Acid- Acids are sour in taste, turn blue litmus red, dissolve in water to release H+ ions.
E.g., HCl (Hydrochloric acid) and H2SO4 (Sulfuric acid)
Base- Bases are bitter in taste, turns red litmus to blue and when dissolved in water releases OH– ions;
e.g., NaOH and KOH.
Chemical Properties of Acids and Bases
1) Acid and Bases in Laboratory
Acid-base indicators are dyes or mixtures of dyes which are used to indicate the presence of acids and bases by change of colour. Some substances whose odour changes in acidic and basic media are called olfactory indicators. The acidic nature of a substance is due to formation of H+ (aq) ions in solution whereas the basic nature is due to formation of OH– (aq) ions in the solutions.
2) How do Acids and Bases React with Metals?
When an acid reacts with a metal, hydrogen gas is evolved and a corresponding salt is formed.
Acid + Metal → Salt + Hydrogen gas
Example:
2 NaOH(aq) + Zn(s) → Na2ZnO2(s) + H2(g)
When a base reacts with a metal, hydrogen gas is evolved and salt is formed which has a negative ion composed of the metal and oxygen.
3) How do Metal Carbonates and Metal Hydrogencarbonates React with Acids?
When an acid reacts with a metal carbonate or metal hydrogencarbonate, it gives salt, carbon dioxide gas and water.
Metal carbonate/Metal hydrogencarbonate + Acid → Salt + Carbon dioxide + Water
Examples:
Na2CO3(s) + 2 HCl(aq) → 2NaCl(aq) + H2O(l) + CO2(g)
NaHCO3(s) + HCl(aq) → NaCl(aq) + H2O(l) + CO2(g)
Limestone, chalk and marble are different forms of calcium carbonate. It can be illustrated from following reaction.
When we pass carbon dioxide gas through lime water then calcium carbonate is formed.
Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)
(Lime water) (White precipitate)
On passing the excess carbon dioxide we get the calcium bicarbonate
CaCO3(s) + H2O(l) + CO2(g) → Ca(HCO3)2(aq)
(Soluble in water)
4) How do Acids and Bases React with each Other?
Acid and base react with each other to form salt and water. This is also known as neutralisation reaction as the effect of base is nullified by an acid and vice versa.
Acid + Base → Salt + Water
Example:
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
5) Reaction of Metallic Oxides with Acids
The reaction between a metal oxide and acid can be written as:
Metal Oxide + Acid → Salt + Water
6) Reaction of a non-metallic Oxide with Base
Metallic oxides are acidic in nature. The reaction can be summarised as:
Nonmetallic Oxide + Base → Salt + Water
Example:
Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)
Similar Properties of Acid and Bases
Acids produce hydrogen ions, H+(aq), in solution which is responsible for their acidic properties.
HCl + H2O → H3O+ + Cl–
Hydrogen ions cannot exist alone, but they exist after combining with water molecules, Thus, hydrogen ions must always be shown as H+(aq) or hydronium ion (H3O+)
Base generate hydroxide (OH–) ions in water. Bases which are soluble in water are called alkalis.
NaOH(s) → Na+(aq) + OH–(aq)
The neutralisation reaction can also be defined as;
H+(aq) + OH–(aq) → H2O(l)
Acid and basic solutions in water conduct electricity because of H+ and OH– ions respectively. The process of dissolving an acid or a base in water is a high exothermic reaction. It results in a decrease in concentration of ions (H3O+/OH–) per unit volume. Such a process is called dilution and the acid or the base is said to be diluted.
Strength of Acids and Bases
Acid-base indicators are used to distinguish between an acid and a base. The strength of an acid or alkali can be tested by using universal indicator or pH scale (0-14). Universal indicator is a mixture of several indicators which shows different colours at different concentration of hydrogen ions in a solution. Similarly, pH scale also give a measure of hydrogen ion concentration in a solution. The p in pH stands for "potenz" which means power. We can measure pH from 0 (very acidic) to 14 (very alkaline) in a pH scale.
A neutral solution has a pH of exactly equal to 7.
An acidic solution has a pH < 7.
A basic solution has a pH > 7.
Importance of pH in Everyday Life
Living organisms carry out their metabolic activities within an optimal pH range.
1) Plants and animals are pH sensitive
Human body works within the pH range of 7.0 to 7.8. The rain water becomes acid rain when pH of rain water is less than 5.6. When this acid rain flows into the rivers in lowers the pH of river water and affect the survival of aquatic life.
2) pH of soil for plant growth
Plants require a specific pH range for their healthy growth which is normally from 5.5 to 7 pH range.
