Changes in Matter

Matter is defined as anything that possesses mass and occupies volume in physical space.
Changes in matter are fundamental processes categorized by whether the underlying chemical identity and composition of a substance are altered.

A physical change is a transformation that alters the physical state, shape, appearance, or size of a substance without changing its chemical composition or molecular structure.
During a physical change, chemical bonds within the molecules remain intact; only intermolecular forces or macroscopic arrangements are affected.
Physical changes are frequently reversible, though certain mechanical alterations are irreversible.

No new chemical substances or compounds are synthesized.
Reversibility is common (e.g., solid water melting to liquid water and re-freezing to solid water).
Changes typically involve physical parameters such as volume, surface area, phase, density, or crystalline structure.

Phase transitions between solid, liquid, gas, and plasma states.
Dissolving a solute into a solvent, such as dissolving sodium chloride ( $NaCl$ ) in liquid water ( $H_2O$ ).
Mechanical deformations, including cutting, tearing, bending, shattering, or grinding substances into powder.
Thermal expansion and thermal contraction resulting from temperature fluctuations.

A chemical change, also known as a chemical reaction, occurs when one or more substances (reactants) undergo rearrangements of atomic bonds to form one or more distinct new substances (products) with unique chemical and physical properties.
Chemical changes fundamentally alter the chemical identity and molecular structure of matter.

Irreversible or distinct color change.
Temperature alteration (heat release in exothermic reactions or heat absorption in endothermic reactions) without external heat manipulation.
Gas production, visible through effervescence, bubbling, or new odor formation.
Precipitation, which is the formation of an insoluble solid when two aqueous solutions react.
Emission of light, sound, or electric potential.

Oxidation reactions, such as the rusting of elemental iron ( $Fe$ ) in the presence of atmospheric oxygen ( $O_2$ ):

$4Fe + 3O_2 \rightarrow 2Fe_2O_3$

Combustion reactions, such as the burning of methane ( $CH_4$ ) gas in oxygen ( $O_2$ ):

$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$

Acid-base neutralization reactions yielding liquid water and ionic salts.
Biological metabolic processes, such as cellular respiration and photosynthesis.

Phase changes represent reversible physical transformations driven by variations in temperature, kinetic energy, and pressure.

Melting: Phase transformation from a solid state to a liquid state.
Freezing (Solidification): Phase transformation from a liquid state to a solid state.
Vaporization: Phase transformation from a liquid state to a gaseous state, encompassing both boiling and evaporation.
Condensation: Phase transformation from a gaseous state to a liquid state.
Sublimation: Direct phase transition from a solid state to a gaseous state without entering an intermediate liquid state (e.g., solid carbon dioxide or dry ice, $CO_2$ ).
Deposition: Direct phase transition from a gaseous state to a solid state without entering an intermediate liquid state.
Ionization: Transition from a gaseous state to a plasma state upon addition of high energy.
Deionization (Recombination): Transition from a plasma state to a gaseous state upon removal of energy.

In any closed system subjected to physical or chemical changes, the total mass of the system remains constant.
Matter and mass are neither created nor destroyed during physical or chemical processes:

$\text{Mass}_{\text{reactants}} = \text{Mass}_{\text{products}}$

In physical changes, total mass is conserved as matter reshapes or shifts phases.
In chemical reactions, the total quantity and mass of each element on the reactant side must equal the total quantity and mass of that same element on the product side.

Elements: Pure substances composed of only one type of atom that cannot be decomposed into simpler substances by chemical processes (e.g., elemental iron, $Fe$ ; oxygen gas, $O_2$ ).
Compounds: Pure substances composed of two or more distinct elements chemically combined in fixed proportions (e.g., water, $H_2O$ ; sodium chloride, $NaCl$ ).

Mixtures: Physical combinations of two or more substances where each retains its chemical identity; physically separable.
- Heterogeneous Mixtures: Non-uniform composition throughout (e.g., sand mixed with water).
- Homogeneous Mixtures (Solutions): Uniform composition throughout (e.g., $NaCl$ dissolved in $H_2O$ ).

Alters physical state, shape, or appearance without changing chemical composition; bonds inside molecules remain intact.
Examples: Phase transitions, dissolving solutes, mechanical cutting, tearing, or grinding.

Reactants undergo atomic bond rearrangements to form distinct new substances (products).
Indicators: Irreversible color change, temperature change, gas production, precipitation, or light emission.
Key Examples:
- Oxidation: $4Fe + 3O2 \rightarrow 2Fe2O_3$
- Combustion: $CH4 + 2O2 \rightarrow CO2 + 2H2O$

Temperature and pressure variations drive reversible phase shifts:
- Melting ( $\text{solid} \rightarrow \text{liquid}$ ) and Freezing ( $\text{liquid} \rightarrow \text{solid}$ )
- Vaporization ( $\text{liquid} \rightarrow \text{gas}$ ) and Condensation ( $\text{gas} \rightarrow \text{liquid}$ )
- Sublimation ( $\text{solid} \rightarrow \text{gas}$ ) and Deposition ( $\text{gas} \rightarrow \text{solid}$ )
- Ionization ( $\text{gas} \rightarrow \text{plasma}$ ) and Deionization ( $\text{plasma} \rightarrow \text{gas}$ )

Total mass remains constant in physical or chemical processes: $\text{Mass}{\text{reactants}} = \text{Mass}{\text{products}}$