Acid: Acid is a compound which gives hydrogen ion in aqueous solution For example Hydrochloric acid, sulphuric acid etc.
Organic acid: Acids which are obtained from the body of living organisms are called organic acids. Example Citric acid Tartaric acid etc.
Inorganic acid: Acids which are obtained from the mineral of the earth crust are called inorganic acids. Example: Sulphuric acid, Hydrochloric acid etc.
Strong acid: The acids which undergo almost complete ionization in aqueous solution and produce high concentration of hydrogen ions in solution are called strong acid. For example: Sulphuric acid Hydrochloric acid etc.
Weak Acids: The acids which donot undergo complete ionization when dissolved in water is called weak acid. Example: Carbonic acid, Citric acid etc
Concentration of acid: Quantity of acid present in a definite volume of water represents the concentration of acids.
Concentrated acid: An acid is called concentrated acid if the quantity of acid present in a definite volume of water is very high.
Dilute acid: An acid is called dilute acid if it contains less quantity of acid in a definite volume of water.
Base: Metallic oxides, metallic hydroxides and ammonium hydroxides are called bases. Example: Sodium oxide, Calcium hydroxide, ammonium hydroxide etc
Alkali: Water soluble bases are called alkalis which give hydroxyl ion (OH–) in aqueous solution. Example: Sodium hydroxide, Potassium hydroxide etc
Strong base: The bases which undergo almost complete ionization in aqueous solution and produce high concentration of hydroxyl ions are called strong base. Example: sodium hydroxide, potassium hydroxide etc.
Weak bases: The bases which undergo partial ionization in aqueous solution and produce low concentration of hydroxyl ions are called weak bases. For example: Calcium hydroxide, Ammonium hydroxide.
Salt: Salt is a compound which is formed by the partial or complete displacement of hydrogen from an acid by a metal or a radical which behaves like a metal. Example: Sodium chloride potassium chloride etc.
Acidic salt: A salt formed by the reaction of a strong acid and a weak base is called acidic salt. Or The salt formed by the partial displacement of hydrogen ions of acid by the metallic radicals of base is called acidic salt. Example: Potassium bisulphate, Calcium bicarbonate etc.
Basic salt: A salt formed by the reaction of strong base and weak acid is called basic salt. Or, the salt formed by the partial displacement of hydroxyl ions of base by the acidic radical of acids is called basic salt. Example: Sodium carbonate, Calcium carbonate etc.
Neutral salt: A salt formed by the reaction of strong acid and strong base or by reaction of weak acid and weak base is called neutral salt. Or, the salt formed by the complete displacement of hydrogen ions of acid is called neutral salt. Example sodium chloride, potassium sulphate etc.
Indicator: The chemical substances which are used to identify the given solution as base, acid or salt by changing their colors are called indicators. Example: Litmus paper, Phenolphthalein and methyl orange etc.
Universal indicators: The mixture of ordinary indicators which is used to determine the strength of the acidic and alkaline solution is called universal indicator.
pH value: The PH value of solution is the negative logarithm to the base 10 of the hydrogen ion concentration expressed in moles per liter.
pH scale: It is a scale with which the strength of acidic and alkaline solutions is measured. The scale starts from pH value 1 and ends at pH 14.
pH meter: It is a device which is used to measure the strength of the acidic and alkaline solution.
REASONABLE FACTS
1. Lemon juice is sour in taste.
Ans- Lemon juice is sour in taste because it is acidic in nature.
2. All the alkalis are bases but all the bases are not alkalis.
Ans- Water soluble bases are called alkalis while the bases which are not soluble in water are not alkalis. So, all the alkalis are bases but all the bases are not alkalis.
3. Hydrogen halides are covalent molecules but conduct electricity in aq. Medium.
Ans- Hydrogen halides are covalent molecules but conduct electricity in aq. Medium because chlorine possesses a partial negative charge and hydrogen acquires positive charge.
4. Ammonium hydroxide is a common alkali but generally differs from other alkalis.
Ans- Ammonium hydroxide is a common alkali but generally differs from other alkalis because it does not contain a metal.
5. NaOH is called a strong alkali but NH4(OH)2 is called a weak alkali.
Ans-NaOH is fully dissociated in water and thus produces a large no. of hydroxyl ions so it is called as a strong alkali or base. But NH4(OH)2 is partially dissociated in water and thus produces a small no. of hydroxyl ions so it is called as a weak alkali or base.
6. The acetic acid and liquor ammonia (or ammonia solution) are weaker acid and base respectively.
Ans- Both acetic acid and liquor ammonia (or ammonium hydroxide) are weaker acid and base respectively because they do not ionize completely.
