ACIDS, BASES & SALTS
1. INTRODUCTION
On the basis of their of their chemical properties, all the compounds can be classified into three groups:
1. Acids,
2. Bases, and
3. Salts
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| Acid bases effect |
2. Indicators for testing Acids and Bases
An indicators is a “dye” that change colour when it is put into an acid or a base. An indicators gives different colours in acid and base.
The three most common indicators to test for acids and bases are: litmus, methyl orange, phenolphthalein.
i. Litmus
a. An acid turns blue litmus to red.
b. A base (or alkali) turns red litmus to blue.
ii. Methyl Orange
a. Methyl orange indicator gives red colour in acid solution.
b. Methyl orange indicator gives yellow colour in basic solution.
iii. Phenolphthalein
a. Phenolphthalein indicator is colourless in acid solution.
b. Phenolphthalein indicator gives pink colour in basic solution.
Litmus is a natural indicator:
Litmus solution is a purple dye which is extracted from a type of plant called ‘lichen’. Lichen is a plant belonging to the division Thallophyta. When litmus solution is neither acidic nor basic (it is neutral), then its colour is purple.
Turmeric is also a natural indicator. Turmeric contains a yellow dye.
3. Acids:
1. Acids are those chemical substances which have a sour taste.
Ex: Mango, raw grapes, lemon, orange, and tamarind (imli).
2. Acids change the colour of blue litmus to red.
3. The acids present in plant materials and animals are called organic acids.
Ex: Acetic acid or ethanoic acid, lactic acid, tartaric acid, oxalic acid and formic acid
Sources of these organic acids are as following:
I. Acetic acid is found in vinegar (sirka).
II. Citric acid is present in citrus fruits such as Lemon and orange.
III. Lactic acid in present in sour milk (or Curd).
IV. Tartaric acid is present in Tamarind and unripe graps,
V. Oxalic acid is present in tomatoes.
VI. Formic acid is present in ant string and nettle leaf sting.
4. Acid solution conduct electricity (they are electrolytes).
5. Acid react with metal to form hydrogen gas.
The acids prepared from the minerals of the earth are called mineral acids.
Ex: hydrochloric acid, sulphuric acid and nitric acid.
Strong acids and weak acids
i. All the mineral acids are strong acids.
Hydrochloric acid, sulphuric acid and nitric acid are strong acid.
HCl H+ + Cl-
Hydrochloric acid hydrogen ion
ii. The organic acids are weak acids.
Acetic acid (ethanoic acid), formic acid, citric acid, tartaric acid and carbonic acid are week acids.
CH3COOH CH3COO‑ + H+
Acetic acid acetic ions hydrogen ions
· Dilute Acid:
Contains only a small amounts of acid and a large amount of water.
· Concentrated Acid:
A concentrated acid contains a large amount of acid and a small amount of water.
1. Acid react with metal to form hydrogen gas.
When an acid reacts with a metal, then a salt and hydrogen gas are formed. That is
Metal + acid salt + Hydrogen gas
When dilute sulphuric acid reacts with zinc metal, then zinc sulphate and hydrogen gas are formed.
Zn (s) + H2SO4 ZnSO4 + H2
Zinc sulphuric acid zinc sulphate hydrogen gas
(a metal) (Diluteacid) (A salt)
Most of the acids reacts with metals to form salts and evolve hydrogen gas. This shown that hydrogen gas is common to all acids.