3) pH in digestive system
Human stomach produces hydrochloric acid (HCl) which help in the digestion of food. Sometimes we suffer from the acidity problem. This is because the stomach produces too much acid during indigestion and it causes pain and irritation. To get rid of this pain, people use bases called antacids.
4) pH change as the cause of tooth decay
Teeth are made up of calcium hydroxyapatite. It does not dissolve in water but is corroded when the pH in the mouth is below 5.5. So, tooth decay starts when the pH is lower than 5.5. Using toothpastes for cleaning the teeth after food can neutralize the excess acid and prevent tooth decay.
5) Self defence by animals and plants through chemical warfare
Honey-bee produces methanoic acid which is also known as formic acid when it stung on human body. Similarly, nettle leaves also produces methanoic acid when someone touches its leaves.
Salts and Their Properties
Here we will learn about the preparation, properties and uses of salts.
1) Family of Salts
Salts having the same positive and negative radicals are said to belong to a family.
Example: NaCl and Na2SO4 belong to the family of sodium salts.
NaCl and KCl belong to chloride salts.
2) pH of Salts
We can identify whether a salt is acidic, basic or neutral in nature.
Salts of a strong acid and a strong base are neutral with a pH value of 7.
Salts of a strong acid and a weak base are acidic with a pH value less than 7.
Strong base and weak acid are basic in nature with pH value more than 7.
3) Chemicals from Common Salt
Seawater contains many salts dissolved in it. Sodium chloride is separated from these salts. The large crystals of solid salts which are brown in colour are called rock salt. Beds of rock salt are formed when the seas of bygone ages dried up. Rock salt is mined like coal.
The common salt is used for various purposes in daily use such as sodium hydroxide, baking soda, washing soda, bleaching powder and many more.
Sodium hydroxide
Chlorine is produced during the electrolysis of aqueous sodium chloride (brine). This process is known as chlor-alkali process because the product formed choir is for chlorine and alkali for sodium hydroxide.
2NaCl(aq) + 2H2O(l) → 2NaOH(aq) + Cl2(g) + H2(g)
Bleaching Powder
The chlorine gas produced in the process of chlor-alkali is used for manufacturing of bleaching powder.
Ca(OH)2 + Cl2 → CaOCl2 + H2O
(Dry slaked lime) (Bleaching powder)
Uses of Bleaching Powder
Bleaching cotton and linen in textile industry
Bleaching wood pulp in paper factory
Beaching washed clothes in laundry
For making drinking water free from germs
Used as an oxidizing agent in many chemical industries
Baking Soda
Baking soda is commonly used in kitchen for making recipe. It is also used for faster cooking. Its chemical name is sodium hydrocarbonate (NaHCO3).
NaCl + H2O + CO2 + NH3 → NH4Cl + NaHCO3
Baking soda is a mild non-corrosive basic salt. When it is heated for cooking from chemical reaction take place;
2NaHCO3 → Na2CO3 + H2O + CO2
Uses of Baking Soda
It is used for making baking powder which is a mixture of baking soda and a mild edible acid such as tartaric acid. Following reaction take place when baking soda is heated.NaHCO3 + H+ → CO2 + H2O + Sodium salt of acid [H+ is taken from any acid]
Used in soda acid fire extinguishers.
Use for neutralizing excess acid in the stomach as it is an ingredient in antacids.
Washing Soda
Recrystallisation of sodium carbonate provides washing soda. It is a basic salt and its chemical formula is Na2CO3.10H2O
Na2CO3 + 10 H2O → Na2CO3.10H2O
(Sodium carbonate)
Uses of Washing Soda
For removing permanent hardness of water
Manufacturing of compounds such as borax
Used in glass, soap and paper industry
Cleaning agent for domestic purposes
4) Are the Crystal of Salts really Dry?
Copper sulphate crystals seem to be dry but they contain water of crystallization. When we heat the crystals, water is removed and the salt turns white. If we moisten the crystals again with water, then blue colour of crystals reappears.
Water of crystallization is the fixed number of water molecules present in one formula unit of a salt. Chemical formula for hydrated copper sulphate is CuSO4.5H2O. There is one other salt which possesses water of crystallization is gypsum. Its chemical formula is CaSO4.2H2O.
Plaster of Paris
When gypsum is heated at 373 K then it comes calcium sulphate hemihydrate which is known as Plaster of Paris. Its chemical formula is CaSO4.H2O. Doctors use it as a plaster for supporting fractures bones. Plaster of Paris can be changed again to gypsum on mixing water to it.
CaSO4.H2O + H2O → CaSO4.2H2O
(Plaster of Paris) (Gypsum)
In the above equation, half a water molecule shown is attached as water of crystallisation.
Uses of Plaster of Paris
Making the toys
Making the decoration material
For making surfaces smooth
By doctor for plastering the facture bones