What Chemistry Studies
Chemistry examines matter and its transformations. It investigates how substances change form and composition, providing insight into the physical world.
Defining Matter and Its Properties
Matter is anything that has mass and occupies space. Mass measures the amount of matter and its inertia, quantifying how much material an object contains and its resistance to acceleration. The SI unit for mass is the kilogram (kg).
Density and Temperature Scales
Density is calculated as mass divided by volume, highlighting the relationship between how much matter is present and the space it occupies.
Temperature scales differ in their reference points: the Celsius scale uses 0 °C as the freezing point of water, while the Kelvin scale uses 0 K as absolute zero, the theoretical point where molecular motion ceases. Absolute zero (0 K) equals –273 °C (more precisely –273.15 °C).
Heat and Phase Changes
Heat is the flow of energy from a higher‑temperature object to a lower‑temperature one. Physical changes involve a change in phase or property without altering chemical identity, such as melting, which converts a solid to a liquid. Direct conversion of a solid to a gas is called sublimation, not melting.
Solids vs. Liquids
Solids have a definite shape and volume, with particles vibrating about fixed positions and unable to move past each other. Liquids retain a definite volume but take the shape of their container, allowing particles to flow while maintaining close proximity.
Physical vs. Chemical Changes
A physical change alters the form or state of a substance without creating a new chemical identity. In contrast, a chemical change rearranges atomic bonds, producing new substances. Chemical bonds are the forces of attraction between atoms that hold molecules together.
Reactants and Products
In a chemical reaction, reactants are the starting materials that are consumed, while products are the newly formed substances that result from the reaction.
Diatomic Molecules
A diatomic molecule consists of two identical atoms bonded together, such as O₂, N₂, and S₈, which exist naturally in this paired form.
Scientific Method Basics
The first step of the scientific method is to make an observation, initiating the inquiry process. A hypothesis must be testable to qualify as scientific. A hypothesis is a provisional, testable guess, whereas a theory is a well‑tested explanation supported by extensive evidence.
Particle Motion in Different States
In solids, particles vibrate about fixed positions and cannot move freely. In gases, particles exhibit rapid, random motion, colliding frequently and moving independently.
Gas Laws Overview
Boyle’s Law describes an inverse relationship between volume and pressure: as pressure increases, volume decreases proportionally.
Charles’s Law states that volume is directly proportional to temperature (V ∝ T) when pressure is constant.
The Ideal Gas Law combines these relationships: PV = nRT, where pressure times volume equals the number of moles times the gas constant times temperature.
Avogadro’s Law relates volume to the amount of gas (V ∝ n), while Charles’s Law relates volume to temperature (V ∝ T). According to kinetic molecular theory, the average kinetic energy of gas particles is proportional to temperature, meaning higher temperatures correspond to greater kinetic energy.
Mixtures, Solutions, and Suspensions
A mixture is a combination of substances that retain their individual properties and can be separated physically.
In a solution, components are fully dissolved, forming a homogeneous mixture at the molecular level. In a suspension, particles are dispersed but not dissolved, remaining visible and separable by filtration.
Noble Gases and Alkali Metals
Noble gases (Group 18) have complete valence electron shells, making them chemically inert and low‑reactivity.
Alkali metals belong to Group 1 and are highly reactive, especially with water.
Transition metals generally react less readily with water compared to alkali and alkaline‑earth metals.
Metric Units for Length
Common metric units for length include kilometer (km), meter (m), and centimeter (cm). One kilometer equals approximately 0.621 miles.