2. Acid reacts with metal carbonates and metal hydrogen carbonates to form carbon dioxide gas
When the acid reacts with a metal carbonate (or metal hydrogen carbonates) then a salt, carbon dioxide gas and water are formed:
Metal carbonates + acid salt + carbon dioxide + water
Metal hydrogen carbonates + acid salt + carbon dioxide + water
i. When dilute hydrochloric acid reacts with sodium carbonate, then sodium chloride, carbon dioxide and water are formed:
Na2CO3 + 2HCl 2NaCl + CO2 + H2O
Sodium carbonate
The carbon dioxide gas is formed in the form of brisk effervescence (the rapid escape of small bubbles of gas from the liquid)
ii. When dilute hydrochloric acid reacts with sodium hydrogencarbonate, then sodium chloride, carbon dioxide and water are formed:
Na2CO3(s) + HCl(aq) NaCl(aq) + CO2 + H2O
Sodium carbonate Sodium chloride
Carbon dioxide reacts with lime water (calcium hydroxide solution) as follows:
a. When carbon dioxide gas is passed through lime water, the lime water turns milky due to the formation of a white precipitate of calcium carbonate.
Ca(OH)2 + CO2 CaCO3 + H2O
Calcium Hydroxide Calcium Carbonate
(Lime Water) (White ppt) (Make lime water milky)
b. If excess of carbon dioxide gas is passed through lime water, then the white precipitate formed first dissolves due to the formation of a solution salt calcium hydrogen carbonates, and the solution becomes clear again:
CaCO3(s) + CO2 H2O + Ca(HCO3)2(aq)
Calcium Carbonate calcium hydrogen carbonate
(White ppt) (Solution in water)
(Insoluble in water)
3. Acids react with base (or alkalis) to form salt and water
When an acid reacts with a base, then a salt and water are formed. That is
Acid + base Salt + water
The reaction between an acid and base to form salt and water is called a neutralisation reaction.
Ex: when hydrochloric acid reacts with sodium hydroxide solution, then a neutralisation reaction takes place to form sodium chloride and water:
NaOH + HCl NaCl + H2O
Sodium hydroxide Hydrochloride Sodium Chloride water
(Base) (Acid) (Salt)
4. Acid react with metal oxide to form salt and water
Acids react with metal oxides to form salt and water
Metal oxide + Acid Salt + water
Copper (II) oxide is a metal oxide. Dilute hydrochloric acid reacts with copper (II) oxide to form copper (II) chloride and water.
CuO + 2HCl CuCl2 + H2O
Cooper (II) Oxide Hydrochloride acid Copper (II) chloride
(Black) (Blue-green)
5. Acid have corrosive nature
The mineral acids cause severe burns on the skin and attack and eat up materials like cloth, wood, metal structures and stonework, so they are said to be corrosive.
The strong bases (or alkalis) such as sodium hydroxide are also very corrosive, and attack and destroy our skin.
6. What do all acids have in common
All acid is a substance which dissociates (or ionises) on dissolving in water to produce hydrogen ions [H+(aq) ions]
For example:
An aqueous solution of hydrochloric acid dissociates (or ionises) to form hydrogen ions (along with chloride ions).
HCl (aq) H+(aq) + Cl- (aq)
Hydrochloride acid hydrogen ions chloride ions
It is the presence of hydrogen ions [H+(aq) ions] in hydrochloric acid sodium which makes it behave like an acid.
Please note that hydrogen ions do not exist as H+ ions in solution, they attach themselves to the polar water molecules to form hydronium ions, H3O+. That is
H+ + H2O H3O+
Hydrogen ion hydronium ion
Sulphuric acid (H2SO4)
H2SO4 2H+ + SO42-
Nitric acid (HNO3)
HNO3 H+ + NO3-
Acetic Acid (CH3COOH)
CH3COOH CH3COO- + H+
Note: Glucose & Alcohol solution does not conduct electricity.
Glucose and alcohol solution do not produce hydrogen ions or some other ions in solution. So due to the absence of ions, glucose and alcohol solution do not conduct electricity.
The aqueous solution of an acid conducts electricity due to the presence of charged particles called ions in it.
For example:
When hydrochloric acid (HCl) is dissolved in water, then its solution contains hydrogen ions, H+ ions and Cl-(aq) ions. A solution of hydrochloric acid conduct electricity.
