Acids

An acid is a substance which produces hydrogen ions, H+, when it is dissolved in water.

All acids contain hydrogen ions, but not all compounds that contain hydrogen are acids, e.g. NH3. This is because NH3 does not produce hydrogen ions in water. It is the hydrogen ions produced that is responsible for the properties of acids.

 

Properties of Acids:

  • Acids have a sour taste.
  • They are proton donors
  • They have a pH below 7.
  • They are corrosive in nature.
  • The lower the pH, the more corrosive they are.
  • They turn blue litmus paper red.
  • They dissolve in water to form solutions that conduct electricity.
  • Acids react with reactive metals to form hydrogen and a salt:
  • Acids react with carbonates to form a salt, carbon dioxide and water:
  • Acids react with bases to form a salt and water:

Acids only show the properties of acids when they are dissolved in water. This is because acids dissociate in water to produce the hydrogen ions which are responsible for the acidic properties.

 

Bases

A base is any metal oxide or hydroxide. This means that a base contains either oxide ions O2- or hydroxide ions OH. Bases are proton accepters and neutralize acids by accepting hydrogen ions.

A base reacts with an acid to give a salt and water only:

Examples of reactions with bases:

Reaction 1
Acid Hydrochloric acid (HCl)
Base Sodium Hydroxide (NaOH)
Reaction
Salt produced The salt sodium chloride (NaCl) is produced.

 

Reaction 2
Acid Sulphuric acid (H2SO4)
Base Copper oxide (CuO)
Reaction
Salt produced The salt copper(II)sulphate (CuSO4) is produced

 

Properties of bases

  • Bases are proton accepters
  • They react with acids by neutralizing them
  • They react with an acid to give a salt and water
  • They have a pH above 7
  • They form soluble bases called as alkalis when dissolved in water
  • Soluble bases (Alkalis) turn red litmus paper blue
  • Insoluble bases have no effect on litmus paper

Alkalis

An alkali is a base that is soluble in water. In an aqueous solution, an alkali produces hydroxide ions OH.

Most bases are insoluble in water. Hence, they aren’t considered as alkalis.

Examples of alkalis include: Sodium hydroxide, copper hydroxide, calcium hydroxide, lithium hydroxide, potassium hydroxide, etc.

Properties of alkalis

  • Alkalis have a bitter taste and a soapy feel.
  • They turn red litmus paper blue.
  • They have a pH above 7.
  • All alkalis produce hydroxide ions (OH) when dissolved in water.
  • All alkalis can react with water to form salt and water only. This is known as neutralization.
  • Alkalis heated with ammonia salts give off ammonia gas:

Neutralization

In the above reactions, the oxide or the hydroxide ions from the bases (OH) react with the hydrogen ions from the acids (H+) to form water (H2O). This reaction is known as neutralization, and it is represented by the ionic equation below:

 

Uses of Acids and bases

Uses of acids
Acid Use
Sulphuric acid Manufacturing of detergents

Manufacturing of fertilizers

In car batteries

Hydrochloric acid Cleaning metals

Processing leather

Ethanoic acid Manufacturing of vinegar

Manufacturing of glue and other adhesives

 

Uses of Bases
Base Use
Calcium oxide In neutralizing acidic soil

In decreasing the acidity of lakes and rivers

Sodium hydroxide In the manufacture of soap and detergents
Ammonia solution In the manufacture of fertilizers

In window cleaning solutions

 

The pH scale

The pH scale shows whether a substance is acidic, alkaline or neutral. It also indicates how acidic, alkaline or neutral it is.

For example:

  1. A substance with a pH of 7 is said to be neutral
  2. A substance with a pH below 7 is said to be acidic
  3. A substance with a pH above 7 is said to be alkaline

The lower the pH of a substance below 7, the more H+ ions it has, hence the more acidic it is.

The higher the pH of a substance above 7, the more OH ions it has, hence the more alkaline it is.

The pH of a substance can be measured using a universal indicator.

Strong and weak acids

Acids can either be strong, or weak. In this section, we’ll be explaining the difference to you.

Strong acids

Strong acids are those that get completely ionised when they are dissolved in aqueous solutions. This means that they are entirely made up of ions and no molecules.

Examples of strong acids are hydrochloric and sulphuric acid.

If we take the example of hydrochloric acid (hydrogen chloride in aqueous state), here is what it consists of:

Weak acids

Weak acids are those which only get partially ionised; they are made up of both ions and molecules.

An example of weak acid is ethanoic acid:

Differences between strong and weak acids

  1. At the same concentration, the stronger acid will have a lower pH.
  2. At the same concentration, the stronger acid will be a better conductor of electricity
  3. At the same concentration and temperature, the stronger acid will react faster with solids such as calcium carbonate.

 

Strong and weak bases

Bases can either be strong, or weak. In this section, we’ll be explaining the difference to you.

Strong bases

Strong bases are those that get completely ionised when they are dissolved in aqueous solutions. This means that they are entirely made up of ions and no molecules.

Examples of strong bases are Sodium hydroxide and potassium hydroxide.

If we take the example of sodium hydroxide, here is what it consists of:

 

Weak bases

Weak bases are those which only get partially ionised; they are made up of both ions and molecules.

An example of a weak base is ammonia in aqueous solution:

Differences between strong and weak bases

  1. At the same concentration, the stronger base will have a higher pH.
  2. At the same concentration, the stronger base will be a better conductor of electricity

Types of Oxides

An oxide is a compound of oxygen and another element. Most oxides can be classified into four types:

  1. Acidic oxides
  2. Basic oxides
  3. amphoteric oxides
  4. Neutral oxides.

Acidic oxides

Non-metals may form acidic oxides. Most acidic oxides dissolve in water to form an acid. Acidic oxides react with alkalis to form salt and water (neutralization reaction).

An example of acidic oxide is sulphur trioxide SO3. it is a gas which, when dissolved in water, forms Sulphuric acid H2SO4.

 

Basic Oxides

The oxides formed by metals are basic oxides. Most basic oxides are insoluble in water. The oxides that are soluble are called alkalis (as seen before)

Basic oxides react with acids to form a salt and water.

An example of basic oxide is sodium oxide. It dissolves in water to form sodium hydroxide, while it reacts with hydrochloric acid or any other acid to form sodium chloride or any other salt, in addition with water.

 

Neutral Oxides

Oxides that react neither with bases nor with acids are called as neutral oxides.

Some examples are water, carbon monoxide and nitrogen (II) oxide.

Amphoteric Oxides

Amphoteric oxides either react with acids or bases to produce salt and water and thus express the properties of both acids and bases.

Some common examples are zinc oxide (or hydroxide) and aluminium oxide (or hydroxide).

 

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