Year 9 Chemical Sciences
Predicting chemical reactions
Precipitation reactions are used to produce solid products from solutions of ionic
substances. Precipitation reactions are used in the removal of impurities during water
treatment and to produce pigments, which are often mixed with oil or
water to produce paints.
A solution of an ionic substance consists of ions separated in a solvent. For example, a
solution of lead(II) nitrate consists of lead (Pb2+ ) ions and nitrate ions (NO3– ) together with
many water molecules (the solvent).
When two solutions are mixed, a solid substance may form. This is because a positive ion
from one solution combines with a negative ion from the other solution to form an insoluble
compound. This insoluble compound is known as a precipitate.
This happens when a solution of lead(II) nitrate is added to potassium iodide—both
colourless solutions—a bright yellow precipitate of lead iodide is formed.
Determining molecular formula using the 3
step Swap and Drop method
Step 1 Identify Ions
Step 2 Swap and Drop
(circle numbers drop them down)
Step 3 Write the final formula (lose the 1’s)
Example:
Lead (ii) Nitrate
Pb 2+ NO3 1Pb 1 (NO3) 2
Pb(NO3)2
When the following ionic compounds are added together a coloured precipitate forms. Your
task is to predict the chemical equation that represents each precipitation reaction. Your
equation must include a correct molecular formula for each ofl the reactants and products.
Iron (II) Nitrate and Potassium Iodide
Magnesium Chloride and Sodium Hydroxide
Potassium Chromate and Copper Sulfate
Remember to follow the 4 step procedure shown in class
Step 1
Determine molecular formulae of reactants
Step 2
Predict the products that form after the precipitation reaction
(write a draft chemical equation)
Step 3
Determine molecular formulae of products
Step 4
Write the final chemical equation