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Elements, Compounds and Mixtures

Chapter 4

Yi Ling Seah

on 31 January 2013

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Transcript of Elements, Compounds and Mixtures

Elements substance Compounds Mixtures Understand what are elements, Elements, Compounds and Mixtures Chapter 4 Objectives compounds and mixtures elements, compounds and mixtures Describe the differences between Classification Elements Compounds Mixtures Pure Cannot Solid, liquid or gas Metal, or non-metal Atoms Argon O 2 O 3 Sodium Chloride CO 2 H O 2 Molecules / Ions be broken down into two or more simpler substances chemically Video: 1:17 - 2:22 represented by chemical symbols (eg. Sodium - Na) substance Pure two or more elements chemically combined in fixed ratio different properties as the elements that it is made up of not Made up of two or more substances . chemically combined homogeneous / heterogeneous salt + water oil + water Both are pure substances One type of atom Two or more elements H Compounds Both are made up of more than one type of elements Mixed in fixed proportion No fixed proportion Different physical and chemical properties as constituent elements Same chemical properties as its components Chemical reaction takes place when formed - energy involved in its formation No chemical reaction when formed - no energy involved in its formation Can be separated into elements by chemical reactions eg electrolysis Can be separated into its components by physical methods eg filtration involves an energy change similar chemical properties as its components Metals Non-Metals Usually high melting / boiling points Usually low melting / boiling point Good conductors of heat and electricity Poor conductors of heat and electricity Malleable and ductile Brittle and non-ductile Shiny and lustrous Dull in appearance Sonorous Non-sonorous Physical differences Chemical differences (Will be covered after learning structure of atoms and bonding) Proton Number metalloids (Behave like both metals and non-metals) Solid at r.t.p (except Hg) Usually gases / liquids at r.t.p Molecules An atom is the simplest substance of an element which can not be further broken down by chemical process. Mon atomic A molecule is made up of two or more atoms chemically combined together. Di Tri atomic atomic Eg. Air (oxygen, carbon dioxide, nitrogen, other gases) Mixture can exist in various ways two elements one element, one compund two compound Hydrogen & Neon Carbon Dioxide & Water Hydrogen & Ammonia The ratios of the components are not fixed. S P E C eparation roperties nergy Change omposition Mixture The components of a mixture can be separated by physical methods, e.g. filtration, distillation or chromatography. No chemical reaction takes place when a mixture is formed — usually there is not fixed physical little or no energy change Alloy An alloy is a mixture of metals with other elements (usually metals but sometimes non-metals such as carbon). Alloys are widely used because they tend to be stronger than pure metals. Pure Metal Alloy Examples of Alloy Steel : iron, carbon
Stainless steel : iron, chromium, nickel, carbon
Brass : copper, zinc
Bronze : copper, tin
Duralumin : aluminium, copper, magnesium
Solder : lead, tin Ions are electrically charged particles (either positive or negative) Example: NaCl (Na and Cl ) + - Chemical Formula of a Compound The chemical formula states
the types of atoms
the ratio of the different atoms H O 2 Hydrogen Oxygen Number of Hydrogen atoms (two) Thus, ratio of H:O is 2:1 and this is fixed Naming Compound 1. A compound made up of two elements has a name that ends in -ide • Sodium chloride — elements sodium and chlorine 2. A compound that contains hydroxide ions , OH– (a negatively charged ion made up of oxygen and hydrogen) is named a hydroxide. • Potassium hydroxide — potassium ions and hydroxide ions 3. A compound that contains a negatively charged polyatomic ion containing oxygen usually has a name ending in –ate. Carbon dioxide — elements carbon and oxygen • Copper(II) sulphate — contains oxygen atoms in the sulphate ion CuSO SO 4 4 2- (sulfate ion) Writing Chemical Formula 1. For many compounds that contain both metallic and non-metallic elements, the symbol of the metallic element is written first. 2. The number of atoms is written as a subscript, to the right of the atom’s symbol. • Water (H O, not H2O or 2HO or HO) Calcium oxide (CaO), Magnesium carbonate (MgCO ) 3 2 2 3. It is not necessary to write the subscript ‘1’. Calcium oxide (CaO, not Ca O ) 1 1 4. The oxygen atom is usually written at the end of the formula. Carbon Monoxide (CO, not OC) Pb(NO ) 3 2 How many atoms of each element are there? Name the compounds. MgO CaCO 3 Heat can be used to break down compounds into elements or simpler compounds. Such a chemical reaction is called thermal decomposition. Mercury(II) Oxide Oxygen Compounds 2
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