Week 2 · 1-2 Chemical Properties of Water: the water molecule, polarity, hydrogen bonds and boiling point, water as a universal solvent (NaCl), ionic equilibrium, hydrolysis of salts, pH and its measurement · 3 lessons
2
Water as a solvent: salt in water and the ions of water
Integrated Science Sec 1 Term 1, Unit 1: pp. 8–15
GoalExplain why water dissolves salt, say what happens to the Na⁺ and Cl⁻ ions, and explain why pure water is neutral.
New words
universal solvent · water, because it dissolves ionic compounds and many polar compoundshydrated ions · ions surrounded by water molecules in a solutionionic equilibrium of water · pure water ionises very slightly into H⁺ and OH⁻, always in equal amounts
Explanation
Water is called the universal solvent because it dissolves ionic compounds and polar compounds. A salt crystal such as sodium chloride is a lattice of positive Na⁺ ions and negative Cl⁻ ions. When it is put in water, the oxygen ends of water molecules (δ−) crowd round the Na⁺ ions and the hydrogen ends (δ+) crowd round the Cl⁻ ions. The lattice breaks down into free hydrated ions, and the water molecules stop the ions from joining again. The ions stay in the solution and do not react with water, so no new substance forms and this is not hydrolysis. Note that a salt crystal has ions, not molecules. Pure water is different: a very small number of its molecules ionise into H⁺ ions, which give acidity, and OH⁻ ions, which give basicity. The two are in equal amounts, so pure water is neutral: it is neutral because H⁺ and OH⁻ balance, not because it has none of them.
NaClin water→Na+ + Cl−
hydrated ions stay separate; no new substance forms
Worked examples
Which of these should dissolve well in water: table salt (ionic), sugar (a polar compound) or cooking oil (not polar)? Salt and sugar dissolve because water molecules can attract their ions or polar ends. Oil has no charged ends, so it does not mix: it floats in drops or a layer. This is an extra fact beyond the book, but it fits the rule.
A learner writes “molecules of NaCl split into Na⁺ and Cl⁻”. What is wrong? Solid NaCl has no molecules; it is a lattice of ions. The ions are already there, and water only pulls them apart. The correct sentence is: the crystal lattice of NaCl breaks into free Na⁺ and Cl⁻ ions in water.
H2O⇄H+ + OH−
tiny amounts, always equal in pure water
In pure water the amount of H⁺ equals the amount of OH⁻. Suppose some acid is added, and the amount of H⁺ rises while OH⁻ falls. Is the water still neutral? No: more H⁺ than OH⁻ means the solution is acidic.
Class activity
Salt, sugar and oil, with a grown-up. Put a spoon of salt, a spoon of sugar and a spoon of cooking oil in three clear cups of cool tap water and stir each one. Describe what you see: which seem to disappear and which stay separate? Do not taste anything. Then set up a bead model of NaCl: make a small square block of alternating red and green beads (Na⁺ and Cl⁻) and pull it apart, putting a ring of blue-bead “water molecules” round each ion.
Practice
1
Which part of a water molecule is attracted to a Na⁺ ion: the oxygen end or the hydrogen end?
The oxygen end, because it is slightly negative.
2
What stops the Na⁺ and Cl⁻ ions from joining again in the solution?
Water molecules surround each ion (they are hydrated).
3
Why is the dissolving of NaCl in water not hydrolysis?
The ions do not react with water, so no acid or base forms.
4
Pure water has H⁺ and OH⁻ ions. Why is it still neutral?
