Elements 1 The nature of substances and chemical Topic overview reactions Elements Elements are substances that cannot be broken down into simpler substances by chemical means and are the basic building blocks of all substances. Elements are made up of only one type of atom. Atoms consist of a nucleus containing protons and neutrons, surrounded by electrons. The number of protons in an element’s nucleus is its atomic number. The mass of one atom of an element is called its relative atomic mass. © Hodder & Stoughton 2017

1 The nature of substances and chemical Topic overview reactions Chemical formulae Every element is given a chemical symbol. The chemical formula of compounds (which contain two or more elements) indicates which elements are present and how many atoms there are of each. © Hodder & Stoughton 2017

Compounds 1 The nature of substances and chemical Topic overview reactions Compounds Compounds are substances made of two or more different types of atom that are chemically joined. Compounds have completely different properties to their constituent elements. Compounds can be ionic or covalent. Ionic compounds are made up of electrically charged particles called ions. © Hodder & Stoughton 2017

Compounds (continued) 1 The nature of substances and chemical Topic overview reactions Compounds (continued) Chemical equations use chemical formulae to show the reactants and products in a chemical reaction. The number of atoms of each element on both sides of an equation must be balanced. The formulae of ionic compounds can be established given the formulae of the ions they contain. © Hodder & Stoughton 2017

Relative atomic mass and relative formula mass 1 The nature of substances and chemical Topic overview reactions Relative atomic mass and relative formula mass Relative atomic mass and relative molecular (formula) mass are used to compare the masses of different elements and different compounds. The mass of an atom of an element is standardised relative to an atom of 12C, which is set at 12. The percentage composition of compounds can be calculated from the chemical formula. © Hodder & Stoughton 2017

Mixtures 1 The nature of substances and chemical Topic overview reactions Mixtures Atoms/molecules in mixtures are not chemically joined. Mixtures can be separated by physical processes such as filtration, evaporation, chromatography and distillation. Filtration separates components on the basis of their size. Evaporation separates a liquid from a solid by evaporating the liquid. Chromatography separates according to their solubility in a solvent, using their Rf value to identify them. Distillation separates liquids which have different boiling points. © Hodder & Stoughton 2017

1 The nature of substances and chemical Topic overview reactions Chemical reactions Chemical reactions involve the rearrangement of the atoms present in the reactants to form one or more products. The products have the same total number of each type of atom as the reactants. Colour changes, temperature changes and effervescence can be used as evidence that a chemical reaction has taken place. If heat is given out, the reaction is described as exothermic. If heat is absorbed, the reaction is described as endothermic. © Hodder & Stoughton 2017

1 The nature of substances and chemical Topic overview reactions Chemical equations Chemical reactions can be represented using word equations. Chemical reactions can be represented using balanced chemical equations, where the total relative mass of reactants and products is equal. © Hodder & Stoughton 2017

Chemical calculations 1 The nature of substances and chemical Topic overview reactions Chemical calculations The formula of a compound can be calculated from reacting mass data. The masses of reactants or products can be calculated from a balanced chemical equation. © Hodder & Stoughton 2017

Calculating the yield of a chemical reaction 1 The nature of substances and chemical Topic overview reactions Calculating the yield of a chemical reaction A chemical equation allows you to calculated the theoretical yield of products. Chemical reactions are never 100% efficient so the actual yield is always less than the theoretical yield. Knowing the theoretical yield and the actual yield allows the calculation of the percentage yield. © Hodder & Stoughton 2017

The Avogadro constant and the mole 1 The nature of substances and chemical Topic overview reactions The Avogadro constant and the mole The Avogadro constant represents the number of atoms in 12 grams of carbon-12. The concept of the mole can be used to convert an amount of a substance in grams into moles and vice versa. © Hodder & Stoughton 2017