Welcome to Chapter 1.10: Solubility
Welcome to your study guide for Solubility, part of CCEA GCSE Chemistry Unit 1! Have you ever stirred sugar into a cup of hot tea and watched it disappear, only to find that if you add too many spoonfuls, the extra sugar just sits at the bottom? That is the science of solubility in everyday life.
In this chapter, you will learn the exact scientific terms used to describe dissolving, master the solubility rules for common compounds, understand how temperature affects solubility curves, and learn how to perform calculations like a chemistry expert. Don't worry if calculations or graphs seem tricky at first — we will break everything down into clear, easy-to-follow steps!
1. Core Vocabulary: The Building Blocks
Before diving into experiments and calculations, let's make sure we have the key definitions down. CCEA examiners love testing these precise words, so learning them thoroughly will guarantee you easy marks.
• Solute: The substance that dissolves in a liquid (solvent). For example, solid salt or sugar.
• Solvent: The liquid in which a solute dissolves. For example, water.
• Solution: The mixture formed when a solute dissolves into a solvent (for example, salt water).
• Saturated Solution: A solution in which no more solute can dissolve at that particular temperature.
• Precipitate: An insoluble solid that emerges from a liquid solution (for example, during a chemical reaction or when cooling a hot saturated solution).
• Solubility: The maximum mass of solid/solute (in grams) that will saturate \(100\text{ g}\) of water (or solvent) at a particular temperature.
Memory Tip for the Exact Definition of Solubility:
Always include all three key ingredients in your exam answer:
1. Maximum mass in grams of solute
2. In \(100\text{ g}\) of water (or solvent)
3. At a particular temperature
Key Takeaway: A solute dissolves in a solvent to make a solution. When the solvent is full and cannot take any more solute at that temperature, it is saturated.
2. Solubility Rules for Common Compounds in Water
In GCSE Chemistry, you need to know whether an ionic compound will dissolve in water or form an insoluble solid. Here is the official CCEA guide to solubility:
Always Soluble:
• All salts of sodium (\(\text{Na}^+\)), potassium (\(\text{K}^+\)), and ammonium (\(\text{NH}_4^+\)) are soluble in water.
• All nitrates (\(\text{NO}_3^-\)) are soluble in water.
Chlorides (\(\text{Cl}^-\)):
• Soluble: Most chlorides are soluble.
• Insoluble exceptions: Silver chloride (\(\text{AgCl}\)) and lead(II) chloride (\(\text{PbCl}_2\)).
Sulfates (\(\text{SO}_4^{2-}\)):
• Soluble: Most sulfates are soluble.
• Insoluble exceptions: Barium sulfate (\(\text{BaSO}_4\)) and lead(II) sulfate (\(\text{PbSO}_4\)).
• Slightly soluble exception: Calcium sulfate (\(\text{CaSO}_4\)) is sparingly/slightly soluble.
Carbonates (\(\text{CO}_3^{2-}\)) and Hydroxides (\(\text{OH}^-\)):
• Insoluble: Most carbonates and hydroxides do not dissolve in water.
• Soluble exceptions: Those containing sodium, potassium, or ammonium (e.g., \(\text{Na}_2\text{CO}_3\), \(\text{KOH}\), \(\text{NH}_4\text{OH}\)).
• Slightly soluble exception: Calcium hydroxide (\(\text{Ca(OH)}_2\)) is slightly/sparingly soluble (limewater is a dilute solution of calcium hydroxide).
Memory Trick: Remember the acronym "SNAP" for compounds that always dissolve: Sodium, Nitrate, Ammonium, Potassium!
Key Takeaway: All nitrates and all group 1/ammonium salts dissolve completely. Most chlorides and sulfates dissolve (with a few heavy-metal exceptions), while most carbonates and hydroxides remain insoluble.
3. Solubility Curves and Graphs
Solubility is measured in units of \(\text{g}/100\text{ g water}\) (grams of solute per \(100\text{ g}\) of water).
A solubility curve is a graph showing how solubility changes as temperature changes:
• \(y\)-axis: Solubility (\(\text{g}/100\text{ g water}\))
• \(x\)-axis: Temperature (\(^\circ\text{C}\))
Reading a Solubility Curve:
1. Points on the line: Any point lying directly on the curve represents a saturated solution at that specific temperature.
2. Points below the line: Any point below the curve represents an unsaturated solution (the water could still dissolve more solute at that temperature).
3. General trend: For almost all solid salts, solubility increases as temperature increases. This is why hot water can dissolve more solid than cold water!
Key Takeaway: The line on a solubility graph represents the saturation limit. Above or on the line is saturated; below the line is unsaturated.
4. Experimental Method: Determining Solubility
How do scientists actually plot a solubility curve in the laboratory? Here is the step-by-step method assessed in Unit 1 and Unit 3 practical skills:
Step 1: Measure a known mass or volume of water into a boiling tube or beaker.
Step 2: Add a known mass of solute in excess, or add solid incrementally with gentle heating until it is completely dissolved.
