ACIDS AND BASES

Definition: (based on Arrhenius Theory)

- An acids is a chemical substance which ionises in water toproduce hydrogen ions, H+ . (text book)

- An acid is a chemical compound that produces hydrogenions, H+ or hydroxonium ions H3O

+ in water. (referencebook)

A substance has acidic properties because of the formationof hydrogen ions or hydroxonium ions in water.

When acids dissolves in water(dissociation/ionisation ofacids in water), hydrogen atoms in the acids are released ashydrogen ions.

Eg:

HCl (g) H+ (aq) + Cl- (aq)

HNO3 (l) H+ (aq) + NO3- (aq)

H2SO4(l) 2H+ (aq) + SO42- (aq)

CH3COOH (l) H+ (aq) + CH3COO- (aq)

H2O

H2O

H2O

H2O

Without the presence of hydrogen ions, a substancedoes not show any acidic property.

Acids can be divided into 2 types:

Types of acid Examples

Mineral acid (obtained from minerals and most do not contain the element carbon)

HCl, H2SO4 , HNO3 .

Organic acid (extracted from living things and contain the element carbon)

CH3COOH, HCOOH, citric acid, tartaric acid, malic acid and ascorbic acid.

Acids can be also classify as a monoprotic and diproticacid based on its basicity.

Basicity – the number of ionisable hydrogen atomsper molecule of an acid.

Monoprotic Diprotic

Hydrochloric acid Sulphuric acid

HCl (aq) H+ (aq) + Cl- (aq)

H2SO4(aq) 2H+ (aq) + SO4

2- (aq)

Get it right!!!

Incorrect Correct

An acid is a substancethat containshydrogen ions.

An acid is a substancethat produceshydrogen ions in thepresence of water.

Definition: (based on Arrhenius Theory)

- A base is a chemical substance which ionises in waterto produce hydroxide ion, OH-. (text book)

- A base is a chemical substance that can neutralize anacid to produce salt and water only. (reference book)

Most of bases are not soluble in water. Bases that aresoluble in water are known as alkalis.

Alkali – a chemical compound that dissolves in waterto produce freely moving hydroxide ions, OH-.

Eg:

NaOH(ag) Na+ (aq) + OH- (aq)

NH3 (aq) NH4+ (aq) + OH- (aq)

Sodium hydroxide dissolves in water and releases itshydroxide ion

When ammonia molecule dissolves in water, ionisationtakes place to produce a hydroxide ion

Bases

Alkalis

(soluble in water)

Bases: all metal oxides or metal hydroxide

Alkalis: only bases that are soluble in water

Get it right!!!

Incorrect CorrectAn alkali is acompound thatcontains hydroxideions.

An alkali is acompound thatproduces hydroxideions in the presence ofwater.

A base is a compoundthat produceshydroxide ions inwater.

A base is a compoundthat neutralises anacid and produces saltand water only.

Acids, bases and alkalis are widely used in our dailylife. They are used in manufacture of variousproducts(food), agriculture, medicine and industries.

Acids Uses

Sulphuric acid To make paints, detergents, polymers,fertilizers, as an electrolyte in lead-acidaccumulators

Hydrochloric acid To clean metals before electroplating

Nitric acid To make fertilizers, explosive substances(such as T. N.T), dyes and plastics

Benzoic acid To preserve food

Carbonic acid To make gassy (carbonated) drinks

Ethanoic acid A component of vinegers

Tartaric acid To make baking powders

Bases Uses

Sodium hydroxide To make soaps, detergents, bleachingagents and fertilizers.

Ammonia To make fertilizers, nitric acid, greaseremover to maintain latex in liquid form

Calcium hyrdoxide To make cements, limewater and toneutralise the acidity of soil

Magnesium hydroxide To make tooth paste, gastric medicine(antacid)

Aluminium hydroxide To make gastric medicine (antacid)

An acids shows its acidic properties only in thepresence of water.

Water is essential for the formation of hydrogen ionthat cause the acidity.

In the absence of water, dry acids or acids dissolves inorganic solvents do not show any acidic propertybecause the acids exist as molecules and are notdissociated into hydrogen ions.

An alkali shows its alkaline properties only in thepresence of water.

Water is essential for the formation of hydroxide ionthat cause the alkalinity.

In the absence of water, an alkali will exist asmolecules and are not show any alkaline properties.

Acids reacts with bases to form salts and water.

Acids react with reactive metals to produce salts and hydrogen gas.

Acids reacts with carbonates to produce salts, water and carbon dioxide.

Alkalis reacts with acids to form salts and water.

When a mixture of an alkali an ammonium salt is heated, ammonia gas is liberated along with salt and water.

Most metal hydroxides are insoluble in water. Hence, adding an alkali to most metal ion solutions will give a precipitate of an insoluble metal hydroxide.