Elements, compounds and mixturesIB MYP Chemistry: Revision notes
Section 1
Elements
An element is a substance made of only one type of atom. There are about 118 elements, and each has its own symbol, such as O for oxygen and Fe for iron. You cannot break an element down into simpler substances by chemical reactions.
In a particle picture, an element is shown as identical particles. They may be separate atoms (as in neon, or in a metal) or molecules made of two or more atoms of the same type joined together, such as oxygen (O₂), nitrogen (N₂) and chlorine (Cl₂).
A molecule is not always a compound. O₂ is a molecule, but it is an element because both atoms are the same type.
Section 2
Compounds
A compound is a substance made of two or more different elements chemically bonded together in a fixed ratio. Examples are water (H₂O), carbon dioxide (CO₂) and sodium chloride (NaCl).
The properties of a compound are different from the properties of the elements in it. Sodium is a reactive metal and chlorine is a toxic gas, but sodium chloride is safe table salt. A compound can only be split into its elements by a chemical reaction, for example by electrolysis or heating.
Section 3
Mixtures
A mixture contains two or more substances that are not chemically bonded. The substances can be elements, compounds or both. Air (nitrogen, oxygen, argon and other gases), seawater and sand with salt are mixtures.
- The proportions are not fixed; you can mix them in any amounts.
- Each substance keeps its own properties.
- The parts can be separated by physical methods such as filtering, evaporating, distilling or using a magnet.
Section 4
Classifying with particle diagrams
In particle diagrams, different elements are shown as different coloured circles. To classify a substance, look at what is in the diagram.
- Element: all the particles are the same type of atom, either separate or joined into identical molecules.
- Compound: all the particles are identical, but each one is made of different atoms joined together.
- Mixture: there are two or more different kinds of particle (separate atoms or molecules) that are not joined to each other.
A substance with only one type of particle is a pure substance; an element and a compound are both pure substances.
First ask 'is there more than one kind of particle?' If yes, it is a mixture. If no, ask whether each particle contains more than one type of atom.
Section 5
Comparing compounds and mixtures
Iron and sulfur show the difference clearly.
- Stirring iron filings with sulfur powder makes a mixture. The iron is still attracted to a magnet and the sulfur is still yellow. Any amounts can be used.
- Heating them strongly makes the compound iron sulfide. It is dark grey, is not attracted to a magnet and contains iron and sulfur in a fixed ratio. Energy is given out as the powder glows.
Summary
- Bonding: a mixture is not chemically bonded; a compound is.
- Composition: a mixture can vary; a compound has a fixed ratio.
- Properties: a mixture has those of its parts; a compound has different ones.
- Separation: a mixture by physical methods; a compound only by a chemical reaction.
Section 6
Worked example: is air a compound?
Air is a mixture. Its main gases are nitrogen (about 78%) and oxygen (about 21%). Oxygen can be removed by passing air over hot copper, which turns black as copper oxide forms, while the nitrogen is left unchanged. If 100 cm³ of air leaves 79 cm³ of gas, then 100 − 79 = 21 cm³ was oxygen, which is 21%.
The gases keep their own properties and the proportions of air vary a little from place to place. This evidence shows that air is a mixture and not a compound.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Elements, compounds and mixtures
- A student examines four sealed containers using a particle model. Container W holds identical separate atoms of one type. Container X holds identical molecules, each made of two atoms of the same type joined together. Container Y holds identical molecules, each made of one carbon atom joined to two oxygen atoms. Container Z holds two different kinds of molecule that are simply mixed together, in a proportion that can vary.Explain why container Z holds a mixture and not a compound.2 marks
- A teacher stirs iron filings and sulfur powder together to make a yellow-grey powder. A magnet pulls the iron filings out of the powder, leaving the yellow sulfur behind. She then heats a fresh sample of the powder strongly. It glows, and a dark grey solid called iron sulfide forms. The iron sulfide is not attracted to a magnet, and it contains iron and sulfur in a fixed ratio.Describe two differences between the mixture of iron and sulfur and the compound iron sulfide.2 marks
- A student hypothesises that air is a mixture and not a compound. She traps 100 cm³ of dry air in a gas syringe and passes it slowly back and forth over heated copper. The copper turns black. When the apparatus has cooled, the syringe reads 79 cm³. A lit splint placed in the remaining gas goes out.Calculate the percentage of the air that reacted with the copper, and state what this shows.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).