Ions in Aqueous Solution (Grade 10 NSC Matric Physical Sciences): Revision Notes

Ions in Aqueous Solution

Introduction to aqueous reactions

Many chemical reactions and all biological processes occur in water. We describe these as aqueous reactions because they take place in aqueous solutions. Water possesses unique properties that make it essential for life on Earth. This is why reactions in aqueous solutions happen so frequently in nature.

Almost all reactions in aqueous solutions involve ions. Understanding how ions behave in water is crucial for studying three main types of aqueous reactions:

Before examining these reaction types, we must first understand how ions exist and behave in aqueous solutions.

Water as a solvent

Water is rarely found in its pure form in nature. Due to water's unique molecular structure, substances dissolve easily in it. This property is vital for life - without water's ability to dissolve substances, life on Earth would be impossible.

In natural water systems like rivers and oceans, dissolved oxygen enables fish to breathe. In plants, dissolved nutrients can be absorbed and transported. In the human body, water carries dissolved substances from one part of the body to another.

Water as a polar molecule

Water is a polar molecule. When we examine water's structure using Lewis diagrams, we can see why this is the case.

The water molecule contains two electron pairs that do not participate in bonding. This region of the molecule has higher electron density than the side where hydrogen atoms are bonded. The oxygen side becomes more negative than the hydrogen side.

We describe the oxygen side as delta negative (δ-) and the hydrogen side as delta positive (δ+). This means one part of the molecule carries a slightly positive charge (positive pole) while another part carries a slightly negative charge (negative pole). A molecule with two poles is called a dipole.

Dissolution of ionic compounds in water

The polar nature of water enables ionic compounds to dissolve in it. Consider sodium chloride (NaCl) as an example. The positive sodium ions (Na⁺) are attracted to the negative pole of water molecules, while the negative chloride ions (Cl⁻) are attracted to the positive pole of water molecules.

When sodium chloride dissolves in water, polar water molecules work their way between individual ions in the crystal lattice. The water molecules surround both the negative chloride ions and positive sodium ions, then pull them away into solution. This process is called dissociation.

Note that the positive side of water molecules attracts negative chlorine ions, while the negative side of water molecules attracts positive sodium ions. When a substance dissociates or dissolves, we say that dissolution has occurred. Dissolving represents a physical change that can be reversed by removing (evaporating) the water.

Key definitions

Chemical equations for dissolution

The dissolution of sodium chloride can be represented by this equation:

$\text{NaCl (s)} \rightarrow \text{Na}^+ \text{(aq)} + \text{Cl}^- \text{(aq)}$

The dissolution of potassium sulphate into potassium and sulphate ions provides another example:

$\text{K}_2\text{SO}_4 \text{(s)} \rightarrow 2\text{K}^+ \text{(aq)} + \text{SO}_4^{2-} \text{(aq)}$

Remember that molecular substances (covalent compounds) may also dissolve, but most do not form ions. Glucose serves as an example:

$\text{C}_6\text{H}_{12}\text{O}_6 \text{(s)} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 \text{(aq)}$

Molecular substances that ionise

Some molecular substances will form ions when they dissolve. Hydrogen chloride provides an example of a molecular substance that ionises to form hydrogen and chloride ions:

$\text{HCl (g)} + \text{H}_2\text{O (ℓ)} \rightarrow \text{H}_3\text{O}^+ \text{(aq)} + \text{Cl}^- \text{(aq)}$

Remember!