (heat) typically drives solid → liquid → gas → plasma; removing energy reverses the chain. Phase changes (melting, freezing, vaporization, condensation, sublimation, deposition) are physical changes — the substance's molecules stay the same; only their arrangement and motion change.
A student finds that 40.5 g of a metal cube raises water in a cylinder from 25.0 mL to 30.0 mL. What is the metal's density?
Molecular Structure and Macroscopic Properties
Metals (e.g., copper, iron, aluminum) form a lattice of positive ions bathed in a sea of delocalized electrons. Those free electrons explain three signature metal properties: electrical conductivity, thermal conductivity, and metallic luster. The lattice can also slide under stress, giving metals malleability and ductility.
Crystals (e.g., sodium chloride, quartz) are built from repeating unit cells held by ionic or covalent bonds. Their ordered structure produces sharp melting points, cleavage planes, and often a nonconductive solid that conducts when molten or dissolved (for ionic crystals).
Polymers (e.g., polyethylene, rubber, proteins) are long chains of repeating monomer units. The flexibility of the chains and the strength of cross-links between them determine whether a polymer is a soft plastic, an elastic rubber, or a stiff fiber. Heating a thermoplastic lets chains slide past one another, so it can be reshaped — a physical change students observe when they recycle a soda-bottle preform.
Periodic Table Regions and Property Trends
The periodic table groups elements by atomic number so that position predicts properties.
| Region | Where | Properties | Examples |
|---|---|---|---|
| Metals | Left and center, below the stair-step | Lustrous, malleable, ductile, good conductors, tend to lose electrons | Fe, Cu, Al, Na |
| Nonmetals | Upper right | Poor conductors, brittle if solid, tend to gain electrons | O, Cl, N, S |
| Metalloids | Along the stair-step (B, Si, Ge, As, Sb, Te) | Mixed; semiconductors | Si (computer chips), B |
| Noble gases | Far right (Group 18) | Inert, full valence shell | He, Ne, Ar |
Groups (families) are vertical columns with the same number of valence electrons and similar chemistry — Group 1 alkali metals react violently with water; Group 17 halogens are reactive nonmetals; Group 18 noble gases are nearly inert. Periods are horizontal rows; moving left to right across a period, metallic character decreases and electronegativity generally increases. Position, therefore, is a reliable predictor of a substance's physical behavior.
Physical vs Chemical Changes
A physical change alters form, state, or size without producing a new substance. Examples: melting ice, freezing water, boiling, dissolving salt, tearing paper, magnetizing iron.
A chemical change produces one or more new substances with different properties. Evidence includes a color change, gas production (bubbles or odor), light or heat release, or formation of a precipitate. Examples: burning wood, rusting iron, cooking an egg, photosynthesis.
Dissolving salt in water is a physical change because the Na⁺ and Cl⁻ ions remain Na⁺ and Cl⁺ — recover the salt by evaporation. Burning the same sodium in chlorine gas to make sodium chloride is a chemical change because a new compound forms.
Cross-Disciplinary Applications
- Life science: Cell membranes rely on the selective solubility of phospholipids; density gradients separate blood components in a centrifuge.
- Earth/space science: Differences in density drive plate tectonics (denser oceanic crust subducts under continental crust) and determine whether an asteroid becomes a meteorite. Phase changes of water drive the water cycle and weather.
- Texas context: The Balcones Escarpment and Texas aquifers illustrate how rock porosity (a physical property) controls water storage; the Gulf Coast salt domes form because rock salt is less dense than surrounding sediment and rises buoyantly — a density-driven process students can model with colored liquids of different densities.
Which of the following is a chemical change rather than a physical change?
Why do metals such as copper conduct electricity so well while nonmetals like sulfur do not?