15. Acids and Bases
Definitions
pH Scale
Difference between Strong and Weak Acids
Reactions of Bases
Effect of pH on soil
Nature of Oxides
Oxides can be classified as acidic, neutral, basic or amphoteric based on metallic/non-metallic character.
- An acid is a compound that produces hydrogen ions (H+) when dissolved in water. H+ ions are responsible for acidic properties.
- A base is a metallic oxide or metallic hydroxide.
- An alkali is a soluble base that produces hydroxide ions (OH-) when dissolved in water.
- The pH is a measure of how acidic or alkaline a solution is in water. The pH scale is numbered from 0 (most acidic) to 14 (most alkaline).
- A universal indicator is a mixture that gives different colours at different pH.
pH Scale
- pH 7 is neutral. [Concentration of H+ ions = Concentration of OH- ions.]
- pH less than 7 is acidic. [Concentration of H+ ions > Concentration of OH- ions.]
- pH more than 7 is alkaline. [Concentration of H+ ions < Concentration of OH- ions.]
Difference between Strong and Weak Acids
- In a strong acid, all the acid molecules are completely ionized when dissolved in water. e.g. HCl
- In a weak acid, the acid molecules are only partially ionized when dissolved in water. e.g. CH3COOH(aq)
- A strong acid contains a higher concentration of H+ ions than a weak acid of the same concentration.
- Acid + metals: Acids react with metals (Mg, Zn and Fe) to form salt and hydrogen.
- Acid + carbonate or hydrogencarbonates: Acids react with carbonates or hydrogencarbonates to produce a salt, carbon dioxide and water.
- Acid + base/ alkali (neutralisation reaction): Acids neutralise base/alkalis to form salt and water only.
Reactions of Bases
- Base + ammonium salt: Bases react with ammonium salts to form a salt, water and ammonia gas.
Effect of pH on soil
- To reduce acidity, calcium oxide (quicklime), calcium hydroxide (slake lime) or calcium carbonate can be added.
- Soluble bases like sodium hydroxide should not be added to neutralise acidic soil as there is a possibility of adding excess of it which will result in soil becoming too alkaline.
Nature of Oxides
Oxides can be classified as acidic, neutral, basic or amphoteric based on metallic/non-metallic character.
- Acidic: Non-metallic oxides - CO2 ,SO2 , NO2, P4O10
- Neutral: Non-metallic oxides - CO , NO, H2O
- Basic: Metallic oxides - Na2O, MgO, CaO, FeO
- Amphoteric: Metallic oxides - Al2O3, PbO, ZnO (exhibit both acidic and basic properties)