7. Acid-base reaction is also called as neutralization reaction.
Ans- Acid-base reaction is also called as neutralization reaction because the characteristic properties of both acids and bases are destroyed and the third compound called as sealt and water which are neutral are formed.
8. Every salt has its parent acid and parent base.
Ans- Every salt has its parent acid and parent base because the characteristic properties of both acids and bases are destroyed to form the salt during the chemical reaction.
9. Universal indicators are more efficient (reliable) than any other common lab. Indicators like litmus.
Ans- Universal indicators are more efficient (reliable) than any other common lab. Indicators like litmus because it is a mixture of simple indicators that helps in identifying the acidic and basic characters of substance with their strength.
10. Sodium bicarbonate is called an acid salt.
Ans- Sodium bicarbonate is formed by the partial displacement of hydrogen atoms of the acid. So, sodium bicarbonate is called an acid salt.
11. Carbonic acid is called a weak acid.
Ans- Carbonic acid is called a weak because it is partially ionized in water and thus produces a small no. of hydrogen ions. Most of its molecules remain in molecular form in solution. H2CO3(aq.)® 2H+(aq.)+CO32-(aq.)
12. Acetic acid is called a weak acid.
Ans-Acetic acid is an acid having low degree of ionization. It produces low concentration of hydrogen ions in its solution. So, Acetic acid is called a weak acid.
13. HCI is called a strong acid.
Ans-HCI is called a strong acid because it is completely ionized in water and thus produces a large no. of hydrogen ions. HCI (aq.)«H+(aq.)+CL(aq.)
14. Acids are called proton donors.
Ans- Acids gives hydrogen ions (H+) in its aqueous solution. Being hydrogen ions electropositive, they are called protons. So, acids are called proton donors.
15. Acids are sour in taste.
Ans- Acid contains hydrogen ions (H+ions). These ions stimulate the taste buds so taste of acid is sour.
16. Pickle cannot be stored in metal vessels for a long time.
Ans- Pickle contain week acids which slowly react with metal vessels and corrode them. Therefore, pickles cannot be stored in metal vessels for long time.
17. A universal indicator is more meaningful then an ordinary indicator.
Ans- A universal indictor not only shows whether a tested solution is acidic or basis but also the strength of the acid or base by giving the PH value. But an indicator shows whether the solution is acidic or basic and it cannot measure the strength of acidity or basicity. Therefore a universal in director is more meaningful then an ordinary indicator.
18. Ammonium hydroxide is different from other bases.
Ans- Generally bases are metallic oxides or hydroxides i.e. they contain metallic ions but ammonium hydroxide is made from the non-metallic radical.
Additional Study to Acids, Bases and Salts
Classification of matter
On the basis of
a) composition – elements, compounds and mixtures
b) state – solids, liquids and gases
c) solubility – suspensions, colloids and solutions
Types of mixtures – homogeneous and heterogeneous
Types of compounds – covalent and ionic
What Is an Acid and a Base?
Ionisable and non-ionisable compounds
An ionisable compound when dissolved in water or in its molten state, dissociates into ions almost entirely. Example: NaCl, HCl, KOH, etc.
A non-ionisable compound does not dissociate into ions when dissolved in water or in its molten state. Example: glucose, acetone, etc.
Arrhenius theory of acids and bases
Arrhenius acid – when dissolved in water, dissociates to give H+ (aq) or H3O+ ion.
Arrhenius base – when dissolved in water, dissociates to give OH− ion.
Examples
Acids
- Hydrochloric acid (HCl)
- Sulphuric acid (H2SO4)
- Nitric acid (HNO3)
Bases
- Sodium hydroxide (NaOH)
- Potassium hydroxide (KOH)
- Calcium hydroxide (Ca(OH)2)
Bronsted Lowry theory
A Bronsted acid is an H+ (aq) ion donor.
A Bronsted base is an H+ (aq) ion acceptor.
Example
In the reaction: HCl (aq) + NH3 (aq) → NH+4(aq) + Cl− (aq)
HCl – Bronsted acid and Cl− : its conjugate acid
NH3 – Bronsted base and NH+4 : its conjugate acid
Physical test
Given are two possible physical tests to identify an acid or a base.
a. Taste
An acid tastes sour whereas a base tastes bitter.
The method of taste is not advised as an acid or a base could be contaminated or corrosive.
b. Effect on indicators by acids and bases
An indicator is a chemical substance which shows a change in its physical properties, mainly colour or odour when brought in contact with an acid or a base.
Below mentioned are commonly used indicators and the different colours they exhibit:
a) Litmus
In a neutral solution – purple
In acidic solution – red
In basic solution – blue
Litmus is also available as strips of paper in two variants – red litmus and blue litmus.