7. Uses of mineral acids in industry
1. Sulphuric acid is used in the manufacture of fertilisers (like ammonium sulphate), paints, dyes, chemicals, plastics, synthetic fibres, detergents, explosives and car batteries.
2. Nitric acid is used for making fertilisers (like ammonium nitrate) explosives (like TNT : tri-Nitro-Toluene), dyes and plastics.
3. Hydrochloride acid is used for removing oxide film from steel objects (before they are galvanised) and removing ‘scale’ deposits from inside the boilers. It is also used in dye-stuffs, textile, and food and leather industries.
4. Bases
Bases are those chemical substance which have a bitter taste. Bases are the chemical opposites of acids.
A base is a chemical substance which can neutralise an acid. All the metal oxide and metal hydroxide are bases.
For example:
Sodium oxide (Na2O) is metal oxide, so it is bases, calcium oxide (CaO) and calcium hydroxide [Ca(OH)2], ammonium hydroxide (NH4OH) is also a base thought it is not a metal hydroxide.
Sodium carbonate (Na2CO3), Calcium carbonate (CaCO3) and sodium hydrogen carbonate (NaHCO3) are also bases.
Water soluble bases: Alkalis
Most of the bases do not dissolve in water but some bases dissolved in water. Those bases which dissolve in water without chemical reaction have a special name. They are called alkalis. Thus a base which is soluble in water is called an alkali.
Some common water soluble bases are: sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide [Ca(OH)2], ammonium hydroxide (NH4OH) and magnesium hydroxide [MG(OH)2].
1. What do all the bases have in common
When a bases is dissolved in water, it always produces hydroxide ions (OH- ions).
NaOH + water Na+ + OH‑
Sodium Hydroxide Sodium Ions hydroxide ions
Similarly:
KOH + water K+ + OH-
And
Mg(OH)2 + water Mg2+ + OH‑
A common property of all the bases is that they all produce hydroxide ions when dissolved in water.
A bases which completely ionises in water and thus produce a large amount of hydroxide ions is called a strong bases.
Ex: NaOH, KOH, etc.
A base which is property ionises in water and thus produces a small amount of hydroxide ions is called a weak base.
Ex: NH4OH, Ca(OH)2, Mg(OH)2 etc.
2. Properties of bases:
1. Bases have bitter taste.
2. Bases feel soapy to touch
3. Bases turn red litmus to blue
4. Bases conduct electricity in solution (they are electrolytes)
5. Bases react with some metal to form hydrogen gas.
6. Base react with acids to form salt and water.
NaOH + H2SO4 Na2SO4 + 2H2O
7. Base react with non-metal oxides to form salt and water.
Non-metal + base Salt + Water
Ex:
Ca(OH)2 + CO2 CaCO3 + H2O
Calcium hydroxide calcium carbonate
(Base) (Salt)
3. Uses of bases:
Some of the important uses of bases are given below:
1. Sodium hydroxide is used in the manufacture of soap, paper and a synthetic fibre called ‘rayon’.
2. Calcium hydroxide is used in the manufacture is bleaching powder.
3. Magnesium hydroxide is used as an ‘antacid’ to neutralise excess acid in the stomach and cure indigestion.
4. Sodium hydrogen carbonate is used as baking soda in cooking food, for making baking powders, as an antacid to cure indigestion and in soda- acid fire extinguishers.
5. Sodium carbonate is used as washing soda and for softening hard water.
5. STRENGTH OF ACID AND BASE SOLUTION: pH SCALE
Water is slightly ionised in to hydrogen ions (H+) and hydroxide (OH‑). In pure water, the concentrations of hydrogen ions and hydroxide ions are equal.
6. Impotence of pH in everyday life
The pH plays an important role in many activities of our everyday life. For example pH of gastric juices in the stomach is important in the process of digestion; the pH changes in mouth can become a cause of tooth decay; the growth of plants and survival of animals also depends on the proper maintenance of pH; and many animals, and some plants make use of acids and basic liquids for sell defence purposes.
