Types of mixtureIB MYP Chemistry: Revision notes
Section 1
Solutions
A solution forms when a solute dissolves in a solvent. The solute is the substance that dissolves (for example sugar or salt) and the solvent is the liquid it dissolves in (usually water). A substance that dissolves is soluble; one that does not is insoluble.
In a solution the solute particles are spread evenly between the solvent particles. They are far too small to see, so a solution is clear (it can be coloured), the particles never settle, and the solute passes through filter paper.
A solution can be coloured and still be clear. Copper sulfate solution is blue but you can see through it.
Section 2
Suspensions
A suspension contains insoluble particles that are large enough to see. The mixture looks cloudy, and the particles settle to the bottom if left to stand. They can be removed by filtration.
Examples are muddy water, chalk shaken in water and sand in water.
Section 3
Colloids and emulsions
In a colloid, the particles are bigger than those in a solution but smaller than those in a suspension. They are spread through another substance and do not settle, and they cannot be filtered out. They scatter a beam of light, so a colloid looks cloudy.
An emulsion is a colloid in which tiny droplets of one liquid are spread through another liquid that they do not mix with. Milk and mayonnaise are emulsions. Oil and water will separate, so an emulsifier (such as egg yolk in mayonnaise) is added to keep the droplets spread out. Other colloids include fog (liquid in gas), jelly and foam.
Section 4
Alloys
An alloy is a mixture of a metal with at least one other element. The elements are mixed, not chemically bonded, so the proportions can vary. Alloys are made by melting the metals together and letting the mixture solidify.
- Brass: copper and zinc.
- Bronze: copper and tin.
- Steel: iron with a small amount of carbon.
Alloys are often harder and stronger than the pure metals. The atoms of the added element are a different size, so the layers of atoms cannot slide over one another as easily.
Section 5
Solubility and saturated solutions
Solubility is the maximum mass of solute that dissolves in a given mass of solvent at a certain temperature. For solids in water it is usually given in grams per 100 g of water.
A saturated solution has dissolved as much solute as it can at that temperature, so any extra solid stays undissolved. An unsaturated solution can still dissolve more solute. For most solids, solubility increases as the temperature rises, but some, such as sodium chloride, change very little.
Section 6
Solubility curves and calculating crystals
A solubility curve is a graph with temperature on the horizontal axis and solubility on the vertical axis. Read the solubility at a chosen temperature from the curve. A steep curve (like potassium nitrate) means temperature has a big effect; a flat curve (like sodium chloride) means it has little effect.
If a hot saturated solution is cooled, the solubility falls, and the extra solute forms crystals.
Worked example: 100 g of water is saturated with potassium nitrate at 60 °C (109 g) and cooled to 20 °C (32 g). Mass of crystals = 109 − 32 = 77 g. For 200 g of water, double it: 154 g.
If the mass of water is not 100 g, scale the solubility values up or down by the same factor before subtracting.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Types of mixture
- A student looks up how much of two solids dissolves in 100 g of water to make a saturated solution. Potassium nitrate: 32 g at 20 °C, 64 g at 40 °C, 109 g at 60 °C and 169 g at 80 °C. Sodium chloride: 36 g at 20 °C, 36 g at 40 °C, 37 g at 60 °C and 38 g at 80 °C.Describe how the effect of temperature on the solubility of sodium chloride differs from its effect on potassium nitrate.2 marks
- A food technologist classifies four products. Muddy river water has soil particles that can be seen and settle to the bottom after an hour. Sweet tea is clear, with the sugar particles too small to see. Mayonnaise has tiny droplets of oil spread through a water-based liquid and stays evenly mixed. A bronze coin is made by melting copper and tin together and letting the liquid solidify.Explain why bronze is classed as an alloy and as a mixture.2 marks
- A student investigates how temperature affects how much potassium nitrate dissolves. She warms 20 g of water to each chosen temperature in a water bath, adds potassium nitrate in small portions while stirring, and records the total mass added when some solid will no longer dissolve. The total masses dissolved in 20 g of water were: 6.4 g at 20 °C, 12.8 g at 40 °C, 21.8 g at 60 °C and 33.8 g at 80 °C.Describe the pattern in the results, using data from the investigation.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).