An acid turns a moist blue litmus paper to red.
A base turns a moist red litmus paper to blue.
b) Methyl orange
In a neutral solution – orange
In acidic solution – red
In basic solution – yellow
c) Phenolphthalein
In a neutral solution – colourless
In acidic solution – remains colourless
In basic solution – pink
Acid-Base Reactions
Reactions of acids and bases
a) Reaction of acids and bases with metals
Acid + active metal → salt + hydrogen + heat
2HCl + Mg → MgCl2 + H2 (↑)
Base + metal → salt + hydrogen + heat
2NaOH + Zn → Na2ZnO2 + H2 (↑)
A more reactive metal displaces the less reactive metal from its base.
2Na + Mg (OH) 2 → 2NaOH + Mg
b) Reaction of acids with metal carbonates and bicarbonates
Acid + metal carbonate or bicarbonate → salt + water + carbon dioxide.
2HCl + CaCO3 → CaCl2 + H2O + CO2
H2SO4 + Mg (HCO3)2 → MgSO4 + 2H2O + 2CO2
Effervescence indicates liberation of CO2 gas.
c) Neutralisation reaction
1. Reaction of metal oxides and hydroxides with acids
Metal oxides or metal hydroxides are basic in nature.
Acid + base → salt + water + heat
H2SO4 + MgO → MgSO4 + H2O
2HCl + Mg (OH) 2 → MgCl2 + 2H2O
2. Reaction of non-metal oxides with bases
Non-metal oxides are acidic in nature
Base + Nonmetal oxide → salt + water + heat
2NaOH + CO2→ Na2CO3 + H2O
Water
Acids and bases in water
When added to water, acids and bases dissociate into their respective ions and help in conducting electricity.
Difference between a base and an alkali
Base:
- Bases undergo neutralisation reaction with acids.
- They are comprised of metal oxides, metal hydroxides, metal carbonates and metal bicarbonates.
- Most of them are insoluble in water.
Alkali:
- An alkali is an aqueous solution of a base, (mainly metallic hydroxides).
- It dissolves in water and dissociates to give OH− ion.
- All alkalis are bases, but not all bases are alkalis.
Hydronium ion
Hydronium ion is formed when a hydrogen ion accepts a lone pair of electrons from the oxygen atom of a water molecule, forming a coordinate covalent bond.
Dilution
Dilution is the process of reducing the concentration of a solution by adding more solvent (usually water) to it.
It is a highly exothermic process.
To dilute acid, the acid must be added to water and not the other way round.
Strength of acids and bases
Strong acid or base: When all molecules of a given amount of an acid or a base dissociate completely in water to furnish their respective ions, H+(aq) for acid and OH−(aq) for base).
Weak acid or base: When only a few of the molecules of a given amount of an acid or a base dissociate in water to furnish their respective ions, H+(aq) for acid and OH−(aq) for base).
Dilute acid: contains less number of H+(aq) ions per unit volume.
Concentrated acid: contains more number of H+(aq) ions per unit volume.
Universal indicator
A universal indicator has a pH range from 0 to 14 that indicates the acidity or alkalinity of a solution.
A neutral solution has pH=7
pH
pH=−log10[H+]
In pure water, [H+]=[OH−]=10−7 mol/L. Hence, the pH of pure water is 7.
The pH scale ranges from 0 to 14.
If pH < 7 → acidic solution
If pH > 7→ basic solution
pH scale
Importance of pH in everyday life
1. pH sensitivity of plants and animals
Plants and animals are sensitive to pH. Crucial life processes such as digestion of food, functions of enzymes and hormones happen at a certain pH value.
2. pH of a soil
The pH of a soil optimal for the growth of plants or crops is 6.5 to 7.0.
3. pH in the digestive system
The process of digestion happens at a specific pH in our stomach which is 1.5 to 4.
The pH of the interaction of enzymes, while food is being digested, is influenced by HCl in our stomach.
4. pH in tooth decay
Tooth decay happens when the teeth are exposed to an acidic environment of pH 5.5 and below.
5. pH of self-defence by animals and plants
Acidic substances are used by animals and plants as a self-defence mechanism. For example, bee and plants like nettle secrete a highly acidic substance for self-defence. These secreted acidic substances have a specific pH.
