2.34 Defining salts
Common "table" salt ( sodium chloride) is found dissolved in large quantities in seawater. Sodium Chloride is just one example of the many compounds which can be called salts. Most salts are crystalline ionic compounds
A salt is defined as :
A compound resulting from a chemical reaction of an acid, in which the acid's hydrogen ions are replaced by other (positive) ions .
Assumed background knowledge
2.34 Activity 1. The best solution?
Students should:
- 2.34 know the general rules for predicting the solubility of ionic compounds in water
Different Ions have different solubilities . When two soluble ions are combined to form a salt the salt will remain in solution. Thus a soluble salt is formed
When two insoluble ions mix to form a salt, the salt forms as a precipitate.
Pretty precipitation:
The video the first reaction shows nitrate reacting with potassium iodide solution - the result is a yellow precipitate of lead iodide. We can write the following equation:
2.34 Activity 2. Saturation points
Use the animation below to find out the concentration ( in Mol/dm3)at which each solute forms a saturated solution.
Collect your results in a suitable table .
PLEASE NOTE : The units for concentration are : Molarity (or M) = mol/liter = mol/dm3
2.34 Activity 3. Predicting precipitates
Use the solubility table to predict what you would observe when you mix the following pairs of solutions:
2.35 Activity 4. Neutralisation
Students should:
- 2.35 understand acids and bases in terms of proton transfer
- 2.36 understand that an acid is a proton donor and a base is a proton acceptor
- 2.38 know that metal oxides, metal hydroxides and ammonia can act as bases, and that alkalis are bases that are soluble in water
2.37 - 2.39 Activity 5. Reacting Acids
Students should:
- 2.37 describe the reactions of hydrochloric acid, sulfuric acid and nitric acid with metals, bases and metal carbonates (excluding the reactions between nitric acid and metals) to form salts
Watch the animation carefully and write word equations giving the names the reactants and products for the four reaction types shown:
2.37. Metal plus Acid (MASH)
This demonstration shows how a lot of ( explosive) hydrogen gas can be made from an acid and a reactive metal such as magnesium.
What does the demonstration tell us about the relative reactivity of magnesium and hydrogen?
What sort of reaction is this?
2.39, 2.40, 2.42 Activity 6. Making Soluble Salts
Students should:
- 2.39 describe an experiment to prepare a pure, dry sample of a soluble salt, starting from an insoluble reactant
- 2.40C describe an experiment to prepare a pure, dry sample of a soluble salt, starting from an acid and alkali
- 2.42 practical: prepare a sample of pure, dry hydrated copper(II) sulfate crystals starting from copper(II) oxide
Study the video and use it to construct a sequence of instructions to describe how to make a soluble salt :
2.41, 2.43 Activity 7. Making Insoluble Salts
Students should:
- 2.41C describe an experiment to prepare a pure, dry sample of an insoluble salt, starting from two soluble reactants
- 2.43C practical: prepare a sample of pure, dry lead(II) sulfate
Watch the video then complete the missing word exercise below.