Impure Products and Solubility of Salts

British & IGCSE › Checkpoint 8 › Checkpoint 8 Science — Complete Course

1. Key words

2. Reactions often give a mixture, not one pure product

In Stage 7 you wrote word equations such as magnesium + hydrochloric acid → magnesium chloride + hydrogen. A word equation makes it look as if you finish with neat, separate products. In a real laboratory, what is left in the flask at the end is almost always a mixture. The product you want is impure, and you must separate and purify it.

Remember from the purity lesson: purity describes how much of the wanted chemical is in a sample. A sample that is 100 % one substance is pure. Anything else mixed in is an impurity.

3. Getting a pure product: making copper sulfate crystals

Here is the standard way to make a pure, dry salt from an insoluble solid and an acid. Learn the reason for each step — Checkpoint often asks "Why did the student…?"

How can you tell if a product is pure? A pure substance melts (and boils) at one fixed temperature. An impure substance melts over a range of temperatures, usually lower than the pure substance. Paper chromatography is another test: a pure substance gives only one spot.

Percentage purity. You can describe how pure a sample is with a percentage:

4. Solubility

When a salt dissolves in water, the salt is the solute, the water is the solvent and the mixture is a solution. If you keep adding salt and stirring, there comes a point when no more will dissolve — some solid stays at the bottom. The solution is now saturated.

The solubility of a substance is the largest mass that will dissolve in 100 g of water at a stated temperature. Its unit is g per 100 g of water. For example, the solubility of sodium chloride at 20 °C is about 36 g per 100 g of water: 100 g of water at 20 °C can dissolve 36 g of salt, but no more.

5. How solubility changes with temperature

Scientists measure the solubility of a salt at several temperatures and plot a solubility curve. The table and graph show approximate values for three salts.

What the graph tells you:

• For most salts, solubility increases as the temperature increases — hot water dissolves more salt than cold water.
• The amount of change is very different for different salts. Potassium nitrate rises very steeply (from 14 g at 0 °C to 167 g at 80 °C — more than ten times as much). Copper sulfate rises steadily.
• Some salts hardly change. Sodium chloride (common salt) dissolves only about 2 g more at 80 °C than at 0 °C — its curve is almost flat.
• A few salts are even slightly less soluble in hot water (lithium carbonate is one example), so never assume — always read the graph.
• Where two curves cross, the two salts have the same solubility at that temperature (potassium nitrate and sodium chloride at about 23 °C).

Using particles: in hotter water the particles move faster and have more energy, so for most salts the water can break up and hold more of the salt's particles. You do not need to explain this in detail at Stage 8, but you must be able to describe the pattern from data.

6. Reading and using a solubility curve

1. Solubility at a temperature: go up from the temperature to the curve, then across to the solubility axis.
2. Temperature for a given solubility: go across from the solubility to the curve, then down to the temperature axis.
3. Comparing salts: at a given temperature, the higher curve is the more soluble salt.
4. Crystals formed on cooling: mass of crystals = solubility at the hot temperature − solubility at the cold temperature (for 100 g of water).
5. Other masses of water: scale up or down. 50 g of water dissolves half as much as 100 g; 200 g dissolves twice as much.

7. Worked examples

Worked example 1 — Sara makes magnesium sulfate by adding magnesium powder to dilute sulfuric acid until no more fizzing happens and some grey magnesium is left. magnesium + sulfuric acid → magnesium sulfate + hydrogen (a) Explain why the mixture in the beaker is not pure magnesium sulfate. [2] (b) Why did Sara add magnesium until some was left over? [1]

1. (a) The magnesium sulfate is dissolved in water (it is a solution) (1), and there is unreacted magnesium left over (1). (The hydrogen escapes as a gas.)
2. (b) So that all the acid is used up — no acid will be left mixed with the product.
3. Common mistake: writing "the hydrogen makes it impure". Hydrogen is a gas and bubbles away; the impurities that stay are the water and the leftover magnesium.

Worked example 2 — A 40 g sample of salt made in a reaction contains 34 g of potassium chloride. Calculate its percentage purity. [2]

1. Percentage purity = (mass of potassium chloride ÷ mass of sample) × 100
2. = (34 ÷ 40) × 100 = 0.85 × 100
3. = 85 %

Worked example 3 — Use the graph in section 5. (a) What is the solubility of potassium nitrate at 40 °C? [1] (b) Which salt is the most soluble at 10 °C? [1]

1. (a) Up from 40 °C to the potassium nitrate curve, then across: 61 g per 100 g of water.
2. (b) At 10 °C the highest curve is the dotted line: sodium chloride (about 36 g). Potassium nitrate is only about 22 g at 10 °C.
3. Common mistake: in (b), choosing potassium nitrate because it is "the most soluble salt". It is only the most soluble above about 23 °C — always read the temperature you are asked about.

Worked example 4 — A saturated solution of potassium nitrate is made with 100 g of water at 60 °C. It is cooled to 20 °C. Calculate the mass of crystals that forms. [2]

1. At 60 °C, 100 g of water holds 106 g of potassium nitrate.
2. At 20 °C it can hold only 32 g.
3. Mass of crystals = 106 − 32 = 74 g (1 mark for both readings, 1 mark for the answer).

Worked example 5 — A saturated solution of copper sulfate is made with 50 g of water at 80 °C and cooled to 20 °C. What mass of crystals forms? [2]

1. For 100 g of water: 55 g at 80 °C and 21 g at 20 °C, so 55 − 21 = 34 g of crystals.
2. 50 g is half of 100 g, so the mass is halved: 34 ÷ 2 = 17 g.
3. Common mistake: giving 34 g — the graph is always for 100 g of water, so scale to the mass you are given.

Worked example 6 — Describe how the solubility of sodium chloride and potassium nitrate change between 0 °C and 80 °C. Use data in your answer. [2]

1. Potassium nitrate: solubility increases a lot, from 14 g to 167 g per 100 g of water (1).
2. Sodium chloride: solubility increases only very slightly / hardly changes, from 36 g to 38 g (1).
3. Exam tip: "Use data" means quote numbers with units from the table or graph. A pattern with no numbers usually loses a mark.

Practice

All pages in this course

Start here

• Stage 8 Science: The Tests and How to Prepare
• Thinking and Working Scientifically: Models, Investigations and Analysing Results

Biology

• Joints and Antagonistic Muscles
• Blood and Its Components
• Breathing, Gas Exchange and Diffusion in the Lungs
• A Balanced Diet, Energy Stores and Lifestyle
• Aerobic Respiration
• Ecosystems, Bioaccumulation and Invasive Species

Chemistry

• The Structure of the Atom
• Purity, Concentration and Chromatography
• Word Equations, Exothermic and Endothermic Reactions
• The Reactivity of Metals
• Impure Products and Solubility of Salts

Physics

• Speed and Distance–Time Graphs
• Balanced and Unbalanced Forces, and Moments
• Pressure, Particles and Diffusion
• Reflection and Refraction
• Dispersion and Colour
• Magnetic Fields and Electromagnets

Earth and space

• The Earth's Magnetic Field and Earth's Resources
• Climate, Weather and Climate Change
• Galaxies and Asteroids

Practice tests

• Stage 8 Science — Practice Test 1
• Stage 8 Science — Practice Test 2
• Stage 8 Science — Practice Test 3