Manufacture of Acids and Bases
Manufacture of acids and bases
a) Nonmetal oxide + water → acid
SO2(g) + H2O(l) → H2SO3(aq)
SO3(g) + H2O(l) → H2SO4(aq)
4NO2(g) + 2H2O(l) + O2(g) → 4HNO3(aq)
Non-metal oxides are thus referred to as acid anhydrides.
b) Hydrogen + halogen → acid
H2(g) + Cl2(g) → 2HCl(g)
HCl(g) + H2O(l) → HCl(aq)
c) Metallic salt + conc. sulphuric acid → salt + more volatile acid
2NaCl(aq) + H2SO4(aq) → Na2SO4(aq) + 2HCl(aq)
2KNO3(aq) + H2SO4(aq) → K2SO4(aq) + 2HNO3(aq)
d) Metal + oxygen → metallic oxide (base)
4Na(s) + O2(g) → 2Na2O(s)
2Mg(s) + O2(g) → 2MgO(s)
e) Metal + water → base or alkali + hydrogen
Zn(s) + H2O(steam) → ZnO(s)+ H2(g)
f) Few metallic oxides + water → alkali
Na2O(s) + H2O(l) → 2NaOH(aq)
g) Ammonia + water → ammonium hydroxide
NH3(g) + H2O(l) → NH4OH(aq)
Salts
A salt is a combination of an anion of an acid and a cation of a base.
Examples – KCl, NaNO3 ,CaSO4, etc.
Salts are usually prepared by the neutralisation reaction of an acid and a base.
Common salt
Sodium Chloride (NaCl) is referred to as common salt because it’s used all over the world for cooking.
Family of salts
Salts having the same cation or anion belong to the same family. For example, NaCl, KCl, LiCl.
pH of salts
A salt of a strong acid and a strong base will be neutral in nature. pH = 7 (approx.).
A salt of a weak acid and a strong base will be basic in nature. pH > 7.
A salt of a strong acid and a weak base will be acidic in nature. pH < 7.
The pH of a salt of a weak acid and a weak base is determined by conducting a pH test.
Preparation of Sodium hydroxide
Chemical formula – NaOH
Also known as – caustic soda
Preparation (Chlor-alkali process):
Electrolysis of brine (solution of common salt, NaCl) is carried out.
At anode: Cl2 is released
At cathode: H2 is released
Sodium hydroxide remains in the solution.
Bleaching powder
Chemical formula – Ca(OCl)Cl or CaOCl2
Preparation – Ca(OH)2(aq)+Cl2(g)→CaOCl2(aq)+H2O(l)
On interaction with water – bleaching powder releases chlorine which is responsible for bleaching action.
Baking soda
Chemical name – Sodium hydrogen carbonate
Chemical formula – NaHCO3
Preparation (Solvay process):
a. Limestone is heated: CaCO3→CaO+CO2
b. CO2 is passed through a concentrated solution of sodium chloride and ammonia:
NaCl(aq)+NH3(g)+CO2(g)+H2O(l)→NaHCO3(aq)+NH4Cl(aq)
Uses:
1. Textile industry
2. Paper industry
3. Disinfectant
Washing soda
Chemical name – Sodium hydrogen carbonate
Chemical formula – NaHCO3
Preparation (Solvay process) –
a. Limestone is heated: CaCO3 → CaO + CO2
b. CO2 is passed through a concentrated solution of sodium chloride and ammonia:
NaCl(aq) + NH3(g) + CO2(g) + H2O(l) → NaHCO3(aq) + NH4Cl(aq)
Uses
1. In glass, soap and paper industries
2. Softening of water
3. Domestic cleaner
Crystals of salts
Certain salts form crystals by combining with a definite proportion of water. The water that combines with the salt is called water of crystallisation.
Plaster of paris
Gypsum, CaSO4.2H2O (s) on heating at 100°C (373K) gives CaSO4. ½ H2O and 3/2 H2O
CaSO4. ½ H2O is plaster of paris.
CaSO4. ½ H2O means two formula units of CaSO4 share one molecule of water.
Uses – cast for healing fractures.
Frequently Asked Questions on Acids, Bases and Salts
When dissolved in water, Arrhenius base is a compound that ……….?
An Arrhenius base is a compound that increases the concentration of OH– ions that are present when added to water.
A solution X has a pH value of 2 and another solution Y has a pH value of 1. What can be inferred regarding the difference in hydrogen ion concentration between them?
pH is a measure of hydrogen ion concentration in a solution. Higher the hydrogen ion concentration, lower is the pH. Acids which give rise to more hydrogen ions are more acidic than the acids which give less hydrogen ions. Thus, lower the pH, higher is the acidic nature of the solution. Thus, Y has more hydrogen ion concentration than X.
Dilute hydrochloric acid (HCl) reacts with metals to evolve __ gas along with the formation of corresponding metal salt.
Acids react with metals to produce the respective metal salt along with hydrogen gas. Therefore, when hydrochloric acid (HCl) reacts with metals it evolve hydrogen gas along with the formation of corresponding metal salt.
