.(The first half of this video is about metals.
Start at time index 10.16 for the section on acids and alkalis)
ACIDS (acidic solutions)
Acids have the following properties:
- Have a pH value less than pH 7
- Turn blue litmus indicator red
- Can neutralise an alkali
- Have a sour taste (WARNING: never taste any chemicals)
examples of common acids:
- citric acid (found in lemons and other citrus fruits) tannic acid (found in tea)
- acetic acid (found in vinegar) formic acid (found in ant stings)
[ Acetic acid is correctly called ethanoic acid and formic acid is correctly called called methanoic acid ]
- sulphuric acid (accumulator acid, found in car batteries)
- hydrochloric acid (in our stomach to kill germs)
- nitric acid (used in the manufacture of explosives and fertilizers)
NOTE: strong acids are a lot more dangerous than weak acids.
Acids are CORROSIVE which means they will attack and weaken many things including metals, paper, clothing and skin.
A concentrated acid or alkali is more dangerous than a dilute one.
To make something more dilute water needs to be added.
CONCENTRATED means without much or without any water added.
DILUTE means a lot of water added.
Alkalis have the following properties:
- Have a pH value more than pH 7
- Turn red litmus indicator blue
- Can neutralise an acid
- Feel soapy to touch (reacts with fat to form soap)
Examples of common alkalis:
- caustic soda (sodium hydroxide): used to clean ovens and drains
- washing soda (sodium carbonate)
- ammonia solution (often used in household cleaners)
- garden lime (calcium oxide): used to neutralise acid in the soil
- indigestion powder (often magnesium hydroxide)
Alkalis are caustic which means they will burn skin and eyes.
A strong alkali (like caustic soda) is VERY dangerous.
A neutral liquid is one with a pH value equal to pH 7.
examples: water, salt (sodium chloride) solution, sugar solution
(Just because a liquid is neutral does not mean that it is safe to drink)
- A scale of numbers ranging from 1 to 14
- The pH number tells you if a liquid is a liquid is an acid or an alkali.
- pH 7 is neutral.
- Less than pH7 is acid.
- More than pH7 is an alkali
There are two indicators you need to know the names of: Litmus indicator and Universal indicator
Litmus can only be two colours
In ACIDS it is RED
In ALKALIS it is BLUE
We use litmus indicator when we want to tell precisely when a liquid is neutral but it can’t tell us how strong an acid or an alkali is
Universal indicator is a liquid that changes colour depending on the pH of the liquid it is added to.
We use universal indicator when we want to know how strong an acid or alkali is but it is difficult to know when a liquid is precisely neutral
Acids and alkalis in the home
This reaction is called NEUTRALIZATION and makes the pH number rise.
The alkali is ‘opposite’ to the acid and cancels it out.
An acid always reacts with an alkali to produce a salt and water
ACID + ALKALI = SALT + WATER
eg Hydrochloric acid will react with sodium hydroxide to produce a solution of sodium chloride (salty water)
If the solution is warmed so the water evaporates crystals of common salt will be left
1. In agriculture:
Lime (or quicklime), which is an alkali, is placed on the soil to neutralise acid soil and so raise its pH.
The soil becomes too acid due to the bacteria which help plants and animals decompose.
Why do we need to neutralise soil?
- Some plants grow better in less acid soils
- Bacteria, which cause plants to decompose and so fertilize the soil, grow better in less acid soils.
2. In medicine:
Indigestion is often caused by too much acid in your stomach.
Indigestion mixture contains a mild alkali which neutralises excess acid in your stomach.
Reactions involving acids
1. Acids react with most metals to release hydrogen gas:
eg zinc + sulphuric acid zinc sulphate + hydrogen
2. Acids react with any metal carbonate (eg copper carbonate or calcium carbonate) to produce carbon dioxide gas
eg calcium carbonate + hydrochloric acid calcium chloride + water + carbon dioxide
3. An acid reacts with an alkali to form a salt plus water
ACID + ALKALI → SALT + WATER
eg hydrochloric acid + sodium hydroxide →sodium chloride + water
This is an example of a neutralization reaction
NOTE: A base is any chemical that can neutralise an acid.
An alkali is a base that is soluble in water.
A salt is formed whenever a base neutralises an acid
eg. sulphuric acid + copper oxide copper sulphate + water
[acid + base → salt + water]