Step 3: Allow the hot solution to cool down slowly while stirring carefully with a thermometer.
Step 4: Watch closely and record the exact temperature at which the first crystals appear (initial crystallisation). At this exact temperature, the solution has just become saturated.
Step 5: Add a measured volume of distilled water to dilute the mixture, re-heat until all crystals dissolve again, and record the new, lower crystallisation temperature as it cools.
Step 6: Repeat this process to gather several data pairs of solubility and temperature, and plot your solubility curve.
Key Takeaway: The temperature at which crystals first start appearing during cooling tells you the exact point of saturation for that mass of solute and water.
5. Quantitative Calculations & Problem Solving
Solubility calculations are very straightforward once you follow the standard formulas. There are two main types of calculations you will meet in CCEA exams.
Type 1: Scaling to Different Masses of Water
Solubility values from a graph or table are always given for \(100\text{ g}\) of water. If a question asks about a different amount of water, scale it proportionally:
\(\text{Mass of solute dissolved in } m \text{ grams of water} = \text{Solubility} \times \frac{m}{100}\)
Worked Example 1:
The solubility of potassium nitrate at \(40^\circ\text{C}\) is \(64\text{ g}/100\text{ g water}\). Calculate the maximum mass of potassium nitrate that will dissolve in \(25\text{ g}\) of water at \(40^\circ\text{C}\).
Step-by-step solution:
1. Identify the solubility for \(100\text{ g}\) of water: \(64\text{ g}\).
2. Identify the mass of water given: \(m = 25\text{ g}\).
3. Apply the scaling factor:
\(\text{Mass dissolved} = 64 \times \frac{25}{100} = 64 \times 0.25 = 16\text{ g}\).
Answer: \(16\text{ g}\) of potassium nitrate.
Type 2: Mass of Crystals Precipitated upon Cooling
When a hot, saturated solution is cooled down, the water can no longer hold as much dissolved solid. The extra solute comes out of solution as solid crystals (precipitate).
\(\text{Mass of crystals precipitated} = (\text{Solubility at } T_{\text{high}} - \text{Solubility at } T_{\text{low}}) \times \frac{m_{\text{water}}}{100}\)
Worked Example 2:
A saturated solution of a salt in \(50\text{ g}\) of water is cooled from \(80^\circ\text{C}\) to \(20^\circ\text{C}\).
• Solubility at \(80^\circ\text{C} = 110\text{ g}/100\text{ g water}\)
• Solubility at \(20^\circ\text{C} = 32\text{ g}/100\text{ g water}\)
Calculate the mass of crystals that will precipitate out of the solution.
Step-by-step solution:
1. Find the difference in solubility per \(100\text{ g}\) of water:
\(\text{Difference} = 110\text{ g} - 32\text{ g} = 78\text{ g}\) per \(100\text{ g}\) of water.
2. Scale this difference to the mass of water used (\(50\text{ g}\)):
\(\text{Mass of crystals} = 78 \times \frac{50}{100} = 78 \times 0.5 = 39\text{ g}\).
Answer: \(39\text{ g}\) of crystals precipitate.
Key Takeaway: When cooling a solution, subtract the lower solubility from the higher solubility, then scale for the mass of water given in the question.
6. Common Examiner Traps & How to Avoid Them
CCEA examiner reports highlight several recurring mistakes students make. Keep these in mind to secure top marks:
1. Incomplete Definition of Solubility:
Mistake: Writing "how much solid dissolves in water".
Fix: Always state "the maximum mass of solute in grams that dissolves in \(100\text{ g}\) of water at a particular temperature".
2. Confusing Solvent Mass with Solution Mass:
Mistake: Dividing by the mass of the entire solution instead of the mass of water.
Fix: Remember that solubility is strictly relative to \(100\text{ g}\) of water (solvent), not total solution.
3. Forgetting to Scale for Water Mass:
Mistake: Calculating the difference between two solubilities from a graph and stopping there, even when the question specifies \(50\text{ g}\) or \(25\text{ g}\) of water.
Fix: Always check the mass of water stated in the question and multiply by \(\frac{m_{\text{water}}}{100}\).
4. Inaccurate Graph Reading:
Mistake: Misreading grid units or guessing points without using a ruler.
Fix: Use a ruler to read perpendicularly from the axes and always double-check the scale on each axis (e.g., whether one small square represents \(1^\circ\text{C}\), \(2^\circ\text{C}\), or \(5^\circ\text{C}\)).
Quick Review Checklist
Make sure you can confidently answer "yes" to these questions before your exam:
• Can I state the complete definition of solubility and saturated solution?
• Do I know which common salts are soluble and insoluble (including exceptions like \(\text{AgCl}\), \(\text{PbCl}_2\), \(\text{BaSO}_4\), and \(\text{PbSO}_4\))?
• Can I describe how to find crystallisation temperatures experimentally?
• Can I calculate the mass of crystals formed when a solution cools down in a given mass of water?