How bonding and structure are related to the properties of substances

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The Three States of Matter

  • The three states of matter are solid, liquid and gas.
  • In a solid, particles are very close together in a regular arrangement and vibrate about fixed positions.
  • In a liquid, particles are close together and randomly arranged, and they move around each other.
  • In a gas, particles are far apart and randomly arranged, and they move quickly in all directions.
  • The simple particle model represents particles as small solid spheres.
  • A substance can usually exist in all three states depending on temperature (and pressure).

Changes of State

  • Melting is a solid changing into a liquid; freezing is the reverse, liquid to solid.
  • Melting and freezing occur at the melting point; boiling and condensing occur at the boiling point.
  • Boiling is a liquid changing into a gas throughout the liquid, forming bubbles.
  • Evaporation is a liquid changing into a gas only at the surface, and it occurs over a range of temperatures below the boiling point.
  • Condensation is a gas changing into a liquid on cooling.
  • Sublimation is a solid changing directly into a gas (e.g. iodine, solid carbon dioxide); the reverse is called deposition or desublimation.
  • Changes of state are physical changes: the particles themselves remain the same, only the forces between them change, and the change is easily reversed.

The state changes

The state changes

Energy and Interconversions

  • The amount of energy needed to change state depends on the strength of the forces between the particles.
  • The stronger the forces between particles, the more energy is needed to overcome them, so the higher the melting point and boiling point.
  • When a substance is heated, particles absorb thermal energy and convert it into kinetic energy, so they vibrate or move more.
  • On a heating or cooling curve, horizontal sections show a change of state where temperature stays constant as energy is absorbed or released.
  • The sloped sections show the substance heating up or cooling down in a single state.
  • The interconversion of states can be shown with a reversible arrow: ⇌.

Predicting Physical State

  • Below the melting point, a substance is a solid.
  • Between the melting point and the boiling point, a substance is a liquid.
  • Above the boiling point, a substance is a gas.
  • For example, a substance with melting point −7 °C and boiling point 59 °C is a liquid at 20 °C and a gas at 100 °C.
  • Individual atoms do not have the bulk properties of the material they make up.

State Symbols

  • State symbols show the physical state of each substance in a chemical equation.
  • (s) = solid, (l) = liquid, (g) = gas, (aq) = aqueous (dissolved in water).
  • State symbols are written after each formula in brackets.
  • Example: CuCO₃(s) + 2HCl(aq) → CuCl₂(aq) + CO₂(g) + H₂O(l).
  • Aqueous means the substance is dissolved in water to form a solution.

Properties of Ionic Compounds

  • Ionic compounds form giant ionic lattices with strong electrostatic forces of attraction in all directions between oppositely charged ions.
  • They have high melting and boiling points because a large amount of energy is needed to overcome the many strong electrostatic forces.
  • The greater the charge on the ions, the stronger the electrostatic forces and the higher the melting point (e.g. MgO has a higher melting point than NaCl).
  • Solid ionic compounds do not conduct electricity because the ions are in fixed positions and cannot move.
  • When molten or dissolved in water, ionic compounds conduct electricity because the ions are free to move and carry charge.

Properties of Small Molecules

  • Substances made of small molecules are usually gases or liquids with relatively low melting and boiling points.
  • They have strong covalent bonds between the atoms within each molecule, but only weak intermolecular forces between molecules.
  • When a simple molecular substance melts or boils, it is the intermolecular forces that are overcome, not the covalent bonds.
  • The intermolecular forces increase with the size of the molecules, so larger molecules have higher melting and boiling points.
  • Small molecular substances do not conduct electricity because there are no free ions or electrons to carry charge.

Polymers

  • Polymers consist of very large molecules made by linking many small molecules called monomers.
  • The atoms in polymer molecules are linked by strong covalent bonds.
  • The intermolecular forces between polymer chains are relatively strong compared with those between simple molecules.
  • Because of these stronger intermolecular forces, polymers are usually solids at room temperature.
  • Examples include polythene and poly(chloroethene), commonly known as PVC.

Monomers joining to form a polymer

Monomers joining to form a polymer

Giant Covalent Structures

  • Giant covalent structures are solids with very high melting points, made of many non-metal atoms bonded by strong covalent bonds.
  • All the atoms are linked by strong covalent bonds, and these bonds must be overcome to melt or boil the substance.
  • Examples include diamond, graphite and silicon dioxide (silica).
  • Diamond: each carbon atom bonds to four others in a tetrahedron; it is extremely hard, has a very high melting point, and does not conduct electricity.
  • Graphite: each carbon atom bonds to three others in layers of hexagons, leaving one free electron per carbon atom, so graphite conducts electricity and is soft and slippery.
  • Silicon dioxide is made from silicon and oxygen atoms and also has a giant covalent structure.

Properties of Metals and Alloys

  • Metals have giant structures with strong metallic bonding – the attraction between positive metal ions and negative delocalised electrons.
  • Most metals have high melting and boiling points because the metallic bonds are strong and need lots of energy to break.
  • Pure metals are malleable because the atoms are arranged in layers that can slide over each other when a force is applied.
  • An alloy is a mixture of two or more metals, or a metal with a non-metal such as carbon (e.g. steel is iron and carbon).
  • Alloys are harder than pure metals because the different-sized atoms distort the layers, making it more difficult for the layers to slide over each other.
  • Metals are good conductors of electricity because the delocalised electrons are free to move through the structure and carry charge.
  • Metals are good conductors of thermal energy because the delocalised electrons transfer energy through the structure.

In an alloy, atoms of a different size (Element Y) are scattered among the host metal's atoms (Element X), distorting the regular layered arrangement that a pure metal has.

In an alloy, atoms of a different size (Element Y) are scattered among the host metal's atoms (Element X), distorting the regular layered arrangement that a pure metal has.

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  1. 1.Which of the following is the name of the change of state when a liquid changes into a solid?

    Easy
    • Acondensation
    • Bevaporation
    • Cfreezing
    • Dmelting

  2. 2.Substance Y melts at -101 °C and boils at -35 °C. What is the state of substance Y at -110 °C?

    Easy
    • Asolid
    • Bliquid
    • Cgas
    • Daqueous

  3. 3.Substance Z has a melting point of -7 °C and a boiling point of 59 °C. What is the state of Z at 0 °C and at 100 °C?

    Medium
    • Asolid at 0 °C and gas at 100 °C
    • Bgas at 0 °C and liquid at 100 °C
    • Cliquid at 0 °C and solid at 100 °C
    • Dliquid at 0 °C and gas at 100 °C

  4. 4.Substances exist as solids, liquids or gases. What change of state occurs at Y?

    Medium
    • Amelting
    • Bfreezing
    • Cboiling
    • Dcondensation

  5. 5.A balloon is filled with helium gas. What is true about the gas particles inside the balloon?

    Medium
    • AThey have little energy
    • BThey vibrate in fixed positions
    • CThey fall to the bottom of the balloon
    • DThey move in random directions

  6. 6.Which of the following is not a chemical change?

    Medium
    • AIron railings becoming rusty
    • BBurning a piece of wood
    • CNeutralising an acid with alkali
    • DDissolving salt in water

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

    Easy

    True or false?

  8. 8.In a solid, the particles are arranged randomly and move quickly in all directions.

    Easy

    True or false?

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