TEAS 7 · Science · 6 min read
States of Matter, Reactions, Solutions and pH
- Differentiate between the physical states of matter, particle behaviors, and the thermal properties of phase transitions.
- Classify major types of chemical reactions, identify oxidation-reduction processes, and explain factors that influence reaction rates and chemical equilibrium.
- Characterize solutions by saturation level, compare acid-base definitions and properties, and interpret the logarithmic pH scale.
States of Matter and Phase Transitions
Matter primarily exists in three distinct physical states: solids, liquids, and gases. Solids possess a definite shape and a definite volume because their particles are tightly packed together and vibrate in fixed positions. Liquids have a definite volume but take the shape of their container, as their particles are closely packed yet can slide past one another. Gases have neither a definite shape nor a definite volume; their constituent particles are separated by large distances and move rapidly and randomly. Across all three states, the average kinetic energy of the particles depends on temperature, rather than solely on whether the substance is a solid, a liquid, or a gas.
Absorbing or releasing thermal energy can cause a substance to change state, but it can also change the substance’s temperature without causing a phase change. Melting is the endothermic phase change that converts a solid into a liquid, whereas freezing is the exothermic phase change that converts a liquid into a solid. Vaporization is the endothermic phase change from a liquid to a gas, while condensation is the exothermic phase change from a gas to a liquid. Transitions can also occur directly between the solid and gas phases without passing through a liquid state: sublimation is the endothermic phase transition directly from a solid to a gas, whereas deposition is the exothermic phase transition directly from a gas to a solid.
During an equilibrium phase change, such as melting or boiling at constant pressure, a pure substance’s temperature remains constant while latent heat is absorbed or released. A phase diagram shows which phases are stable at different temperatures and pressures; a heating curve shows the constant-temperature intervals during which latent heat is absorbed or released. On this diagram, the triple point represents the exact temperature and pressure at which solid, liquid, and gas coexist simultaneously in equilibrium. The critical point marks the endpoint of the liquid, gas coexistence curve; above this critical temperature and pressure, a substance can exist as a supercritical fluid without distinct liquid and gas phases.
| Phase Transition | Initial State | Final State | Thermal Nature |
|---|---|---|---|
| Melting | Solid | Liquid | Endothermic |
| Freezing | Liquid | Solid | Exothermic |
| Vaporization | Liquid | Gas | Endothermic |
| Condensation | Gas | Liquid | Exothermic |
| Sublimation | Solid | Gas | Endothermic |
| Deposition | Gas | Solid | Exothermic |
Common mistakes
- Assuming temperature rises during equilibrium melting or boiling; at constant pressure, a pure substance maintains a constant temperature during these transitions while absorbing or releasing latent heat.
- Confusing oxidation with reduction; oxidation is strictly the loss of electrons, while reduction is the gain of electrons.
- Treating the pH scale as linear rather than logarithmic, thereby forgetting that each one-unit decrease in pH represents a tenfold increase in hydrogen ion activity or concentration.
- Believing catalysts are used up during a reaction, when in reality catalysts are neither consumed nor permanently altered by the reactions they facilitate.
Key points
- Solids have definite shape and volume; liquids have definite volume but variable shape; gases have neither definite shape nor definite volume.
- Phase changes between states are endothermic (melting, vaporization, sublimation) or exothermic (freezing, condensation, deposition); a pure substance’s temperature remains constant during an equilibrium phase change at constant pressure.
- Mass is conserved in chemical reactions, requiring balanced equations where atoms on reactant and product sides are identical.
- Reaction rates increase with higher temperature, higher concentration, greater solid surface area, higher gaseous pressure, or the addition of a catalyst that lowers activation energy.
- Solutions consist of a solute dissolved in a solvent such as water, and are classified as unsaturated, saturated, or supersaturated based on dissolved solute capacity.
- Acids donate protons or produce hydronium ions, while bases accept protons or produce hydroxide ions; at 25 °C, pH values below 7 are acidic, 7 is neutral, and above 7 are basic.
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