Monday Revision test 1 – Solubility - Test and mark scheme

Our test on Monday was drawn from some past paper questions on solubility. These questions were variously gathered from past papers, including some paper 1, 2 and 6 and were designed to test your knowledge on solubility from different angles.

The results you got from this test will tell you how much you know and how much you need to learn. To help you fill in the gaps I’ve posted the paper here and the mark scheme, complete with comments on where most of you dropped marks.

Test paper
Page 1
Page 2
Page 3
Page 4
Page 5
Page 6

Mark Scheme
Click and save

Importantly there are a few major gaps I picked up, where all of you are slipping up.

The first is to do with the formation of crystals from solution.

We made quite a few crystals during Acids, bases and salts, and during that unit I tried to get you to make sure that the crystals were made properly.

When making crystals from a solution there are some stages we must follow

Filter our mixture to remove any excess residue that has not reacted. Usually our salts are made by adding a compound or metal to an acid. Let’s take Magnesium and Hydrochloric acid for example. What is left in the filter paper is any magnesium that has not reacted. The salt that is made remains in solution (Magnesium chloride) to go through the filter paper
In order to get the Magnesium Chloride (MgCL2) out of solution we have to evaporate it. SLOWLY. This is NOT in order to make big crystals (although it is true that the slower the crystals are cooled the bigger the crystals made) but it is in order to make crystals AT ALL
The solution that gets through the filter (the filtrate) is heated on a low flame until crystals are seen to form around the top of the solution. Then take it OFF the flame and leave it to evaporate.

Explanation
I didn’t teach you this the first time round as it’s not in the syllabus but now that we’ve finished the syllabus we’ll do a practical on this on Sunday.

Crystals are often held together by water. This so-called “Water of crystallisation” is loosely bonded to the molecules of the crystal and this loose bonding can be seen in the formula for example in the question on Page 3 of the test. MgSO4.7H2O shows that each MgSO­4 molecule is surrounded by 7 water molecules.

These water molecules can be removed quite easily by heating the crystal, which causes the crystal to break down to powder Magnesium Sulphate, releasing the water as steam.

In Sunday’s lesson we will see the effect that heating away the water of crystallisation has on Copper Sulphate crystals.

If we heat the crystals too much the water of crystallisation evaporates and we don’t make crystals at all.

The reason we heat it is to speed up the process of evaporation, but we don’t want to heat it too far.

Ideally, we want to evaporate off enough water to leave us with a warm, saturated solution. As we know, warm water holds more solute than cold. If we can make a warm saturated solution, when it cools down the crystals in solution will crystallise out, with enough available water to hold the crystals together.