0.2 Tg physical science - chapter contexts  (Page 4/10)

Chemical change

Physical and chemical change

This section starts by distinguishing between physical and chemical changes of matter. Matter does not change during a physical change, it is the arrangement of molecules that change. Matter changes during chemical changes through decomposition and syntheses reactions. Physical and chemical changes are compared with respect to the arrangement of particles, conservation of mass, energy changes and reversibility. The role of intermolecular forces during phase changes (a physical change) is highlighted. Understanding of concepts is enhanced by examples which include diagrams, experiments and investigations.

Representing chemical change

As a Physical Sciences educator you will welcome this section as it will bridge the gap learners might have in conceptual understanding and skills to represent chemical change. The content revised includes: common chemical symbols, writing chemical formulae and balancing chemical equations by applying the law of conservation of mass. The four labels used to represent the state (phase) of compounds in the chemical equation are:

• (g) for gaseous compounds
• (l) for liquids
• (s) for solid compounds and
• (aq) for an aqueous (water) solution

Learners will develop the skills to balance chemical equations when they study and apply the steps discussed in the text. Learners need to do the proposed investigation and work through the examples and exercises to assess understanding and consolidate learning.

Reactions in aqueous solutions

Many reactions in chemistry and all reactions in living systems take place in water (or aqueous solutions). In almost all these reactions ions are present. We explore:

• ions in aqueous solutions;
• electrical conductivity; and
• the three main types of reactions that occur in aqueous solutions, namely precipitation, acid-base and redox reactions.

Ions in aqueous solutions

Learners need to understand why water is a polar molecule, to apply their knowledge in further discussions. It is this unique property that allows ionic compounds to dissolve in water. In plants and animals water is the carrier of these dissolved substances making life possible. The process of dissociation is thoroughly explained using words, a definition, image and an equation. The equation for the dissolution of sodium chloride is:

$\mathrm{NaCl}(s)\to {\mathrm{Na}}^{+}(\mathrm{aq})+{\mathrm{Cl}}^{-}(\mathrm{aq})$

Electrolytes, ionisation and conductivity

Concepts are explored using: definitions, equations and experiments.

• Conductivity is a measure of the ability of water to conduct an electric current. The more ions in the solution, the higher its conductivity.
• An electrolyte is a material that increases the conductivity of water when dissolved in it. Electrolytes are divided into strong and weak electrolytes.
• A non-electrolyte is a material that does not increase the conductivity of water when dissolved in it. The substance that goes into a solution becomes surrounded by water molecules separate from each other, but no chemical bonds are broken. This is a physical change. In the oxygen the reaction is reversible because oxygen is only partially soluble in water and comes out of solution very easily.