Lessons 12–13 · 2 lessons · S. Masharipov, A. Mutalibov, E. Murodov, H. Islomova. General chemistry, Grade 11, 1st edition. G‘afur G‘ulom Publishing and Printing House, Tashkent, 2018
12
The concept of a solution
Textbook: pp. 62–65
GoalExplain solution, solvent and solute, distinguish a solution from a mixture and from a chemical compound, and state the importance of solutions.
New words
solution · eritmasolvent · erituvchisolute · erigan moddahydrated ion · gidratlangan ion
Explanation
A solution is a homogeneous system formed by the interaction of the particles of a solvent and a solute: its composition and properties are the same in every part. Dissolving is not simple mixing but a physical–chemical process: solvent molecules surround the particles of the solute (hydration in water), so heat may be released or absorbed and the volume may change. Sugar spreads through water as molecules, while NaCl splits under the action of water into hydrated Na⁺ and Cl⁻ ions. Because its composition is variable and its parts can be separated by physical means (evaporation), a solution resembles a mixture; because it is homogeneous and its formation has a heat effect, it resembles a compound. More solute raises the density of a solution and lowers its freezing point. Natural waters, blood, lymph, tea, air (a solution of gases) and alloys are examples of solutions; many reactions run quickly in solution because the substances are broken down to molecules or ions. Only distilled water is chemically pure.
Worked examples
Iodine tincture: the solute is iodine I₂ and the solvent is ethyl alcohol. Sugar in water: the solute is sugar, the solvent is water. Air is a gaseous solution of oxygen and other gases in nitrogen, the most abundant component.
Sea water does not freeze at 0 °C because dissolved salts lower the freezing point, while pure water freezes at 0 °C. This is an example of a solution’s properties differing from those of its solvent.
Class activity
Observation (with the teacher): put sugar into one glass of water and table salt into another and stir. Describe the clarity of the solutions, then evaporate a little and look at the residue. Never taste anything in the laboratory.
Practice
1
Compare a solution with a mixture and with a compound, and say in what respects it resembles each.
Like a mixture: variable composition, separable by physical means. Like a compound: homogeneous, heat is released or absorbed on formation.
2
Why does the dissolving of KMnO₄ crystals in water show the chemical side of dissolution?
The crystal splits into ions that become hydrated and the solution takes on a new colour — this is more than mechanical mixing.
3
What role do solutions play in the body?
Nutrients pass into solution and are absorbed into the blood; blood and lymph are aqueous solutions.
4
Why do many reactions run fast in solution?
The substances are broken down to molecules or ions, so the particles collide easily.