Elements, compounds and mixturesIB MYP Sciences: Revision notes
Section 1
Elements, compounds and mixtures
- An element is made of one type of atom only (for example iron, oxygen). It cannot be broken down chemically.
- A compound contains two or more elements chemically combined in a fixed ratio (for example water, H₂O).
- A mixture contains two or more substances that are not chemically combined, in any proportion (for example air, seawater).
- A pure substance is a single element or compound with nothing else mixed in.
A compound can only be separated into its elements by a chemical reaction. A mixture can be separated by physical methods.
Section 2
Compounds versus mixtures
Iron and sulfur show the difference.
- Mixture of iron filings and sulfur: the iron can be pulled out with a magnet, and each keeps its own properties.
- Compound iron(II) sulfide, made by heating them together: the elements are chemically combined, the solid is black and is not magnetic. It has different properties from the elements.
The properties of a compound are very different from the elements in it, but the properties of a mixture are a mixture of the properties of its parts.
A compound is not just a mixture of two elements. Its atoms are bonded together, so it has new properties.
Section 3
Solutions and solubility
A solute is a substance that dissolves. A solvent is the liquid it dissolves in. Together they form a solution, for example salt (solute) in water (solvent).
A substance that dissolves is soluble; one that does not is insoluble. Solubility is how much of a solute dissolves in a given amount of solvent at a given temperature. Most solids are more soluble in hot water.
Section 4
Filtration, evaporation and crystallisation
- Filtration separates an insoluble solid from a liquid. The solid stays on the filter paper as the residue; the liquid that passes through is the filtrate. Use it to remove sand from water.
- Evaporation heats a solution until all the solvent has gone, leaving the solute. Use it to get salt from salt solution.
- Crystallisation: heat the solution to evaporate some solvent, then leave it to cool so that crystals form. Use it when the solute might decompose if heated to dryness, such as copper sulfate.
Section 5
Simple and fractional distillation
Simple distillation separates a solvent from a solution. The solution is heated, the solvent boils, and the vapour passes into a condenser where it cools and turns back into a liquid. The solute stays in the flask. Use it to get pure water from salt water.
Fractional distillation separates a mixture of liquids with different boiling points, such as ethanol (78 °C) and water (100 °C). The liquid with the lowest boiling point evaporates first and moves up a fractionating column; the higher-boiling vapour condenses and returns to the flask. It is used to make ethanol fuel and to separate crude oil.
Choose the method from the question: insoluble solid and liquid = filtration; solvent from solution = simple distillation; liquids with different boiling points = fractional distillation.
Section 6
Chromatography and Rf values
Chromatography separates dissolved substances, such as inks and dyes. A pencil line is drawn near the bottom of the paper (pencil does not dissolve) and spots are placed on it. The paper stands in a solvent with the level below the line. As the solvent rises, each substance moves a different distance.
The Rf value = distance moved by the substance ÷ distance moved by the solvent.
Worked example: a spot travels 2.7 cm and the solvent travels 9.0 cm. Rf = 2.7 ÷ 9.0 = 0.30.
A pure substance gives one spot. Matching Rf values (in the same solvent) suggest the same substance.
Section 7
Purity and melting points
A pure substance melts at one fixed, sharp melting point and boils at a fixed boiling point. A mixture or impure substance melts over a range of temperatures, and its melting point is lower than the pure substance.
This is how chemists test purity. Pure water freezes at exactly 0 °C; salt water freezes below 0 °C.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Elements, compounds and mixtures
- A chemistry teacher in Nairobi gives her class two samples. Sample 1 is a mixture made by stirring iron filings and yellow sulfur powder together. Sample 2 is iron(II) sulfide, a black solid made by heating iron and sulfur together strongly.Explain why a magnet cannot be used to remove the iron from sample 2.2 marks
- A small salt works on the coast of Gujarat in India fills shallow pools with seawater. The Sun evaporates the water and salt crystals form. The workers also want to collect pure water from some of the seawater, and one sample of seawater they collected contains some sand.Explain how simple distillation can be used to obtain pure water from seawater.2 marks
- A food scientist uses paper chromatography to find out which dyes are in a blue-green sweet coating. She draws a pencil line near the bottom of a strip of chromatography paper and places a spot of the sweet's dye extract and spots of three known dyes (P, Q and R) on the line. She stands the paper in a beaker of water, with the water level below the line. She stops the experiment when the water has risen 9.0 cm above the pencil line.The sweet extract produced two spots, at 3.6 cm and 6.3 cm above the pencil line. The known dyes have Rf values of 0.40 (P), 0.55 (Q) and 0.70 (R). Calculate the Rf value of each spot and state which known dyes are present in the sweet coating.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).