1. pH in our digestive system:
pH level of our body regulates our digestive system. In case of indigestion our stomach produces acid in a very large quantity because of which we feel pain and irritation in our stomach. To get relief from this pain antacids are used. These antacids neutralises the excess acid and we get relief.
2. pH of Acid Rain :
When pH of rain water is less than 5.6 it is called Acid Rain. When this acidic rain flows into rivers these also get acidic, which causes a threat to the survival of aquatic life.
3. pH of Soil :
Plants require a specific range of pH for their healthy growth. If pH of soil of any particular place is less or more than normal than the farmers add suitable fertilizers to it.
Our body functions between the range of 7.0 to 7.8 living organisms can survive only in the narrow range of pH change.
4. Tooth decay and pH:
Bacteria present in the mouth produce acids by degradation of sugar and food particles remaining in the mouth. Using toothpaste which is generally basic can neutralise the excess acid and prevent tooth decay.
Bee sting or Nettle sting contains methanoic acid which causes pain and irritation. When we use a weak base like baking soda on it we get relief.
Neutral Salts: Strong Acid + Strong base
pH value is 7
eg. NaCl, CaSO4
Acidic Salts: Strong Acid + weak base
pH value is less than 7
eq. NH4Cl, NH4NO3
Basic Salts: Strong base + weak acid
pH value is more than 7
eg. CaCO3, CH3COONa
7. Chemicals from Common Salt
– Sodium chloride is called as common salt used in our food. It is derived from seawater.
– Rock Salt is the brown coloured large crystals. This s mined like coal.
– Common Salt is an important raw material for many materials of daily use such as.
Sodium hydroxide
Washing Soda
Bleaching Power.
Sodium Hydroxide
Preparation: Prepared by the method called chlor-alkali Called chlor-alkali because we get chlorine and a base in this.
2NaCl(aq) + 2H2O (l) 2NaOH(aq) + Cl2(g) + H2(g)
Used in industry, medicines and cosmetics Household, Bleaching fabric
Bleaching Power
Preparation
Ca(OH)2 + Cl2 CaOCl2 + H2O
Calcium hydroxide chlorine bleaching water
Powder
Uses in textile, factories and laundry, used as disinfectant
Baking Soda
Common name – Sodium Hydrogen Carbonate
Preparation
NaCl + H2O + CO2 + NH3 NH4Cl + NaHCO3
Sodium Water Carbon Ammonia Sodium hydrogen
Chloride dioxide carbonate
On heating NaHCO3 produces:
NaHCO3 Na2CO3 + H2O + CO2
Sodium Hydrogen carbonate sodium carbonate water carbon dioxide
CO2 produced causes dough to rise and make cakes, pastries spongy.
Uses:
In household, ingredients of antacid
In making baking power
On heating baking powder produces
NaHCO3 + H+ CO2 + H2O + Sodium Salt of acid
Washing Soda
Preparation:
Recrystallization of sodium carbonate
Na2CO3 + 10H2O Na2CO3. 10H2O
Uses:
– Used in glass, soap and paper industry
– Cleaning agent for domestic purposes.
– Removal of hardness of water.
– Manufacturer of borax.
Water of crystallisation:
Fixed no. of water molecules present in one formula unit of a salt.
On heating copper sulphate crystals water droplets appear, formula of hydrated copper sulphate – CuSO4. 5H2O.
Plaster of Paris
Gypsum also contains water of crystallisation.
Formula of gypsum (Plaster of Paris) – CaSO4.2H2O
On heating gypsum at 373k it becomes CaSO4.½H2O is plaster of paris.
Plaster of Paris is used as plaster for fractured bones.
When plaster of Paris is mixed with water it changes to gypsum.
CaSO4.½ H2O + 1½ H2O CaSO4.2H2O
Uses of plaster of Paris:
Making toys, decorative material and smooth surfaces