Chemistry Unit 8: Acids, Bases and Salts

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Unit 8.1 The characteristic properties of acids and bases

Before I start going into the syllabus, let's define acids and bases clearly!

Acids: a substance that dissolves in water to produce hydrogen

Base: a substance that neutralises an acid

A lot of the time, you'll hear the word alkali. An alkali is a type of base that can dissolve in water. An alkali is a base, but a base is not an alkali.

1. Describe neutrality and relative acidity and alkalinity in terms of pH (whole numbers only) measured using full-range indicator and litmus.

When talking about pH, you've got the whole pH scale in mind, where you identify the pH according to a certain colour. You can use litmus paper or universal indicator (UI) to check a substance's pH. 

Neutral: pH 7, turns UI green, turns litmus paper purple

Acid: low pH (less than 7), turns UI red-yellow, turns litmus paper red

Alkali: high pH (greater than 7), turns UI blue-purple, turns litmus paper blue

Below is an image displaying the different colours universal indicator turns according to pH levels, and also what substances are of that pH level.

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2. Describe the characteristic reactions between acids and metals, bases (including alkalis) and carbonates.

Here is a handy-dandy four-step system to this:

● acid + alkali --> salt + water

● acid + metal --> salt + hydrogen

● acid + carbonate --> salt + water + carbon dioxide

● acid + oxides (bases) --> salt + water

3. Describe and explain the importance of controlling acidity in the environment (air, water and soil)

Soil: Soil is used to grow crops, so it is important for it to be neutral. If it happens to be too alkaline or acidic, the crops tend to grow poorly. Acidity is usually the problem for soil, so a base can help neutralise it. Bases include limestone, slaked lime, or quick lime (more on that later!).

Water: Factory waste is often acidic, and it can leak into water. To prevent this from happening, it needs to be neutralised. Again, slaked lime is used for this. 

Air: Burning fossil fuels releases gases into the air, such as nitrogen oxides and sulphur dioxide. They react with water and air, leading to acid rain. As you can tell by the name, it's not that great; it causes buildings to erode and will negatively affect soil and water. 

Unit 8.2 Types of oxides

1. Classify oxides as either acidic or basic, related to metallic and non-metallic character of the other element.

First, let's define oxide: it is a compound made up of oxygen and another element. Next, acidic oxide: a compound of oxygen and a non-metal, most commonly a gas. Lastly, basic oxide: a compound of oxygen and a metal.

Here are some examples of each type of oxide:
acidic oxide: sulphuric acid (H2SO4), nitric acid (HNO3)
basic oxide: sodium hydroxide (NaOH), copper oxide (CuO)

So now that you know what the different oxides are, it's easier to classify them into groups. 

Acidic oxides:
● react with water to give and acid
i.e carbon dioxide + water --> carbonic acid
     CO2 + H2O --> H2CO3
● react with bases to form a salt
i.e. sulphuric acid + copper oxide --> copper sulphate + water
     H2SO4 + CuO --> CuSO4 + H2O

Basic oxides:
● react with acids to form a salt and water
i.e. copper oxide + hydrochloric acid --> copper chloride + water

     CuO + 2HCl --> CuCl2 + H2O


2. Further classify some oxides as neutral, given relevant information.

Some oxides are neither basic or acidic, so they are neutral oxides. They don't react with acids or bases either. Examples of neutral oxides include nitrous oxide (N2O) and carbon monoxide (CO). 




Unit 8.3 Preparation of salts

1. Describe the preparation, separation and purification of salts using techniques selected from section C2.1 and the reactions specified in C8.1.

Preparing the salt:
1. Add an excess of your carbonate to your acid
2. Test the pH with U.I. paper to check if the solution is neutral
3. Filter the solution to get rid of the excess carbonate
4. Heat the liquid until most of it has evaporated and you are left with salt crystals

Separating the salt:
Let's say we have a mixture of salt and pebbles
1. Add water to your mixture so the salt dissolves
2. Filter the mixture
3. Evaporate the liquid

Purifying the salt:
If it's an insoluble salt: filtration
If it's a soluble salt: distillation

2. Suggest a method of making a given salt from suitable starting materials, given appropriate information. 

Remember that handy-dandy four-step system from above? Here, it comes to use.

Acid + alkali --> salt + water
Hydrochloric acid + sodium hydroxide --> sodium chloride + water
HCl + NaOH --> NaCl + H2O
1. Add 30cm2 of sodium hydroxide to a flask
2. Add two drops of the indicator phenolphthalein 
3. Add the hydrochloric acid to the flask
4. The solution will turn colourless when neutralised
5. Heat the solution to evaporate the liquid and obtain the salt

Acid + metal --> salt + hydrogen
Sulphuric acid + zinc --> zinc sulphate + hydrogen
H2SO4 + Zn --> ZnSO4 + H2
1. Add zinc to a flask of the acid
2. When the zinc dissolves, hydrogen bubbles will appear
3. Filter the solution to remove the excess zinc
4. Heat the solution to evaporate the liquid and obtain the salt

Acid + base --> salt + water
Sulphuric acid + iron(II) oxide --> iron sulphate + water
H2SO4 + FeO --> FeSO4 --> H2O
1. Add and excess of iron oxide to the sulphuric acid
2. Filter the solution to remove the excess iron oxide
3. Heat the solution to evaporate the liquid and obtain the salt




Unit 8.4 Identification of ions and gases

1. Use the following tests to identify aqueous cations, anions, and gases.

Aqueous cations:
● ammonium
● copper(II)
● iron(II)
● iron(III)
● zinc 

Cation	Test	What happens if cation is present?
Ammonium	Add dilute sodium hydroxide, warm up	Ammonia gas is released, damp red litmus paper turns blue
Copper(II)	Add dilute sodium hydroxide or ammonia solution	A blue precipitate forms
Iron(II)	Add dilute sodium hydroxide or ammonia solution	A pale green precipitate forms
Iron(III)	Add dilute sodium hydroxide or ammonia solution	A red-brown precipitate forms
Zinc	Add dilute sodium hydroxide or ammonia solution	A white precipitate forms

Anions:

● carbonate

● chloride

● nitrate

● sulphate

Anion	Test	What happens if anion is present?
Carbonate	Add dilute hydrochloric acid	Bubbles that give off gas turn limewater a milky-white
Chloride	Add the same volume of nitric acid as chloride, add aqueous silver nitrate	A white precipitate forms
Nitrate	Add sodium hydroxide, then aluminium	Ammonia gas will be given off
Zinc	Add dilute sodium hydroxide or ammonia solution	A white precipitate forms

Gases:

● ammonia

● carbon dioxide

● chlorine

● hydrogen

● oxygen

Anion	Test	What happens if gas is present?
Ammonia	Damp red litmus paper	Paper turns blue
Carbon dioxide	Limewater	Limewater turns milky-white
Chlorine	Damp blue litmus paper	Paper turns white
Hydrogen	Lighted splint	A loud POP
Oxygen	Glowing splint	Splint relights