Double Award - Science for GCSE

Chapter 14

Particles, reactions and equations

1.	Classifying materials Materials and substances can be classified in different ways. As naturally-occurring and man-made. As elements, compounds and mixtures. As solids, liquids and gases. Solids, liquids and gases are called the three states of matter. We can change the state of a substance by heating it or cooling it. Different substances change state at different temperatures.
2.	The kinetic theory of matter tells us that substances are made up of incredibly small moving particles. Small particles move faster than larger particles at the same temperature. As the temperature rises, the particles have more energy and move around faster. In a solid - The particles are very close with strong forces between them. (Solids have a high density and they cannot be compressed.) - The particles can only vibrate about fixed positions. (Solids have a fixed volume and a fixed shape and they cannot flow.) In a liquid - The particles are further apart and the forces between them are not as strong as in solids. (Liquids have a medium density and they can be compressed slightly.) - The particles can roll around each other. (Liquids flow easily, they can change shape but keep a fixed volume.) In a gas - The particles are very far apart and the forces between them are almost zero. (Gases have low densities and they can be compressed a lot.) - The particles move very fast and fill all the space they can find. (Gases flow very easily and fill the whole of their container.)
3.	Atoms, ions and molecules In different substances, the particles are either atoms, molecules or ions. Atoms, ions and molecules can be represented using symbols for different elements (e.g. the symbol for oxygen is O and that for copper is Cu). In copper the particles are copper atoms. In water the particles are water molecules. In salt (sodium chloride), the particles are sodium and chloride ions .
	An atom is the smallest particle of an element. A molecule has two or more atoms chemically joined together. An ion is formed from an atom by the loss or gain of one or more electrons.
4.	Compounds When non-metals react, they form compounds containing molecules e.g. , When metals react with non-metals, they form compounds containing ions, e.g. sodium chloride (salt) , copper sulphate . All metal ions are positive and all non-metal ions are negative. Most metal ions have a charge of 2+. The only common metal ions with a charge of 1+ are . (Remember this by saying .) The only common metal ions with a charge of 3+ are . (Remember this by saying )
5.	Relative atomic masses Atoms of carbon are given a relative atomic mass of 12. The relative atomic masses of other atoms are then obtained by comparison with carbon. The relative atomic mass of oxygen is 16.0 and that of copper is 63.5. That is . These values are sometimes just written as . Using relative atomic masses, it is possible to calculate relative formula masses. e.g. the relative formula mass of carbon dioxide The relative atomic mass of an element in grams is sometimes called one mole. Finding formulas Using relative atomic masses and knowing the masses of elements which combine, it is possible to calculate the formulas of compounds. e.g. A sample of methane was found to contain 0.16 g of carbon and 0.04 g of hydrogen. What is the formula of methane?
6.	Equations A chemical equation is a summary of the reactants and products in a reaction. Balanced chemical equations can be obtained from word equations by: – writing symbols and formulas – then balancing the number of atoms of each element on both sides of the equation Calculating reacting masses Using equations and relative atomic masses, it is possible to calculate the masses of reactants and products in a reaction.