Solutions - Study Notes

Chapter Summary

Solutions are homogeneous mixtures formed when a solute dissolves uniformly at the molecular or ionic scale within a solvent. This chapter presents a detailed investigation of binary systems, classifying them based on the physical state of the components, solvent polarity (aqueous and non-aqueous), and solute concentration (saturated, unsaturated, and supersaturated). It defines solubility as the maximum mass in grams of solute that can saturate 100 grams of solvent at a specific temperature and pressure, and details how the nature of the substances, temperature, and pressure influence these thresholds. Solution concentrations are quantitatively expressed using mass percentage for solid-in-liquid systems and volume percentage for liquid-in-liquid systems, the latter being temperature-dependent due to thermal liquid expansion. Finally, the chapter covers water of crystallization in hydrated salts, like blue copper sulphate, and compares hygroscopic materials, which absorb atmospheric moisture without dissolving, to deliquescent materials, which absorb enough water to dissolve completely into a liquid solution.

Learning Objectives

• Explain the definition and components of solutions, distinguishing between solute and solvent.
• Classify binary solutions based on physical state, solvent type, and saturation level.
• Analyze how temperature, pressure, and the nature of components influence solubility limits.
• Perform precise calculations for solubility, mass percentage, and volume percentage.
• Describe the role of water of crystallization in hydrated salts and explain the effects of heating.
• Distinguish between hygroscopic and deliquescent compounds using chemical examples.

Key Concepts and Definitions

Homogeneous Mixture

A mixture where the constituent particles are distributed uniformly at the molecular level, making it impossible to visually distinguish the individual components.

Binary Solution

A solution consisting of exactly two components: a single solute (dissolved substance) and a single solvent (dissolving medium).

Aqueous and Non-Aqueous Solutions

An aqueous solution employs liquid water as the solvent. A non-aqueous solution uses an organic liquid, such as carbon disulfide (\(CS_2\)), benzene (\(C_6H_6\)), or ether, to dissolve solutes.

Saturated, Unsaturated, and Supersaturated States

A saturated solution contains the maximum amount of solute that can dissolve in a given solvent at a specified temperature. An unsaturated solution contains less than the maximum limit, while a supersaturated solution holds more solute than its normal saturation limit and is highly unstable.

Solubility

The maximum number of grams of a solute that can dissolve completely in 100 grams of a solvent to form a saturated solution at a specified temperature and pressure.

Water of Crystallization

The fixed number of water molecules chemically combined in a definite stoichiometric ratio within the crystal lattice structure of hydrated salts.

Hygroscopic Substances

Substances that absorb water vapor from the atmosphere at ordinary temperatures without changing their physical state or dissolving into a solution.

Deliquescent Substances

Crystalline solids that absorb so much water vapor from the atmosphere at ordinary temperatures that they eventually dissolve completely in the absorbed moisture, transitioning from a solid to a liquid state.

Worked Methods

Method 1: Calculating Solubility from Mass Data

To find the solubility of a solute in a solvent, use the formula:

\[\text{Solubility} = \frac{\text{Mass of solute (g)}}{\text{Mass of solvent (g)}} \times 100\]

Example: If 18 grams of a salt dissolves completely in 50 grams of water to form a saturated solution at 25°C, calculate its solubility.

• Mass of solute = 18 g
• Mass of solvent = 50 g
• Calculation: \[\text{Solubility} = \frac{18}{50} \times 100 = 36\text{ g per 100 g of water}\]

Method 2: Expressing Concentration by Mass Percentage

To calculate the mass percentage of a solute in a solution, use the formula:

\[\text{Mass Percentage} = \frac{\text{Mass of solute}}{\text{Mass of solution}} \times 100 = \frac{\text{Mass of solute}}{\text{Mass of solute} + \text{Mass of solvent}} \times 100\]

Example: A solution is made by dissolving 15 grams of sodium chloride in 85 grams of water. Find its mass percentage.

• Mass of solute = 15 g
• Mass of solvent = 85 g
• Total mass of solution = 15 g + 85 g = 100 g
• Calculation: \[\text{Mass Percentage} = \frac{15}{100} \times 100 = 15\% \text{ w/w}\]

Method 3: Expressing Concentration by Volume Percentage

To calculate the volume percentage of a liquid solute in a liquid solvent, use the formula:

\[\text{Volume Percentage} = \frac{\text{Volume of solute}}{\text{Volume of solution}} \times 100 = \frac{\text{Volume of solute}}{\text{Volume of solute} + \text{Volume of solvent}} \times 100\]

Example: Calculate the volume percentage of a solution containing 10 mL of ethanol dissolved in 90 mL of water.

• Volume of solute = 10 mL
• Volume of solvent = 90 mL
• Total volume of solution = 10 mL + 90 mL = 100 mL
• Calculation: \[\text{Volume Percentage} = \frac{10}{100} \times 100 = 10\% \text{ v/v}\]

Common Exam Traps

• Denominator Confusion in Percentage Calculations: A very common error is dividing the mass of the solute by only the mass of the solvent instead of the total mass of the solution. Always remember that the denominator must be the total solution mass (solute + solvent).
• Temperature Effects on Volume Percentage: Exams often test whether mass percentage and volume percentage behave similarly with temperature. Remember that volume percentage decreases as temperature rises because liquid solvents expand when heated, whereas mass percentage remains completely unchanged because mass is temperature-independent.
• Hygroscopic vs. Deliquescent State Changes: Students often lose marks by not specifying state changes. Hygroscopic substances absorb moisture but *do not* change their physical state (they remain solid or liquid). Deliquescent substances absorb moisture and *do* change their physical state, melting from crystalline solids into liquid solutions.
• Neglecting Water of Crystallization in Mass Calculations: When solving problems involving hydrated salts like Blue Vitriol (\(CuSO_4 \cdot 5H_2O\)), do not forget that the chemically bound water molecules contribute to the molar mass of the salt and must be accounted for in absolute chemical calculations.

Exam Tips

• Include Temperature and Pressure in Solubility Definitions: Solubility is not a constant; it varies with temperature and pressure. Therefore, any definition of solubility must end with the phrase "at a specified temperature and pressure" to receive full credit.
• Memorize Standard Solubility Benchmarks: Always remember that the solubility of sodium chloride (\(NaCl\)) in water at 25°C is exactly 36 grams per 100 grams of water. This standard value is highly useful for solving quick word problems.
• Identify Hydrated Salts by Common and IUPAC Names: Be prepared to identify Blue Vitriol as Copper(II) sulphate pentahydrate (\(CuSO_4 \cdot 5H_2O\)), Epsom Salt as Magnesium sulphate heptahydrate (\(MgSO_4 \cdot 7H_2O\)), and Green Vitriol as Iron(II) sulphate heptahydrate (\(FeSO_4 \cdot 7H_2O\)).
• Use Correct Units for Final Answers: Ensure concentration percentage answers are labeled as either "% w/w" for mass percentage or "% v/v" for volume percentage to show clear scientific understanding.