Elements,Compounds and Mixtures

Transcription

1 BASIC CONCEPTS: Elements,s and s 1. The smallest fundamental particle of an element that retains the chemical properties of the element is called an atom. 2. A pure substance that cannot be split up into two or more simpler substances by physical processes or by chemical processes is called an element. 3. A pure substance that contains two or more elements chemically joined together is called a compound. 4. A molecule is a group of two or more atoms that are chemically joined together. 5. A diatomic molecule is a molecule with two atoms. 6. The chemical formula of a compound is the notation which shows the relative numbers of atoms of the different elements present in the compound. 7. A mixture is combination of two or more substances that are not chemically joined together. ANALYSE: Describe the differences between elements, compounds and mixtures. State the symbols of the elements and formulae of the compounds. 1.1 Classification of Substances: Matter can be classified in terms of solid, liquid and gas. Matter can also be classified in terms of element, compound or mixture. Matter Pure substance Element Element Element Element Metal Non-metal 1.2 Elements:

2 1. An element is a pure substance that cannot be split up into two or more simpler substances by physicalprocesses or by chemical processes. 2. Each element has a symbol.the table below gives the names and symbols of some common elements. Element Carbon Symbol C Oxygen Hydrogen Aluminium Chlorine Copper Manganese O H Al Cl Cu Mn 3. Most elements are metals. Examples of metals are aluminium (Al), copper (Cu) and zinc (Zn). Only 18 elements are non-metals. Examples of non-metals are sulfur (S) and phosphorus (P). 4. The atom is the smallest particle of an element that has the chemical properties of the element. 5. A molecule is a group of two or more atoms that are chemically combined. Example: An ozone molecule is made up of three oxygen atoms chemically combined. A hydrogen molecule is made up of two hydrogen atoms chemically combined. 6. Many non-metals exist as diatomic molecules. For example, hydrogen, oxygen and chlorine exist as diatomic molecules. The chemical formulae of these gases are H, O and C respectively. 7. Metals and non-metals have different physical and chemical properties. The table below shows the differences in physical properties of metals and non-metals Property Metals Non-metals Melting and boiling points High (except the alkali metals; Low (except carbon) see Section 1.3 Group I elements). Physical state Solids at room temperature (except mercury) Either gases, volatile liquids or solids with low melting point at room temperature Electrical conductivity Good conductor Poor conductor (except graphite) Thermal conductivity Good conductor Poor conductor (except diamond and graphite) Malleability ( hammering into different shapes without Malleable Not malleable, brittle if solid (easily broken when hammered)

3 breaking) Ductility ( drawing into wires) Ductile Not ductile TIP FOR STUDENTS: In the exam, you will be given a Periodic Table. From the position of the element in the Periodic Table, you will know which element is a metal and which element is a non-metal. COMMON ERROR h metals and non-metals exist as molecules. ACTUAL FACT All metals exist as atoms. Most non-metals exist as molecules. 1.3 s: 1. A compound is a pure substance that contains two or more elements chemically combined in a fixed ratio. 2. A compound has different properties from the elements that form it. Example: Water (H O) is a compound formed by joining together two atoms of hydrogen and one atom of oxygen (ratio of hydrogen atoms to oxygen atom in water is always 2 :1) 2 hydrogen atoms 1 oxygen atom Hydrogen is a colourless gas that burns with a pop sound. Oxygen is a colourless gas that supports burning. 1 molecule of water Water is a colourless liquid that does not burn. 3. A compound has a fixed composition by mass. For example, a water molecule (H O) contains 11.1% hydrogen and 88.9% oxygen by mass. 4. A compound can be represented by a chemical formula. 5. A chemical formula of a compound is written by putting together the chemical symbols of the elements that make up the compound. 6. The chemical formula of a compound tells us (a) the types of atoms present in the compound, (b) the ratio of different atoms present in the compound.

4 COMMON ERROR Only compounds, but not elements, exist as molecules ACTUAL FACT Many non-metallic elements exist as molecules. For example, hydrogen and oxygen exist as H and O molecules respectively. 1.4 s: 1. s are formed when two or more substances are added together without chemical bonds being formed. 2. The components of a mixture are not fixed. They can be present in any proportion. Each component retains its chemical and physical properties. 3. The diagram below shows the arrangement of particles in (a) a mixture of two elements, (b) a mixture of two compounds, (c) a mixture of one element and one compound (a) A mixture of two elements, (b) A mixture of two (c) A mixture of one element e.g neon (Ne) and compounds, e.g. water and one compound, hydrogen (H ) vapour (H O) and carbon e.g. hydrogen (H ) and dioxide (CO ) ammonia (NH ) 4. Some examples of different types of mixtures are given in the table below. Element Element compound Element compound Example Alloys are mixtures of metals. Brass is a mixture containing copper and zinc. Hydrochloric acid is a mixture of hydrogen chloride in water. Hydrogen chloride and water are compounds. Air is a mixture of nitrogen, oxygen and carbon dioxide. Nitrogen and oxygen are elements while carbon dioxide is a compound. 5. A mixture can sometimes be converted into a compound. For example, when a mixture of iron and sulfur is heated, iron (II) sulfide is obtained. 6. Many compounds break down on heating to form elements or compounds. Examples:

5 (a) On heating mercury (II)oxide,the elements mercury and oxygen are produced. (b) Similarly, calcium carbonate breaks down to form calcium oxide and carbon dioxide on heating. heat Calcium carbonate calcium oxide carbon dioxide (compound) (compound) (compound) This type of reaction is called thermal decomposition. 7. The difference between a mixture and a compound are summarised below. Separation of components Composition Chemical properties Melting and boiling points Energy change Can be separated by physical methods. For example, a mixture of iron and sulfur can be separated by a magnet. Iron (but not sulfur) is attracted by the magnet The proportion of constituents can vary Same as the chemical properties of the constituents in the mixture. Vary according to the composition. No chemical reaction take place when a mixture is formed usually there is little or no energy change. Can only be separated by chemical reactions or by using electricity. Contains elements in fixed proportion by mass. Physical and chemical properties are different from those of its elements. Fixed melting and boiling points. A chemical reaction takes place when a compound is formed usually there is an energy change, e.g. the reactants get hot or cold. Common Error Tap water is a pure compound. Actual Facts Tap water is not a pure compound. It is a mixture that contains water and dissolved substances such as chlorine and other minerals.