solubility curves

Solubility curves graphically represent the relationship between temperature and the solubility of a substance in a solvent, typically illustrating how solubility increases with temperature for most solids. In contrast, the solubility of gases generally decreases with an increase in temperature, highlighting the differing behaviors of solids and gases in solution.

Solubility Curves

Understanding Solubility

  • Definition: Solubility is the maximum grams of solute that will dissolve in 100 g of solvent at a given temperature.

  • Solubility varies with temperature and is based on a saturated solution.

Temperature Effects on Solubility

  • General Rule: Solubility of most solids increases with the temperature of the solvent.

  • A solubility curve visually represents this relationship.

  • Key Point: A higher temperature often leads to higher solubility for ionic solids.

Solubilities of Solids vs. Temperature

  • Observation: Most salts have greater solubility in hot water.

  • Some salts show a negative heat of solution (exothermic), decreasing in solubility when temperature increases.

Solubility of Gases

  • General Rule: The solubility of most gases decreases with increasing temperature.

  • Gases dissolve better at higher pressures.

  • For gases, solubility and temperature are inversely proportional.

  • Example: NH3 is identified as a gas in the graph provided.

Features of Solubility Curves

  • A typical solubility curve shows the mass of solute (in grams) that dissolves in 100 g of water at different temperatures.

  • Saturation Points:

    • Saturated Solution: If the amount of solute dissolved is on the curve (solid line).

    • Supersaturated Solution: Above the curve.

    • Unsaturated Solution: Below the curve.

Analogy to Understand Solubility

  • Comparing solubility calculations to adjusting a recipe for Kool-Aid:

    • Water = 100 mL (same as 100 g), solute = Kool-Aid powder.

    • Amount of Kool-Aid powder varies based on temperature, affecting how "sweet" or concentrated the Kool-Aid is:

      • Unsaturated: Too little powder (light pink).

      • Saturated: Just right (perfect red).

      • Supersaturated: Too much (very sweet).

Practical Applications of Solubility Curves

Using the Curve

  • Results from solubility curves indicate a saturated solution describing ___ g solute in 100 g of H2O.

  • To find solute in amounts other than 100 g, use proportions based on:

    • g solute = g solute/100 g H2O * g H2O.

  • Note: 1 g H2O = 1 mL H2O.

  • Important reminders about the graph:

    • Graph based on 100 mL or 100 grams of water.

    • The line represents saturation points.

    • Above the line = supersaturated; below = unsaturated.

Example Problems with Solubility Curves

Example 1: Evaluating Saturation

  • Given: A solution at 60ºC with 80 g NaNO3 in 100 g of water.

  • Question: Is it saturated, supersaturated, or unsaturated?

    • Answer: Unsaturated (intersects below the curve).

Example 2: Determining Saturation Mass

  • Question: What amount of NaCl makes a saturated solution at 80ºC?

    • Answer: 39 grams (refer to the curve).

Example 3: Finding Dissolution Temperature

  • Question: How much KCl will dissolve in 300 mL at 50.0ºC?

    • The graph indicates 80 g KCl dissolves in 100 g H2O.

    • Calculation: 240 g KCl for 300 mL based on proportions.

Practice Problems

Problem Set

  1. 120 g of sodium nitrate in 100 mL at RT: saturated, unsaturated, or supersaturated?

  2. a) Amount of KNO3 to saturate 100 g of water at 50°C?b) If 65 g KNO3 is present, how much more to saturate?

  3. For 1000 g of KCl at 50°C, what volume of water is needed?

    • Expected answers: 80 g, 15 g, and 2500 g of water respectively.

Answers to Practice Problems

  1. Result: Supersaturated

  2. a) 80 g KNO3b) 15 g more could be added.

  3. To saturate 1000 g of KCl at 50°C, require 2500 g water.