Measuring Solubility Revision Notes for HSC SSCE Chemistry

Measuring Solubility

Introduction to solubility

Temperature affects how much of a substance can dissolve in a solvent. Solubility describes the maximum amount of a substance (in grams) that can dissolve in $100\text{ g}$ of solvent at a specific temperature.

Understanding solubility is crucial in chemistry because it helps us predict how substances will behave when mixed with solvents at different temperatures.

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Practical Applications

Solubility concepts are essential for understanding:

Crystallization processes in laboratories and industry
Purification of compounds through recrystallization
Solution formation and precipitation reactions
Pharmaceutical formulation and drug delivery

Types of solutions

Saturated solutions

When you add sugar to a cup of water, you'll eventually reach a point where no more sugar can dissolve at that particular temperature. Any extra sugar will remain at the bottom of the cup. This is called a saturated solution - it contains the maximum amount of solute that can dissolve at that temperature.

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A saturated solution represents an equilibrium state where the rate of dissolution equals the rate of crystallization. At this point, the solution cannot hold any more solute at the current temperature.

Unsaturated solutions

Before reaching the saturation point, you have an unsaturated solution. This means the solution can still dissolve more solute at the current temperature. There is still "room" for more solute particles to be accommodated by the solvent molecules.

Supersaturated solutions

A supersaturated solution contains more dissolved solute than a saturated solution at the same temperature. This creates an unstable state.

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Creating a Supersaturated Solution

To create a supersaturated solution, follow these steps:

Step 1: Heat a saturated solution to dissolve more solute

Step 2: Carefully cool the solution without disturbing it

Step 3: The extra solute remains dissolved, creating a supersaturated state

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Instability of Supersaturated Solutions

If the supersaturated solution is bumped, a crystal is added, or the container is scratched, the excess solute will rapidly precipitate out. This occurs because the solution is thermodynamically unstable.

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Real-world Example: Toffee Manufacturing

Toffee is a supersaturated solution of sugar. To prevent crystallization and maintain its smooth texture, manufacturers use a mixture of different sugars or add fats to stabilize the solution.

Solubility curves

What are solubility curves?

Solubility curves are graphs that display the relationship between temperature and the amount of solute that can dissolve. These curves provide valuable information about how solubility changes with temperature.

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Understanding the Graph Components

The graph shows:

x-axis: Temperature (measured in °C, typically from $0$ to $100\text{°C}$ )
y-axis: Solubility (measured in grams of salt per $100\text{ g}$ of water)
Curves: Each line represents a different substance

General trends

For most salts, solubility increases as temperature rises. This is shown by upward-sloping curves on the graph. However, some substances show unusual behaviour:

Sodium chloride (NaCl): Shows minimal change in solubility with temperature (nearly horizontal line)
Cerium sulfate ( $\text{Ce}_2(\text{SO}_4)_3$ ): Demonstrates retrograde solubility - its solubility actually decreases as temperature increases (downward-sloping curve)

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Retrograde Solubility

Retrograde solubility is unusual behaviour where solubility decreases with increasing temperature. This is rare among common salts and has important implications for crystallization processes.

Different substances show different rates of solubility increase:

Potassium nitrate ( $\text{KNO}_3$ ): Steep curve, very sensitive to temperature changes
Potassium chlorate ( $\text{KClO}_3$ ): Moderate increase with temperature
Sodium nitrate ( $\text{NaNO}_3$ ): Steep positive slope, highly temperature-dependent

Reading solubility curves

To use a solubility curve effectively, you need to understand how to extract information from it. Let's work through an example using potassium dichromate ( $\text{K}_2\text{Cr}_2\text{O}_7$ ).

Worked example: Potassium dichromate

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Worked Example: Determining Solubility at 20°C

Question 1: How much potassium dichromate will dissolve at $20\text{°C}$ ?

Solution:

Step 1: Locate $20\text{°C}$ on the temperature (x) axis

Step 2: Draw a vertical line upward until it meets the $\text{K}_2\text{Cr}_2\text{O}_7$ curve

Step 3: From this intersection point, draw a horizontal line to the y-axis

Step 4: Read the solubility value

Answer: Approximately $12\text{ g}$ of $\text{K}_2\text{Cr}_2\text{O}_7$ will dissolve in $100\text{ g}$ of water at $20\text{°C}$ .

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Worked Example: Calculating Additional Solubility

Question 2: How much extra will dissolve at $80\text{°C}$ ?

Solution:

Step 1: Repeat the process for $80\text{°C}$

Step 2: At $80\text{°C}$ , approximately $58\text{ g}$ will dissolve

Step 3: Calculate the difference: $58\text{ g} - 12\text{ g} = 46\text{ g}$

Answer: An extra $46\text{ g}$ of $\text{K}_2\text{Cr}_2\text{O}_7$ will dissolve when the temperature increases from $20\text{°C}$ to $80\text{°C}$ .

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Important Application: Crystallization

This calculation also tells us how much potassium dichromate would crystallize out if a saturated solution at $80\text{°C}$ were cooled to $20\text{°C}$ . The answer is $46\text{ g}$ per $100\text{ g}$ of water.

