Molecules and Compounds Some elements exist as individual atoms. Molecules consist of two or more elements joined together. If more than one type of atom is in a molecule, we call it a compound .
Chemical Reactions Chemical reactions happen when the atoms in one or more molecules split up and join together in new ways. For example, hydrogen can react with oxygen :
When a reaction takes place, things happen. There may be a gas given off, a color change or a solid material appearing. There could also be changes we can’t see, such as a temperature change.
The mass of the reactants (the chemicals we start with) is always the same as the mass of the products (the chemicals we end with).
Law of Conservation of Mass
On the left are the reactants – the starting materials
On the right are the products – the new substances that are formed
An arrow connects the two sides – read as yields or produces
AgNO 3 + NaCl AgCl + NaNO 3 Read left to right like a sentence in chemical terms
4 Types of Reactions Synthesis Decomposition Single Replacement Double Replacement
Synthesis Means to put together or combine occur when two substances (generally elements ) combine and form a compound. In general: A + B AB Fe + S FeS reactant + reactant 1 product
Decomposition In this type of reaction, a substance breaks down into simpler parts when it is heated. 1 Reactant Product + Product In general: AB A + B CuCO 3 heat Cu 2 O + CO 2 For example, copper carbonate will decompose when heated :
Single Replacement In a displacement reaction, one metal ‘kicks out’ a less reactive one. The iron wants to be with the sulphate more than the copper does. We say that iron is more reactive than copper. element + compound element + compound
A + BC AC + B
Fe + CuSO 4 Cu + FeSO 4
Double Replacement Occurs when one metal trades places with another one. AgNO 3 + KCl AgCl + KNO 3 Compound + compound product + product In general: AB + CD AD + CB