C8: Acids, bases, and salts

C8.1: The characteristic properties of acids and bases

Acids and bases are distinguished by their pH measured by the universal indicator

Acids

Are substances with a pH ranging from 0-7
Are sour and corrosive
Acidity is tested through litmus paper. When blue litmus paper is exposed to acids, the paper turns red
Acids can neutralise bases to form a salt and water
When acids react, they lose electrons to gain H+ ions
The presence of H+ ions makes a substance acidic
An example of an acid is hydrochloric acid, which completely separates into H+ and Cl- ions

Bases

Are substances with a pH ranging from 7-14
Bases that can dissolve in water are called alkali
Acidity is tested through litmus paper. When red litmus paper is exposed to acids, the paper turns blue
When alkalis react, they gain electrons to gain OH^- ions
An example of a base is sodium hydroxide, which completely separates into Na+ and OH- ions

The pH scale

It is a scale from 0 to 14, with 0 being extremely acidic, 14 being extremely basic, and 7 being neutral
All acids have pHs lower than 7 and all bases have pHs higher than 7
The lower the pH, the more acidic the substance is, and the higher the pH, the more basic the substance is

Universal Indicator

The pH level of a substance can be assessed by the universal indicator
A drop of the universal indicator can be added to a solution which changes its colour
Each colour represents a certain pH

Diagram of the universal indicator

Importance of controlling acidity in soil

Most plants grow the best at a pH of 5-8
If the acidity of the soil changes, the pH of the plant may exit this range. This causes a decrease in plant growth and less crop produced
The acidity of the soil can be affected by acid rain, the excessive use of fertilisers, and excessive breakdown of organic matter by bacteria
Crushed limestone (a base) is added to the soil to neutralise the soil
However, if too much limestone is added, the soil will become to basic and harm the plants instead

Types of Oxides

Oxides are compounds which consist of an element bonded with one or more oxygen atoms. They are different types of oxides, which include...

Oxides that react with both acids and bases and act as both acidic and basic oxides to form a salt and water
If the other reactant is a base, the amphoteric oxides act as acidic oxides, and if the other reactant is an acid, the amphoteric oxides act as basic oxides

Preparation of salts

A salt is a compound that is made up of a positive ion of the base and a negative ion of the acid
Some salts can be extracted by mining, but some need to be prepared in the lab
Salts can be insoluble and soluble in water, and different preparation methods are needed depending if the salt is water-soluble
There are different methods to prepare salts

Filtration

Used to separate a liquid and insoluble solid
When the mixture is poured on filter paper, the smaller molecules of the liquid will pass through while the larger solid will not and remain on the paper as residue.
Method

Set up the experiment so that filter paper is placed inside of the filter funnel, which should be placed in the hole of a container such as the conical flask in the diagram
Pour the mixture in the filter funnel

Crystallisation

Heat the solution and wait for the solvent to evaporate
Eventually, the maximum amount of solvent will be dissolved. This is called the saturated solution.
Dip a glass rod in the solution. When it is saturated, crystals will form on it.
The solid will become less soluble (less able to dissolved), and the crystals will therefore grow
The crystals can be collected through filtration

Distillation

Heat the solution to evaporate the pure liquid into a vapour
The vapour will condense in the condenser, cooling to turn back into a pure liquid in a beaker
The rest of the mixture will be left behind and therefore separated in the flask containing the original mixture

Precipitation

Depending on whether the salt is insoluble or soluble, you can select one of the methods above