Year 9 Chemistry – Worksheet 4
1.3. Pure substances and mixtures of substances
Chemists separate substances into different groups. First off they differentiate between pure
substances and mixtures of substances. This means that all substances are either pure substances or mixtures of substances.
A pure substance consists of only one type of substance. Through experiments it can be determined whether a substance is a pure substance or not. One of the main characteristics for pure substances are their constant properties: melting- and boiling temperatures, color, smell etc. are always the same.
Here are examples for pure substances: alcohol, coal, sulfur, chalk, salt, sugar, citric acid, iron, gold, distilled water, fine sand, …
Pure substances can be either elements1 (e.g. iron, gold, oxygen) or compounds2 (e.g. water, salt, alcohol). Elements can again be grouped into
metals (e.g. lead, aluminum, gold, silver) and non-metals (e.g. oxygen, chlorine, carbon).
A mixture of substances consists of several types of substances. Here we distinguish homogeneous and heterogeneous mixtures.
Homogeneous mixtures are mixtures of substances, whose individual components cannot be distinguished anymore. Depending on the substances involved there are three different types of homogeneous mixtures:
A mixture consisting of only gases is a gas mixture (e.g. air).
If the mixture consists of different solids, e.g. metals, it is called an alloy (e.g. brass: zinc and copper, bronze: tin and copper).
If a solid, a liquid or a gas is dissolved in a liquid, it is referred to as a solution (e.g. sugar water).
Heterogeneous mixtures are mixtures whose individual components can be distinguished by the naked eye or using equipment such as microscopes. These mixtures can also be further split up:
Bubbles of gas, e.g. carbon dioxide, in a liquid or solid are referred to as foam.
Small and spread out solid particles in a liquid are called a suspension (e.g. wall paint).
If two liquid which cannot be mixed are poured together they form an emulsion (e.g.
milk is an emulsion of mainly fat and water).
A solids mixture is a mixture of different solids (e.g. pepper and salt).
Aerosols are mixtures containing a gas. The second component can be either solid particles (smoke) or liquid (fog).
Tasks
1. Read through the text carefully and underline important parts.
2. Lay out the small cards together with your neighbor in a structure which makes sense to you.
3. Use the empty cards to write down examples of substances and fit these into your structure.
4. Add the correct English terms from the text above to the German vocab sheet.
1 An element is a pure substance which cannot be separated by any chemical means.
2 A compound is a pure substance which can be separated into its elements by chemical means.
Year 9 Chemistry – Worksheet 4
1.4. Separation processes
Your group will receive a beaker of the substance mixture named on your group card.
1. Come up with properties that the two substances are different in.
_______________________________________________________________________________
_______________________________________________________________________________
2. Which of these properties can be used to separate the two? If you are not sure, check the hint card in the green envelope on the teacher’s desk.
_______________________________________________________________________________
3. Plan an experiment in which you separate the two substances into the individual components.
List all equipment you need to perform the experiment. If you are unsure, use the hint card in the yellow envelope on the teacher’s desk.
Draw a sketch of your experimental setup. Check your setup using hint card in the red envelope from the desk.
This is the equipment we need from the teacher’s desk:
_______________________________________________________
_______________________________________________________
Sketch:
4. Check in with your teacher. Then pick up the equipment you need and perform your experiment.
But don’t use all your mixture, please leave some.
5. On the teachers board there are several different names for separation processes. Pick the fitting description for your experiment.
6. Present your results to the entire class!
