Pure Substances, Mixtures, Density & Phase Changes

Pure Substance

• A pure substance contains only one kind of matter, whether an element or compound.
• It cannot be separated into simpler pieces by physical methods.

Mixtures

• A compound is a pure substance made of two or more elements.
• In a heterogeneous mixture, components can be easily identified, like sand and water.
• A homogeneous mixture has a uniform composition, like salt and water.

Density

• Density is the mass per unit volume of a substance.

Phase Changes

• Melting point refers to the temperature at which a solid changes to a liquid phase.
• Melting occurs during process in which a solid changes to a liquid due to heat.
• The melting point of a solid is the same as the freezing point of its liquid form for a given substance.
• Factors such as purity and surrounding pressure affect the melting/freezing points.
• Freezing is the process of liquid to solid phase change by heat removal.
• The freezing point is the temperature at which a liquid, releases heat and becomes a solid.
• Boiling point is the temperature at which a liquid's vapor pressure equals the external pressure.
• Typically, boiling points are measured at sea level.
• Boiling points decrease at higher altitudes due to lower atmospheric pressure; water boils at 71 °C on Mount Everest.
• Condensation is when a gas or vapor turns into a liquid, either by cooling or increasing pressure.
• Evaporation occurs when a liquid changes into a vapor at a temperature below the boiling point, specifically at the surface
• During evaporation, molecules with high kinetic energy escape, lowering the average kinetic energy and temperature of the liquid.
• Sublimation is the process where a solid changes directly into a gas, skipping the liquid phase.
• Deposition is where a gas transforms into a solid, with sublimation being its reverse.

Separation Technique

• Chromatography separates and identifies substances.

Brownian Motion

• Brownian motion describes the random movement of microscopic particles in a liquid or gas.
• This movement is caused by collisions with molecules of the liquid or gas.
• It is named after Scottish botanist Robert Brown (1773-1858).

Diffusion

• Diffusion refers to the movement of atoms or molecules from an area of higher concentration to one of lower concentration.

Electrical Properties

• An electrical conductor allows the flow of charge.
• An electrical insulator does not allow the flow of charge.

Thermal Properties

• A thermal conductor allows heat to pass through easily.
• A thermal insulator does not allow heat to pass through easily.

Elements

• An element is a pure chemical substance with one type of atom.
• Metalloids/semi-metals have properties between metals and non-metals.

Atomic Structure

• The atomic number (Z) indicates the number of protons in the nucleus of an atom.
• Number of protons in the nucleus of an atom
• Mass number (A) is the total of protons and neutrons in the nucleus.
• Protons are positive particles in the atomic nucleus.
• Neutrons are neutral particles in the atomic nucleus.
• Nucleons are protons and neutrons inside the nucleus.
• Electrons are negative particles occupying space around the nucleus.
• Isotopes are atoms of the same element, with the same atomic number, but a different number of neutrons, leading to different mass numbers.

Electrons and Energy

• An electron's excited state is when it gains energy and moves to a higher energy level.
• The ground state is the lowest energy state of an electron.
• Atomic orbital is the most probable region around the nucleus where electrons are located.
• Valence electrons are the outer electrons in the highest filled energy level of an atom.
• Each energy level is quantised, having specific amounts of energy.

Atomic Properties

• Atomic radius is the distance from the atomic nucleus to the outermost stable electron.
• Electron affinity measures the energy released when an electron attaches to an atom or molecule to form a negative ion.
• Electronegativity is the tendency of an atom to attract bonding electrons in a molecule.

Ions and Ionization

• An ion is a charged particle formed when an atom gains or loses electrons.
• Ionisation energy is the energy required to remove electron(s) from an atom in the gaseous phase.
• The first ionisation energy refers to the energy needed to remove the first electron from a gaseous atom.

Hund's Rule and Pauli's Exclusion Principle

• Hund's rule states that there should be no pairing in p orbitals unless each p orbital has at least one electron.
• Pauli's exclusion principle: a maximum of two electrons can occupy an orbital, provided they have opposite spins.

Relative Atomic Mass

• The relative atomic mass is the mass of an atom on a scale where carbon-12 has a mass of 12.

Periodic Table

• A group is a vertical column in the periodic table, some with specific names.
• A period is a horizontal row in the periodic table.
• Periodicity refers to the repetition of similar properties in elements based on their position in the periodic table, reflecting periodic variation in electron configuration with increasing atomic number.

Wave Properties

• A transverse pulse is a wave where particles move perpendicular to the wave's direction.
• Amplitude is the maximum disturbance of a particle from its rest position due to the pulse.
• Pulse length is the distance over which a single disturbance (pulse) extends.

Wave Interference and Superposition

• Constructive interference occurs when displacements of pulses are in the same direction, adding together for a larger amplitude.
• Destructive interference occurs when displacements are in opposite directions and subtract, potentially cancelling each other out if they have equal magnitude.
• Superposition is when two or more pulses/waves meet which leads to the resulting displacement which is the sum of individual displacements.