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Chemistry Compounds and Mixtures 11of of20 36 © Boardworks Ltd 2005 2004 Contents 8F Compounds and Mixtures Comparing elements and compounds Mixtures Word equations Summary activities 12of of20 36 © Boardworks Ltd 2005 2004 Properties of elements and compounds Why is it safe to put sodium chloride on fish and chips… …but not safe to use sodium and chlorine? 13of of20 36 © Boardworks Ltd 2005 2004 Atoms in elements and compounds Elements are materials made up of one type of atom only. The element, hydrogen, exists as molecules. Each hydrogen molecule is made up of two hydrogen atoms joined together. H H Compounds contain two or more different types of atom. The compound, water, exists as molecules. Each water molecule consists of two hydrogen atoms joined to one oxygen atom. 14of of20 36 © Boardworks Ltd 2005 2004 Element or compound? 15of of20 36 © Boardworks Ltd 2005 2004 Making a compound – water A compound is made when atoms of different elements react and join together. For example, water is produced from the chemical reaction between hydrogen and oxygen. hydrogen + oxygen water H H + O O H H Two hydrogen molecules react with one oxygen molecule to produce two molecules of water. Why does water have different properties to its elements? 16of of20 36 © Boardworks Ltd 2005 2004 The Pop Test 17of of20 36 © Boardworks Ltd 2005 2004 Properties of a compound – carbon dioxide A compound has different properties to the elements from which it is made because the atoms are joined differently. carbon + oxygen carbon dioxide Colourless gas in which many substances burn. Colourless gas used in fizzy drinks and fire extinguishers. + Black solid used as fuel. C 18of of20 36 + O O © Boardworks Ltd 2005 2004 Element or compound? • Look at the pictures below - is it an element or a compound? Element 19of of20 36 Element Compound Element Compound © Boardworks Ltd 2005 2004 Element or compound? • How about this - element or compound? Actually this is neither an element or a compound - it is a mixture! 1 10ofof20 36 © Boardworks Ltd 2005 2004 Contents 8F Compounds and Mixtures Comparing elements and compounds Mixtures Word equations Summary activities 1 11ofof20 36 © Boardworks Ltd 2005 2004 What is a mixture? A mixture contains two or more substances that are mixed together but have not reacted with each other. Sea water is a mixture of salts, water and other substances. A mixture is not the same as a compound: 1. The proportions of the substances in a mixture are not fixed. 2. The properties of a mixture are often an “average” of the properties of its ingredients (e.g. a mixture of a black and white powder is grey). 3. The substances in a mixture are just mixed, not chemically joined, and so it is usually quite easy to separate the ingredients (e.g. it is easy to get salt from sea water). 1 12ofof20 36 © Boardworks Ltd 2005 2004 Mixture Example E.g. a mixture of iron powder and sulphur powder will show properties of both iron and sulphur! This means our mixture will contain grey magnetic bits of iron + yellow bits of sulphur 1 13ofof20 36 © Boardworks Ltd 2005 2004 A compound is not a mixture The atoms in a mixture of hydrogen gas and oxygen gas, which have not reacted with each other, look like this… The atoms in water, the compound made when hydrogen and oxygen react and their atoms become chemically joined to each other, look like this… 1 14ofof20 36 © Boardworks Ltd 2005 2004 Element, compound or mixture? 1 15ofof20 36 © Boardworks Ltd 2005 2004 Element, compound or mixture? 1 16ofof20 36 © Boardworks Ltd 2005 2004 Contents 8F Compounds and Mixtures Comparing elements and compounds Mixtures Word equations Summary activities 1 17ofof20 36 © Boardworks Ltd 2005 2004 Writing a word equation A word equation can be used to describe any chemical reaction, i.e. any process in which atoms become joined in different ways. The steps for writing a word equation are: 1. On the right-handside, put the name of the reactant(s). If there are two or more reactants, link them with a + sign. 2. In the middle, write down an arrow (). 3. On the right-handside, put the name of the product(s). If there are two or more products, link them with a + sign. 1 18ofof20 36 © Boardworks Ltd 2005 2004 What is the word equation? Lead reacts with oxygen to form lead oxide. What is the word equation for this reaction? lead + oxygen lead oxide Why is lead oxide so different to both lead and oxygen? The lead and oxygen don’t mix, they react to form lead oxide. This means that the lead and oxygen atoms in the product are joined differently to the atoms in the reactants . 1 19ofof20 36 © Boardworks Ltd 2005 2004 What is the word equation? Write out word equations for the following chemical reactions. 1. Limestone (calcium carbonate) is heated to make calcium oxide and carbon dioxide. calcium carbonate calcium oxide +carbon dioxide 2. Magnesium reacts with hydrochloric acid to make magnesium chloride and hydrogen. magnesium + hydrochloric acid magnesium chloride + hydrogen 3. Methane is burnt to make carbon dioxide and water. methane + oxygen carbon dioxide + water 1 20ofof20 36 © Boardworks Ltd 2005 2004 Contents 8F Compounds and Mixtures Comparing elements and compounds Compound names and formulae Word equations Mixtures Summary activities 1 21ofof20 36 © Boardworks Ltd 2005 2004 Glossary atom – The smallest particle that can exist on its own. boiling point – Temperature at which a pure liquid becomes a gas. compound – Substance made up of two or more different types of atoms that are chemically joined together. element – Substance made up of only one type of atom. formula – The symbols and numbers that represent the ratio of different atoms in a substance. immiscible – Liquids which do not mix. miscible – Liquids which do mix. mixture – Two or more substances that are mixed but not chemically joined together. 1 22ofof20 36 © Boardworks Ltd 2005 2004 Anagrams 1 23ofof20 36 © Boardworks Ltd 2005 2004 True or false? 1 24ofof20 36 © Boardworks Ltd 2005 2004 Multiple-choice quiz 1 25ofof20 36 © Boardworks Ltd 2005 2004