ACIDS, BASES, AND SALTS

ACIDS

An acid is a substance that gives/donates hydrogen ions H+, as the only positive ions
when it is dissolved in water.

Acids split into ions in solution and this is called ionization.

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

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

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

NB: All acids are covalent compounds and they show acidic properties in aqueous
solution.

CLASSES OF ACIDS

Strong acids- they are acids which ionize completely in solution giving H+ ions. All
molecules separate into ions.

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

HCl is a strong acid because all HCl molecules present split into ions.

Examples of strong acids

Name of acid formula

Hydrochloric acid

Sulphuric acid

Nitric acid

Phosphoric acid

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Weak acids: they are acids which dissociate/ionize partially in aqueous solution.ie They
do not ionize completely in solution. Some acid molecules remain unseparated. This is
indicated by a reversible equation as follows;

H2CO3 2H+ + CO32-

CH3COOH H+ + CH3COO-

Examples of weak acids

Name Formula

Carbonic acid

Sulphurous acid

Ethanoic acid

Tartaric acid

Ascorbic acid

Citric acid

PROPERTIES OF ACIDS

➢ Have a pH less than 7

➢ Have a sour taste
➢ They all dissolve to give hydrogen ions
➢ They are corrosive

CHEMICAL REACTIONS

ACIDS AND ALKALIS

Acids react with bases to form salt and water only
Examples

HCl + NaOH
H2SO4 + KOH

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ACIDS AND METALS
Acids react with metals to form a salt and hydrogen gas
Examples

HCl + Zn
H2SO4 + Mg

Test for H2 gas

Test: use a burning splint
Results: produces a ‘POP’ sound.

ACIDS AND CARBONATES

Acids react with carbonates to form salt, water and carbon dioxide

HCl + ZnCO3
H2SO4 + MgCO3

Test for CO2 gas

Test: bubble the gas in lime water
Results: the gas turns lime water milky.

Uses of acids
➢ Strong acids are used to remove rust from metals.
➢ Weak acids are used to preserve food and other things. e.g. Vinegar, citric acid
and benzoic acid.
➢ Manufacture of; paints, fertilizers and detergents.
➢ Sulphuric acid is used as an electrolyte in car batteries.
➢ Medicine (ascorbic acid or vitamin C)
➢ Flavouring food (vinegar)

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BASES

They are the hydroxides and oxides of metals. They react with acids giving salt and
water only.
Examples include,

Base Formula
Copper (ii) oxide

Iron (ii) oxide

Zinc oxide

Sodium hydroxide

Potassium hydroxide

Calcium hydroxide

Classification of bases

Soluble bases
They are oxides which dissolve in water giving hydroxide solutions called ALKALIS.
e.g. Potassium Hydroxide, Calcium Hydroxide, Sodium Hydroxide

Another unusual alkali is called aqueous Ammonia or Ammonium hydroxide

NB: All oxides of group I elements are soluble, calcium oxide is partially soluble.

Insoluble bases
do not dissolve in water. (mostly oxides of transition elements).
e.g.

ALKALIS

An alkali is a substance that accept H+ ions from acids. Alkalis form OH- ions in
solution.

e.g. NaOH (aq) OH- + Na+

Strong alkalis: they ionize completely in solution giving OH- ions.

KOH ( ) ……………………………….

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Weak alkalis: they ionize partially in solution giving OH- ions

NH3 + H2O NH4+ + OH-

STRONG ALKALIS WEAK ALKALIS

Sodium hydroxide ( ) Calcium hydroxide ( )

Potassium hydroxide ( ) ammonium hydroxide ( )

Properties of alkalis and bases

➢ They have a pH more than 7

➢ Strong alkalis are soapy to touch; this is because they react with natural oils in the
skin making soap.
➢ Strong alkalis are corrosive ( e.g. NaOH, KOH)
➢ Alkalis turn red litmus paper blue
➢ All alkalis dissolve in water and give solutions which contains hydroxide ions.
➢ Alkalis will react with most metal ions to form insoluble precipitates (ppt).

Eg Cu(NO3)2 ( ) + NaOH ( ) Cu(OH)2 ( ) + NaOH ( )

Copper (ii) hydroxide is a blue precipitate.

➢ Bases neutralize acids. When a base neutralizes an acid, the process is called a
Neutralization reaction and water is always produced.

e.g. NaOH + H2SO4 ……………....................................

__________________________________________________________(ionic equation)

➢ Strong alkalis displace ammonia gas from ammonium salts.

i.e. ammonium salt + alkali salt + ammonia gas + water

e.g. ………………………………………………………………………………………….

USES OF BASES

✓ Treatment of ingestion (milk of magnesia and antiacid tablets)

✓ In toothpaste to neutralize acid in the mouth [ Mg(OH)2]

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 ✓ Dissolve dirt and grease from surfaces
✓ Manufacture of detergents ( cleaning chemicals)
✓ Treatment of acid soils. e.g. lime( )
✓ Treatment of insect stings.

STRENGTH AND CONCENTRA TION

CONCENTRATION

It is the amount of solute in a given volume of solvent. Measured in mols/dm3.

STRENGTH

It is the extent to which an acid or an alkali separates/dissociates into ions in solution.

RE: A strong acid is the one which dissociates completely in water while a Weak acid
dissociates partially in water.

INDICATORS

The presence of acids or alkalis may be shown using indicators. An indicator changes
from one colour when mixed with an acid or alkali.

Indicator Acid alkali

Red litmus paper Red Blue

Blue litmus paper Red Blue

Methyl orange Pink Yellow

Phenolphthalein Colourless Pink

Screened methyl orange violet green

Universal indicator pink blue

Universal indicator: it is a mixture of several indicators used to distinguish strong and

weak acids as well as alkalis.

red Orange Yellow green Light blue Dark blue

Strong acid Weak acid Weak neutral Weak alkali Strong alkali
acid

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PH scale

It measures how acidic or alkaline a solution is. i.e. It measures the strength of an acid
and an alkali. It ranges from zero to fourteen.

