Stoichiometric Relationships - Introduction to the particulate nature of matter and chemical change

Matter is defined as anything that has mass and occupies space. It is composed of particles (atoms, molecules, or ions) whose arrangement and movement determine the state of matter: solid ($s$), liquid ($l$), and gas ($g$).

A $pure\ substance$ has a constant chemical composition and distinct chemical properties. It can be an $element$ (e.g., $O_2$, $Fe$) or a $compound$ (e.g., $H_2O$, $NaCl$).

A $mixture$ contains two or more substances that are not chemically combined. $Homogeneous\ mixtures$ (solutions) have a uniform composition (e.g., $NaCl(aq)$), while $heterogeneous\ mixtures$ have non-uniform compositions (e.g., a mixture of $S(s)$ and $Fe(s)$).

Phase changes are physical processes: $melting$ ($s \rightarrow l$), $freezing$ ($l \rightarrow s$), $vaporization$ ($l \rightarrow g$), $condensation$ ($g \rightarrow l$), $sublimation$ ($s \rightarrow g$), and $deposition$ ($g \rightarrow s$).

Chemical changes involve the reorganization of atoms to form new substances. These are represented by chemical equations where the $Law\ of\ Conservation\ of\ Mass$ must be obeyed, meaning the number of atoms of each element must be equal on both sides of the equation.

The $Kinetic\ Molecular\ Theory$ postulates that particles in a gas are in constant random motion, have negligible volume compared to the container, and undergo perfectly elastic collisions.