A13 Solubility Rules

1. A salt is soluble if it contains any of the following ions:

NH_4­⁺, Li⁺, Na⁺, K⁺ or NO₃^-

Examples

LiCl, Na₂SO₄, KBr, Ca(NO₃)₂

2. SO₄^2- (sulphate) salts. Salts with SO₄^2-  are soluble, but not if the positive ion is:

Ba²⁺, Pb²⁺, Hg²⁺, Ca²⁺

Examples

Soluble	Insoluble
MgSO4	BaSO4 PbSO4

3. Other Salts. Other salts containing CO₃^2-, PO₄^3-, S^2- and OH^- are not soluble

Examples

Soluble	Insoluble
Na2CO3 K2S	CaCO3 CuS

A10 Insoluble Salts

Describe how to prepare insoluble salts with the precipitation method.

A soluble salt is an ionic compound that dissolves in water, an insoluble salt is one that does not dissolve in water

When the two colourless solutions KI and PbN are mixed together they form a yellow precipitate, this is the salt PbI. When know that this salt is insoluble because no matter how much we stir the solution it will not dissolve to form a clear liquid, the Lead Iodide will settle on the bottom.

A9 Soluble Salts

Describe how to prepare soluble salts using;

1. Metal + Dilute acid
2. Metal oxide + Dilute acid
3. Alkali + acid

and experimental techniques such as filtration, evaporation, crystallisation and titration.

In the diagram below, an experiment is being shown:

1. Add 50cm³ Sulphuric acid (H₂SO₄) and 10g Copper Oxide (CuO) into a conical flask.
2. Apply a small amount of heat and stir/shake
3. Filter the contents of the flask into another conical flask
4. Place liquid into a evaporating dish and boil off some of the water and leave the rest to crystallise.
5. What you will result with is a salt, this salt is Copper Sulphate (CuSO₄)

A8 Hydrogen Replacement

Describe a salt as a substance formed when the hydrogen in an acid is replaced.

We know that an acid only shows its acidic properties when it has dissociated in water, if you add a base to this “acid” we know that water and a salt is formed. What this means is the lone H⁺ are now being used to form the H₂O; they are being replaced into a new molecule that results in the formation of salt with the molecules not used to form water.

A7 Salts

Name the salts formed by reacting metals, metal oxides and metal carbonates with HCl, H₂SO₄ and HNO₃

A chemical reaction happens if you mix together an acid and a base.

The equation of the salt is given by the metal from the metal oxide and the chemicals in the acid, minus the Hydrogen:

When mixing a carbonate with an acid an extra element is formed, this is Carbon Dioxide (CO₂)

A6 Proton Transfer

Define acids and alkalis in terms of proton transfer, acids as a source of H­⁺ and alkalis a source of OH^-

Acidity is caused by the presence of H⁺ ions, these ions form when a chemical dissociates in water. What this means is that the water separates the Chemical i.e. HCl into H⁺ ions and Cl^- ions, as shown in the below diagram. To do this the HCl gas is bubbled through the water

It is the presence of these H⁺ ions that make a substance acidic, the more H+ ions in relation to the other ions, the more acidic the solution will be.

This is why pH stands for Percentage Hydrogen, or Part Hydrogen, meaning that it shows how much Hydrogen there is in the substance (Hence why the H is capitalised because it is the elemental symbol)

Alkalinity is basically the same principle, except instead of dealing with H⁺ ions we are now talking about OH^- ions. So for example if we take Sodium Hydroxide and place it in water we can see that it dissolves and dissociates to form OH^- ions and Na⁺ ions, as shown in the diagram below.

Bases

Bases are any Metal oxide, hydroxide or carbonate, for example copper oxide. What you should know is that all alkalis are bases, every single one, but as the diagram below should explain, not all bases are alkalis.

So what’s the difference between a base and an alkali?

Well an alkali is a soluble base; this means a base that will dissolve in water. You can do a simple experiment to show this. Place Calcium carbonate, Copper oxide and Sodium hydroxide into three separate test tubes, half fill the test tubes with water, observe. What you should see is that only the Sodium Hydroxide dissolves, this means only this one is an Alkali.

A5 Universal Indicator

Describe the use of universal indicators to measure pH of a solution

Universal indicator is perhaps the main way in which the pH of a solution is tested, this is because not only does it tell you whether the solution is an acid, alkali or neutral it also tells you how acidic or alkaline the solution is.

As you know each number on the pH scale is given a colour, ranging from deep red to deep purple. These colours represent the colours universal indicator goes when presented with such a chemical. For example putting universal indicator into an acid solution we may find that the indicator turns an orangey red and comparing with our pH scale we can easily see that this chemical has a pH of roughly 2. And obviously this works for alkalis too.

You must remember that Universal indicator solution is green on its own so when presented with a neutral solution it will stay green. But Universal indicator paper is often a yellow colour, so when presented with a neutral colour will change to be green.

A4 Solutions

Describe solutions which have a pH value less than 7 as acidic, those with a pH value of more than 7 as alkaline and those with a pH of 7 as neutral.

Any solution with a pH value that is less than 7 is classed as acidic, the further away from 7 it is, the more acidic it is, i.e. pH 2 is more acidic than pH 5.

Any solution with a pH that is greater than 7 is called an alkali, and as with acids, the further away from the number 7 it is the more alkaline it is i.e. pH 14 is much more alkaline than pH 9

A solution with a pH of 7 is neutral, there aren’t many neutral substances, but pure water is a good example, it is neither acidic nor alkaline.