01 2 3 456 7 8 9 10 11 12 13 14

Strong acid Weak acid Neutral Weak alkali Strong alkali

red Orange to green Light blue Dark blue

yellow

BASICITY OF AN ACID

It is the number of hydrogen ions that can be produced by one molecule of an acid.

i. monobasic acid : produces one H+ ion. E.g. HCl

ii. dibasic acid : produces two H+ ion. E.g. H2SO4
iii. tribasic acid: produces three H+ ions. E.g. H3PO4

OXIDES

They are compounds with oxygen atoms or oxide ions attached to other elements. They
are formed when elements are burned.

Examples

Oxide Formula
carbon monoxide

nitrogen monoxide

copper (ii) oxide

carbon dioxide

sulphure dioxide

sulphure trioxide

dihydrogen oxide

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Types of oxides

When oxides are dissolved in water and tested with universal indicator it shows that they
can be classified as acidic, basic, amphoteric and neutral oxides.

a) Acidic oxides

Non-metallc oxides which when combined with water produce an acid. They neutralize
base forming salt and water only.

e.g

SO2 + H2O H2SO3

SO3 + H2O H2SO4

CO2 + H2O H2CO3

NO2 + H2O HNO3

b) Basic oxides

They are metallic oxides which react with acid giving salt and water only.

e.g CuO, Fe2O3, Na2O, MgO, etc.

MgO + HCl ………………………………….

c) Amphoteric oxides

They are metallic oxides which react with both acids and alkalis forming salt and water.

e.g. Aluminium oxide and Zinc oxide.

ZnO + HCl ZnCl2 + H2

ZnO + NaOH Na2[Zn(OH)4] (sodium zincate)

Al2O3 + HCl AlCl3 + H2O

Al2O3 + NaOH Na[Al(OH)4] ( )

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d) Neutral oxides

Oxides which are neither basic nor alkaline. They do not react with acids or alkalis and
are mostly monoxides.

e.g H2O, CO, NO.

SALTS

A salt is a substance formed when acidic hydrogen of an acid is replaced by a metal or an

ammonium ion.

e.g.

KOH + HCl KCl + H2O

NH4OH + H2SO4 (NH4)2SO4 + H2O

TYPES OF SALTS

1. Acidic salts

An acidic salt is formed when acidic hydrogen of a dibasic or tribasic acid is partially
replaced. ie. One hydrogen remains as part of the salt and make the salt acidic.

NAME OF SALT FORMULA

Calcium hydrogen carbonate

Sodium hydrogen carbonate

Magnesium hydrogen sulphate

2. Neutral/Normal salts

A neutral salt (PH = 7) is formed when an acidic hydrogen of an acid is completely

replaced by a cation.

e.g. NaOH + H2SO4 Na2SO4 + H2O

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1. PREPARATION OF SOLUBLE SALTS

The salts are prepared by;

✓ Mixing and dissolving correct reactants (Acid + another substance in excess).

✓ Filtration to remove unused (excess) substance.
✓ Heating to concentrate the salt solution (remove excess water)
✓ Crystallization by leaving the concentrated salt solution to dry slowly in air(keeps
water of crystallization).

Procedure

1. Warm the acid

- To speed up the reaction

2. Add solid Y in excess

- To make sure that all the acid has dissolved

3. Filter
- To remove excess solid Y

4. Crystalize the filtrate

- To obtain the salt crystals

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Questions

a. For the preparation of copper(ii) sulphate, name suitable chemicals to use as acid
X and solid Y.

Acid X: ……………………………………….

Solid Y: ……………………………………….

b. Write a balanced chemical reaction with state symbols for the reactants above.

………………………………………………………………………………………

c. State two observations made during the reaction

………………………………………………………………………………………

………………………………………………………………………………………

d. What two observations will be made to confirm that all the acid has reacted?

………………………………………………………………………………………

………………………………………………………………………………………

REACTIONS FOR SALT PREPARATION

1. Acid and a metal

2. Acid and a carbonate
3. Acid and an alkali

NB: -Titrate an acid with an alkali using an indicator to determine the exact end
point.

-To obtain a pure salt (not contaminated by an indicator), use new solutions and
titrate without using an indicator.

Water of crystallization

Most salts contain a fixed percentage of water in their crystal Lattice when they
crystallize. It is essential for their shape as well as colour and is called water of
crystallization. If it is removed, then the colour of a salt changes and it becomes a
powder.

e.g Copper (ii) sulphate pentahydrate

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Cobalt (ii) chloride hexahydrate

Na 2 CO3 .10 H 2 O

Na 2 SO4 .10 H 2 O

2. PREPARATION OF INSOLUBLE SALTS

They must be prepared by mixing solutions containing the correct ions, for example, the
preparation of lead iodide; this may be done by adding lead ions to iodide ions.

The method is called precipitation.

Precipitate(ppt)-it is a suspension of solid particles in solution formed by insoluble

solids.

e.g. Ba(NO3)2 ( ) + CuSO4( ) BaSO4( ) + Cu(NO3)2

Barium sulphate is a white precipitate. It can be separated by filtration followed by

drying.

REACTION SCHEME FOR INSOLUBLE SALT PREPARATION

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USES OF SALTS

SALT USE
Silver bromide photography
Sodium chloride Flavouring food
Calcium carbonate Making cement, glass, extraction of iron
Sodium carbonate Glass making, softening water
Ammonium sulphate In fertilizers
Potassium nitrate In fertilizers and gunpowder
manufacture

QUALITAIVE ANALYSIS

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