Forming salts by neutralisation
What Is pH?
pH is a numerical scale that shows how acidic or alkaline (basic) a substance is. It ranges from 0 to 14.
pH 0–6: Acidic
pH 7: Neutral
pH 8–14: Alkaline (basic)
What pH Measures
pH measures the concentration of hydrogen ions in a solution.
More hydrogen ions → lower pH → stronger acid
Fewer hydrogen ions → higher pH → stronger alkali
How pH Is Measured
1. Universal Indicator
Changes the solution’s color. Compare it to a pH colour chart.
Red → Strong acid
Yellow → Weak acid
Green → Neutral
Blue → Weak alkali
Purple → Strong alkali
2. pH Probe / Meter
Gives precise numerical values.
Why pH Matters
Biology: Blood stays around pH 7.4
Agriculture: Soil pH affects plant growth
Medicine: Stomach acid has very low pH
Everyday life: Soaps, shampoos, foods all have different pH levels
Chemistry: Neutralisation reactions aim to reach pH 7
pH in Neutralisation
When an acid reacts with a base:
pH moves up toward 7 if the acid is neutralised
pH moves down toward 7 if the base is neutralised
If both react in exact amounts, the final pH = 7.
What Is a Base?
A base is a substance that can react with an acid to neutralise it and form a salt + water.
A base usually:
produces hydroxide ions (OH⁻) in water
has a pH greater than 7
feels slippery/soapy
tastes bitter (DON’T taste chemicals in real life — this is just theory)
Base vs Alkali
These two words are related but not the same:
Base: Any substance that reacts with acids.
Alkali: A soluble base (a base that dissolves in water).
So all alkalis are bases, but not all bases are alkalis.
Examples of alkalis:
Sodium hydroxide
Potassium hydroxide
Calcium hydroxide
Examples of bases that are not alkalis (because they don’t dissolve well):
Copper(II) oxide
Magnesium oxide
Iron(III) oxide
Properties of Bases
pH > 7
Weak bases: around 8–11
Strong bases: around 12–14
Turn red litmus paper blue
This is a classic test for bases.
React with acidsBase + Acid → Salt + Water
Produce hydroxide ions in waterThis is why solutions of alkalis are alkaline.
Examples of Bases
Common strong bases:
Sodium hydroxide
Potassium hydroxide
Common weak bases:
Ammonia solution
Calcium hydroxide
Metal oxides as bases:
Copper(II) oxide
Magnesium oxide
Zinc oxide
These don’t dissolve well, but they still react with acids.
Reactions With Acids (Word Equations)
Base + Acid → Salt + Water
Examples:
Sodium hydroxide + Hydrochloric acid → Sodium chloride + Water
Calcium hydroxide + Nitric acid → Calcium nitrate + Water
Copper(II) oxide + Sulfuric acid → Copper(II) sulfate + Water
Everyday Bases
You see them all the time:
Soap
Baking soda
Toothpaste
Bleach
Washing powder
Oven cleaner
They feel slippery because bases react with the oils on your skin.
What Happens During Neutralisation?
Acids contain hydrogen ions.
Bases (or alkalis) contain hydroxide ions.
When they react, these ions join to form water.
Whatever ions are left over combine to make a salt.
Examples (word equations):
Hydrochloric acid + Sodium hydroxide → Sodium chloride + Water
Sulfuric acid + Calcium hydroxide → Calcium sulfate + Water
Nitric acid + Magnesium oxide → Magnesium nitrate + Water
Hydrochloric acid + Copper(II) oxide → Copper(II) chloride + Water
What Happens to pH During Neutralisation?
The acid’s pH starts below 7.
The base’s pH starts above 7.
As they react, pH moves towards 7.
If exact amounts are used, the final pH is neutral (7).
Where Neutralisation Is Used
Treating indigestion
Making salts in chemistry
Treating acidic soil in farming
Treating acidic industrial waste
Making products like toothpaste and soap
What Are Ions?
Ions are atoms or groups of atoms that have an electric charge because they have gained or lost electrons.
If they lose electrons, they become positively charged → called positive ions.
If they gain electrons, they become negatively charged → called negative ions.
Positive Ions (Cations)
These are formed when atoms lose electrons.
Examples of positive ions:
Sodium ion
Calcium ion
Magnesium ion
Hydrogen ion
These usually come from metals or acids.
Negative Ions (Anions)
These are formed when atoms gain electrons.
Examples of negative ions:
Chloride ion
Sulfate ion
Nitrate ion
Hydroxide ion
Carbonate ion
These often come from non-metals, acids, or bases.
Why Ions Matter in Chemistry
Ions are important because:
Acids release hydrogen ions.
Bases (alkalis) release hydroxide ions.
These ions react together during neutralisation.
Salts are made of positive and negative ions.
A salt is always made from:
a positive ion from the base or metal
a negative ion from the acid
Example:Sodium ion + Chloride ion → Sodium chloride (a salt)
Ions in Solutions
When salts dissolve in water, they split into ions.
For example:
Table salt splits into sodium ions and chloride ions.
Calcium chloride splits into calcium ions and chloride ions.
This is why solutions can conduct electricity — ions carry charge.
Ions in Everyday Life
Toothpaste contains fluoride ions
Sports drinks contain electrolytes (ions like sodium, potassium, chloride)
Blood contains many ions to control pH
Batteries rely on ions to move charge
What Are Alkalis?
An alkali is a type of base that dissolves in water.When an alkali dissolves, it produces hydroxide ions, which make the solution alkaline.
So in simple terms:
A base reacts with acids.
An alkali is a base that can dissolve in water.
All alkalis are bases, but not all bases are alkalis.
What Alkalis Do
When an alkali dissolves in water, it releases hydroxide ions.This causes the solution to have a pH greater than 7.
Common Alkalis
These are the main alkalis you will use in school chemistry:
Sodium hydroxide
Potassium hydroxide
Calcium hydroxide
Aqueous ammonia (ammonia dissolved in water)
Properties of Alkalis
Have a pH above 7
Turn red litmus paper blue
Can neutralise acids
Feel slippery due to reaction with oils on skin
Strong alkalis can be corrosive
Reactions of Alkalis
Alkalis react with acids to form salt and water.
Examples (word equations):
Sodium hydroxide + Hydrochloric acid → Sodium chloride + Water
Potassium hydroxide + Nitric acid → Potassium nitrate + Water
Calcium hydroxide + Sulfuric acid → Calcium sulfate + Water
Everyday Alkalis
Some everyday substances contain alkalis, such as:
Soap
Toothpaste
Bleach
Washing powder
Certain cleaning sprays