State symbols and phase changes

State symbols and phase changes


In this lesson, we will learn:
  • The difference between chemical and physical changes as it affects chemical reactions.
  • The different phases of matter, the state symbols and the different terms for them.
  • The terms used to describe phase changes.

  • In a chemical reaction, the bonds between atoms and molecules get re-arranged, breaking some and creating other new ones. Chemical substances are defined by the bonds between the atoms they’re made of, so changing the bonding arrangement changes the substance and its properties.
    • An example would be carbon dioxide, where the bonding arrangement is a carbon atom making a double bond to two different oxygen atoms (hence the formula CO2). If you change this bonding at all by breaking one of the C=O bonds, you will not have carbon dioxide anymore; it is now a different substance.

    But it’s important to know that chemical changes have nothing to do with the phase of the substance – whether it is solid, liquid, gas or aqueous. The phase can affect how reactive a substance is, but changing phase (a physical change) is not the same as changing the substance (a chemical change).

  • A fully detailed chemical equation will show the state (or phase) of matter that the atoms or molecules are in.
  • These states are:
    • Solid, given the symbol (s)
    • Liquid, given the symbol (l)
    • Gas, given the symbol (g)
    • Aqueous, meaning dissolved in water, and given the symbol (aq)
    Remember that water can’t be ‘dissolved in water’ so as a liquid it is always H2O (l), never H2O (aq). ‘Aqueous water’ is just water!

    Phase is a bulk property, which means it describes how the atoms/molecules behave as a large group, not as individual particles.
    • For example, solid CO2 is when CO2 molecules are densely packed with little movement of particles, and in gaseous CO2 the particles are energetic and are very spread out. Both are CO2, both have exactly the same bonding arrangement within the CO2 molecule.

  • The phase is important for chemists when planning an experiment. Knowing the phase of the product helps plan how you can collect it after the reaction. For example, an aqueous product would need to be evaporated, and a solid product would need to be filtered to help isolate it from the rest of the reaction mixture.

  • There are different ‘forms’ of some chemical phases and descriptive terms used:
    • Crystals, powder and precipitate are all solids.
    • Vapour is particles of a substance becoming a gas.
    • A solution of a substance is describing the substance being dissolved in a solvent. For example, “a solution of NaOH” means the NaOH is dissolved, probably in water. In this case, it would be shown in a reaction as NaOH (aq).

  • There are specific terms used for phase changes. Although these are not chemical reactions, we will look at these terms here:
    • Freezing is the change from liquid to solid; melting is from solid to liquid. The temperature this happens at is called the melting or freezing point. They are the same temperature, their use depends on which direction the change is going in.
    • Boiling is the phase change from liquid to gas; condensation is gas to liquid.
    • Sublimation is the change from solid to gas. This is quite rare in nature but when pressure is changed many substances can sublime. Deposition is the change from gas to solid.
    • Any phase becoming a solution is just called dissolving. All three phases can become a solution: solids and liquids can be mixed in and gases can be bubbled into the solvent.
  • Introduction
    Building on chemical equations
    Chemical and physical changes.

    Chemical phases and state symbols.

    Other key phase/state language.

  • 1.
    Recall the different states of matter from descriptions of chemicals.
    Write "solid", "liquid", "gas", or "aqueous", next to each term below to show which state it is describing.






  • 2.
    Write full chemical equations using information from descriptions of laboratory experiments.
    Read the following experiment notes and write a balanced chemical equation, with state symbols, to describe what is happening.
    Strips of magnesium were weighed out and placed in a beaker. Shortly afterwards, a solution of hydrochloric acid was added to the beaker to produce magnesium chloride (MgCl2_2) and gaseous hydrogen.

    In a reaction vessel, chlorine gas reacts with chunks of sodium metal to form sodium chloride in a reaction releasing a lot of heat energy.

    When hot steam is passed over iron filings, gaseous hydrogen and solid iron oxide is produced.

    Aqueous hydrochloric acid and sodium carbonate (Na2_2CO3_3) powder react to form sodium chloride in solution, water and bubbles of carbon dioxide.