AP Chemistry 3.2: Properties of Solids, Liquids, and Gases

AP Chemistry commonly divides solids into ionic solids, metallic solids, molecular solids, and covalent-network solids . Khan Academy's current Unit 3 has lessons for all four categories.

• The important skill is not merely naming the solid. You should be able to connect the particles and forces inside the solid to its observable properties .

Ionic solids consist of positive and negative ions arranged in a repeating crystal lattice.

• Examples include NaCl, MgO, CaF₂, and many other salts.

• Ionic solids are held together by strong electrostatic attractions between oppositely charged ions .

• Stronger ionic attractions occur when the ion charges are larger and/or the ions can get closer together.

• This follows Coulombic reasoning: larger charge magnitude and smaller separation lead to stronger attraction.

• Ionic solids usually have relatively high melting points because significant energy is required to separate ions from the lattice.

• Ionic solids are often hard because the ions are strongly held in fixed positions.

• They are also commonly brittle .

• When stress shifts one layer of an ionic crystal, ions with the same charge can become aligned near one another.

• Like charges repel strongly, causing the lattice to split or crack.

• Solid ionic compounds generally do not conduct electricity well because their ions are locked into fixed lattice positions.

• When an ionic solid melts, its ions become free to move, so the molten substance can conduct electricity.

• If an ionic compound dissolves in water and forms mobile ions, the resulting aqueous solution can also conduct electricity.

Metallic solids consist of metal atoms/positive metal cores interacting with delocalized valence electrons.

• Metallic bonding is often described with the simplified model of positive metal cores surrounded by a sea of mobile electrons .

• Because the electrons can move through the structure, metals generally conduct electricity well.

• Mobile electrons also contribute to good thermal conductivity .

• Metals are generally malleable , meaning they can be hammered or shaped.

• They are also commonly ductile , meaning they can be drawn into wires.

• Unlike an ionic crystal, shifting metal atoms does not necessarily line up rigid positive and negative ions. Delocalized electrons can continue holding the metal structure together as layers shift.

Molecular solids consist of individual molecules held together by intermolecular forces.

• Examples can include ice, solid CO₂, and I₂.

• The bonds inside the molecules may be strong covalent bonds, but melting a molecular solid usually requires overcoming intermolecular attractions , not breaking the molecules apart.

• Because IMFs are often weaker than ionic or network covalent bonding, molecular solids commonly have lower melting points than ionic or covalent-network solids.

• Molecular solids generally conduct electricity poorly because they usually lack mobile charged particles.

• Their melting points can vary substantially depending on molecular size and intermolecular-force strength.

• A molecular solid capable of strong hydrogen bonding may have a noticeably higher melting point than a comparable nonpolar molecular solid.

Covalent-network solids consist of atoms connected by an extended network of covalent bonds.

• They are not made of separate molecules in the ordinary sense. Instead, covalent bonding extends throughout a large part or all of the structure.

• Examples include diamond and silicon dioxide, SiO₂ .

• Because melting or significantly deforming these solids requires disrupting strong covalent bonds, they generally have very high melting temperatures and can be extremely hard.

• Most network covalent solids are poor electrical conductors because their electrons are localized in bonds.

Diamond is an important example. Each carbon atom forms four strong covalent bonds in a three-dimensional network.

• This rigid structure explains diamond's extreme hardness.

• Diamond does not conduct electricity well because its valence electrons are localized in covalent bonds.

Graphite is also made entirely of carbon but has completely different properties because it has a different structure.

• In graphite, carbon atoms form extended two-dimensional layers .

• Covalent bonds within each layer are strong, but attractions between the layers are much weaker.

• The layers can slide over one another, making graphite much softer than diamond.

• Graphite also contains delocalized electrons that can move along the layers, making graphite electrically conductive.

• Diamond vs. graphite is a perfect example of the AP Chemistry principle: structure determines properties .

When given an unknown solid, use its properties to identify the likely type.

• A solid that conducts well while remaining solid and is malleable is probably metallic .

• A solid that does not conduct while solid but conducts when molten is likely ionic .

• A low-melting, nonconducting solid may be molecular .

• An extremely hard, very high-melting, usually nonconducting material may be a network covalent solid .

The three common states of matter are solid, liquid, and gas .

• State depends largely on the balance between particle kinetic energy and attractive forces between particles .

In a solid , particles remain very close together and occupy relatively fixed positions.

• Solid particles are not motionless. They vibrate around their equilibrium positions.

• Solids generally have a definite shape and definite volume .

In a liquid , particles are still close together, but they have enough freedom to move past neighboring particles.

• Liquids therefore have a definite volume but no definite shape . They take the shape of the portion of the container they occupy.

In a gas , particles are separated by much larger distances and move freely throughout the container.

• Gases have no definite shape and no definite volume .

• They expand to fill the available container.

General particle spacing follows approximately solid < liquid << gas .

• Gas particles are extremely far apart relative to their own particle sizes.

• This helps explain why gases are highly compressible .

• Solids and liquids contain particles already packed relatively closely, so they are much less compressible.

Particle diagrams are important in AP Chemistry.

• A solid should normally be shown with particles close together in an organized or fixed arrangement.

• A liquid should show particles close together but disordered and able to move around one another.

• A gas should show particles far apart and distributed throughout the entire container.

• Khan Academy's current lesson explicitly includes representing solids, liquids, and gases using particulate models .

Phase changes involve changes in particle arrangement and intermolecular attractions.

• Melting: solid → liquid.

• Freezing: liquid → solid.

• Vaporization: liquid → gas.

• Condensation: gas → liquid.

• Sublimation: solid → gas.

• Deposition: gas → solid.

Moving particles farther apart generally requires energy to overcome attractions.

• Therefore melting, vaporization, and sublimation are endothermic processes.

Bringing particles into more strongly attracted arrangements releases energy.

• Therefore freezing, condensation, and deposition are exothermic .

During a phase change, energy can go primarily into changing the potential energy associated with intermolecular attractions rather than immediately raising average kinetic energy.

• This is why temperature can remain approximately constant during a phase transition even while heat is being transferred.

A crystalline solid has long-range, repeating particle order.

• Many ionic solids and metals are crystalline.

An amorphous solid lacks long-range repeating order.

• Khan Academy also includes crystalline and amorphous polymers in this topic.

Polymers are long-chain molecules built from repeating units.

• Polymer chains may form ordered crystalline regions or disordered amorphous regions.

• A real polymer can contain both kinds of regions.

• Greater chain alignment and stronger attractions can alter properties such as rigidity, density, and melting/softening behavior.

For particle-model questions, focus on spacing, arrangement, movement, and attractions , not just memorized drawings.