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Exam Tip: Accurate Graph Reading

When reading solubility curves, always use a ruler or straight edge to draw accurate lines. Small errors in reading the graph can lead to incorrect calculations.

Experimental determination of solubility curves

Understanding how to determine solubility curves experimentally is important for both practical skills and understanding the concept of solubility.

Investigation: Deriving the solubility curve for potassium chloride

Aim

To gather experimental data and construct a solubility curve for potassium chloride.

Principle

The experiment works by creating a saturated solution at a high temperature, then cooling it until crystals first appear. The temperature at which crystals begin to form is the saturation temperature for that particular concentration.

Equipment needed

Potassium chloride (solid)
Distilled water ( $10\text{ mL}$ per group)
Large test tube
$250\text{ mL}$ beaker (water bath)
Thermometer ( $-10$ to $110\text{°C}$ )
Stirring rod
Bunsen burner with tripod and gauze mat
Balance
Boss head and clamp
Safety glasses

Method

Step 1: Weighing: Accurately weigh the assigned mass of potassium chloride (ranging from $3.00\text{ g}$ to $5.00\text{ g}$ depending on your group)

Step 2: Preparation: Place the weighed potassium chloride into a large test tube and add exactly $10.00\text{ mL}$ of distilled water

Step 3: Water bath setup: Half-fill a beaker with tap water. Clamp the test tube securely so it is immersed in the water bath

Step 4: Heating: Gently warm the water bath using a Bunsen burner. Continuously stir the contents of the test tube with the stirring rod until all the potassium chloride has completely dissolved

Step 5: Cooling and observation: Allow the test tube to cool while continuing to stir. Watch carefully for tiny crystals to start appearing

Step 6: Recording: Note the exact temperature when crystals first appear (the recrystallization temperature). This is the saturation temperature for your concentration

Safety considerations

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Risk: Bunsen burners get very hot

Management: Ensure long hair is tied back, keep the flame away from flammable materials, and never use a burner with a damaged gas tube. If burned, immediately place the affected area under cold running water for 20 minutes and inform your teacher.

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Risk: Broken glassware causes cuts

Management: Inspect all glassware before use and discard any chipped or cracked items. If glass breaks, use a brush and dustpan to clean up - never use your fingers.

Results

Group	Mass of potassium chloride (g)	Mass of water (g)
1	3.00	10.00
2	3.40	10.00
3	3.80	10.00
4	4.20	10.00
5	4.60	10.00
6	5.00	10.00

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Calculating Solubility

To calculate solubility in $\text{g}/100\text{g}$ water from your results, multiply the mass of potassium chloride by 10 (since you used $10\text{ g}$ of water, not $100\text{ g}$ ).

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Worked Example: Solubility Calculation

If Group 1 used $3.00\text{ g}$ of KCl in $10.00\text{ g}$ water, the solubility would be:

$\text{Solubility} = \frac{3.00\text{ g}}{10.00\text{ g}} \times 100 = 30.0\text{ g}/100\text{g H}_2\text{O}$

Analysis

After collecting class data, plot a graph with:

x-axis: Temperature ( $0-100\text{°C}$ )
y-axis: Solubility ( $\text{g}/100\text{g}$ water)

Join the points with a smooth curve. This is your experimental solubility curve for potassium chloride.

Evaluating accuracy

You can calculate the percentage error of your experiment using:

$\text{Percentage error} = \frac{\text{Experimental value} - \text{True value}}{\text{True value}} \times 100$

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Worked Example: Calculating Percentage Error

If the theoretical solubility of potassium chloride at $50\text{°C}$ is $50\text{ g}/100\text{g}$ , and your experiment gave $48\text{ g}/100\text{g}$ :

$\text{Percentage error} = \frac{48 - 50}{50} \times 100 = -4\%$

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Common Sources of Error

Difficulty in identifying exactly when crystals first appear
Heat loss during cooling leading to faster temperature drop
Impurities in the potassium chloride or water
Incomplete dissolution of the salt before cooling
Inaccurate temperature readings

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Experimental Improvements

To improve accuracy:

Cool the solution more slowly for better observation
Use a digital thermometer for more accurate temperature measurement
Repeat the experiment multiple times and take an average
Ensure thorough stirring throughout the process

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Exam Tip: Recording Crystallization Temperature

In practical exams, make sure you record the temperature the moment you see the first crystals appear. Waiting too long will give an inaccurately low temperature reading.

Remember!

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Key Points to Remember:

Solubility is the maximum mass (in grams) of a substance that can dissolve in $100\text{ g}$ of solvent at a specific temperature
A saturated solution contains the maximum amount of dissolved solute at that temperature, while an unsaturated solution can dissolve more solute
Supersaturated solutions contain more solute than a saturated solution at the same temperature and are unstable
Solubility curves are graphs showing how solubility changes with temperature, with most salts showing increased solubility at higher temperatures
To read a solubility curve, draw vertical and horizontal lines from the temperature and curve to find the solubility value at any given temperature

Measuring Solubility (HSC SSCE Chemistry): Revision Notes