What is a solution, and what controls how much solute dissolves?
Solutions, solubility and concentration: describe solutes, solvents and the dissolving process, the factors that affect rate of dissolving and solubility, and how to read a solubility curve.
A focused Virginia SOL Chemistry answer on solutions under CH.5: solute and solvent, the dissolving process and like dissolves like, the factors that change the rate of dissolving and solubility, saturated and unsaturated solutions, and reading a solubility curve.
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What this topic is asking
Standard CH.5 begins with solutions. Virginia expects you to describe a solute dissolving in a solvent, to explain the factors that change the rate of dissolving and the solubility, to classify solutions as unsaturated, saturated or supersaturated, and to read a solubility curve. Solutions underpin acid-base chemistry and the molarity calculations that follow.
Solute, solvent and dissolving
Dissolving happens when solvent particles surround and separate solute particles. Whether a solute dissolves depends on polarity: water, a polar solvent, dissolves polar and ionic substances (such as salt and sugar) because their charges attract water molecules, but it does not dissolve nonpolar substances (such as oil). This is the rule "like dissolves like", which ties solubility back to intermolecular forces.
Rate of dissolving versus solubility
These two ideas are easy to confuse. Stirring and crushing make a solid dissolve faster but do not let more of it dissolve overall. For most solids, solubility increases with temperature; for gases, solubility decreases as temperature rises (warm soda goes flat) and increases with pressure.
Saturation and the solubility curve
A solution is unsaturated when it holds less solute than the maximum (more can dissolve), saturated when it holds the maximum at that temperature (no more dissolves and undissolved solute remains in contact), and supersaturated when it temporarily holds more than the maximum (an unstable state that drops the excess if disturbed).
A solubility curve plots the mass of solute that dissolves per g of water against temperature. A point on the line is a saturated solution; below the line is unsaturated; above the line is supersaturated. Reading the curve tells you how much will dissolve at a temperature and how much excess will crystallize if the solution is cooled.
Try this
Q1. Name two ways to speed up the dissolving of a sugar cube in water. [1 point]
- Cue. Any two of: stir the water, heat the water, crush the cube (increase surface area).
Q2. State whether a gas becomes more or less soluble in water as the temperature rises. [1 point]
- Cue. Less soluble; gas solubility decreases with increasing temperature.
Exam-style practice questions
Practice questions written in the style of VDOE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SOL (multiple choice)1 marksWhich change will generally increase the rate at which a solid dissolves in water? (A) using larger pieces of solid (B) cooling the water (C) stirring the mixture (D) using less waterShow worked answer →
The answer is (C) stirring the mixture.
Stirring brings fresh solvent into contact with the solid surface and carries dissolved particles away, speeding dissolving. Larger pieces (A) reduce surface area and slow it; cooling (B) usually slows the dissolving of a solid; using less water (D) does not increase the rate. The three ways to speed dissolving a solid are stirring (agitation), heating, and increasing surface area (smaller pieces).
The trap is confusing rate of dissolving with how much can dissolve; this question is about speed, not total solubility.
SOL (tech-enhanced, fill in the blank)2 marksA solubility curve shows that g of a salt dissolves in g of water at to give a saturated solution. (a) Classify a solution containing g of the salt in g of water at . (b) State what happens if a saturated solution is cooled and the solubility falls.Show worked answer →
A 2-point solubility-curve item.
(a) Unsaturated (1 point): g is less than the g maximum at , so more could dissolve.
(b) On cooling (1 point): as solubility falls, the excess solute can no longer stay dissolved, so it crystallizes (comes out of solution) as a solid.
Markers reward comparing the amount present with the curve value and recognizing that cooling a saturated solution forces out the excess solute.
Related dot points
- Molarity and solution stoichiometry: calculate molarity, prepare and dilute solutions, and use molarity in solution stoichiometry.
A focused Virginia SOL Chemistry answer on concentration under CH.5: molarity as moles per liter, calculating molarity, the dilution equation M1V1 = M2V2, and using molarity to find moles in solution stoichiometry.
- Acids, bases and the pH scale: describe the properties and definitions of acids and bases, the pH scale, and the relationship between pH and hydrogen ion concentration.
A focused Virginia SOL Chemistry answer on acids and bases under CH.5: the Arrhenius and Bronsted-Lowry definitions, the properties of acids and bases, the pH scale from 0 to 14, and how pH relates to hydrogen ion concentration and strength.
- Neutralization and titration: write neutralization reactions that form a salt and water, and use titration data to find an unknown concentration.
A focused Virginia SOL Chemistry answer on neutralization under CH.5: the acid plus base gives salt plus water reaction, the role of indicators and the equivalence point, and using titration data with M1V1 = M2V2 to find an unknown concentration.
- Polarity and intermolecular forces: determine molecular polarity from shape and bond polarity, and compare dispersion, dipole-dipole and hydrogen-bonding forces and their effect on properties.
A focused Virginia SOL Chemistry answer on polarity under CH.3: how bond polarity and molecular shape combine to make a molecule polar or nonpolar, the three intermolecular forces (dispersion, dipole-dipole, hydrogen bonding), and how they set boiling and melting points and solubility.
- Phase changes and heating curves: name the phase changes and their energy changes, and interpret a heating or cooling curve including the plateaus.
A focused Virginia SOL Chemistry answer on phase changes under CH.4: the names and energy direction of melting, freezing, vaporization, condensation and sublimation, and how to read a heating curve, including why temperature stays constant during a phase change.
Sources & how we know this
- 2018 Science Standards of Learning - Chemistry — Virginia Department of Education (2018)
- Chemistry Curriculum Framework — Virginia Department of Education (2018)