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.

A3 The pH Scale

Describe the pH scale, running from 0-14, as a scale of strength acidity and alkalinity.

The pH scale is a scale running from 0 to 14 measuring the acidity or alkalinity of a substance; 0 being the most acidic and 14 being the most alkaline. The pH scale also comes in a scale of colours, acidic being red through brown (0-6) and alkaline being the blues and purples, a neutral substance has a pH of seven and is represented by a green colour. Below is a common pH scale showing the full range from 0 to 14 and their corresponding colours.

A2 Testing for Acids and Alkalis

Recall the colours produced by the following indicators in acidic solution and alkaline solution: litmus, phenolphthalein, methyl orange and universal indicator.

Blue Litmus Paper: Blue

•          In an Alkali: No change
•          In an Acid: Turns Red

Red Litmus Paper: Red

•          In an Alkali: Blue
•          In an Acid: No change

Universal indicator: Green solution (Yellow Paper)

•          In an Alkali: Green to Purple
•          In an Acid: Red to Green
•          In a Neutral solution: Green

Phenolphthalein: Colourless

•          In an Alkali: Violet
•          In an Acid: Colourless

Methyl Orange: Orange

•          In an Alkali: Orange
•          In an Acid: Pink/Red

A1 Testing for acidity and alkalinity

Topic 5, Physical Chemistry

Recall how to test for acidity and alkalinity, using suitable indicators

There are several indicators that can be used to test the acidity of a substance. The ones that are needed in the IGCSE course are: Litmus paper (Blue and Red), Universal indicator, Phenolphthalein and Methyl Orange. Each of these colour tests are explained in the next objective (A2)

UPDATE

From now on I will be posting every single topic objective for all the Topics that are covered this year: Topic 5, Physical Chemistry and Topic 6 Electricity and Chemistry.
If I have time over the course of the year I may also write out explanations for all the previous topics and post them here. I hope this helps everyone that needs it.

A6. Cracking

6. Describe how long-chain hydrocarbons are cracked to give more short-chain hydrocarbons using silica or alumina as a catalyst at a temperature in the range of 600-700°C


Because fractional distillation forms more long-chain molecules than the more in-demand short-chain molecules, a process called Cracking is used to break down the longer molecules.


The longer molecules are heated to a temperature in the range of 600-700°C and passed over a catalyst like alumina or silica, this breaks down the molecules into smaller ones.

A5. Too many long chains...

5. Recall that fractional distillation of crude oil produces more long-chain and fewer short-chain hydrocarbons than required.

Fractional distillation produces many more of the long chain molecules e.g. bitumen than the useful short-chain molecules, and so the molecules need to be shortened in length to produce the more in-demand short-chain molecules.

A4. Properties of fractions

4. Describe the trend in physical properties and main uses of the main fractions (focus on boiling point and viscosity)


Small molecules (i.e. Petroleum to Kerosene)

• Low boiling point
• Light in colour
• Easy to light and burn well (Flammable)
• Runny

Large molecules (i.e. Diesel to Bitumen)

• High boiling point
• Dark in colour
• Hard to burn/light (reasonably inflammable)
• Viscous or thick (Bitumen is a solid at room temperature)

Basically, the longer the molecule, the higher the boiling point, the darker the colour, the viscosity increases and they become increasingly harder to set alight.

A3. Hydrocarbons of crude oil

3. Recall the names and the uses of fractions obtained from crude oil

• Petroleum gases: (such as propane and butane): Cooking and Heating
• Gasoline (petrol): Car fuel
• Kerosene (paraffin): Aeroplane/Jet fuel
• Diesel: Fuel for larger vehicles and cars
• Lubricating oil: surprisingly... used as a lubricant
• Heavy fuel oil: used as fuel for large ships
• Bitumen: used for road surfaces

A2. Fractional distillation of crude oil

2. Describe how the process of fractional distillation can be used to separate the hydrocarbons in crude oil.


The oil is heated up until it vaporises.
It then passes through a fractionating column with a high temperature at the base, and a much lower temperature at the top
As the vapours pass up the tower they cool, and the larger molecules condense to liquid and are collected
And the smaller ones rise until they condense
Separating the crude oil into many useful oils.

A1. Crude oil is a mix of Hydrocarbons

1. Recall that crude oil is a complex mixture of hydrocarbons

Crude oil is a complex mixture of hydrocarbons.
Hydrocarbons are compounds consisting of Hydrogen + Carbon only.
These different hydrocarbons can be separated by the process of fractional distillation.