1. Introduction to Acids
Acids are substances we come across in our daily life—like the sour taste of lemon, curd, oranges, or tamarind.
In Chemistry, an acid is defined by the ions it releases in water.
2. Scientific Definition of Acids
In Chemistry, an acid is a substance that releases hydrogen ions \( H^+ \) when dissolved in water. This presence of hydrogen ions gives acids their characteristic properties like sour taste, ability to react with metals, and effect on indicators.
2.1. Arrhenius Definition
According to Arrhenius, an acid is a substance that increases the concentration of \( H^+ \) ions when dissolved in water.
Example: Hydrochloric acid (HCl) dissociates as:
\( \text{HCl} \rightarrow \text{H}^+ + \text{Cl}^- \)
2.2. Bronsted–Lowry Definition
Bronsted and Lowry defined an acid as a proton donor. This means an acid is a substance that can donate \( H^+ \) ions to another substance.
2.3. Lewis Definition
According to Lewis, an acid is a substance that can accept an electron pair. This concept is useful for advanced chemical reactions, especially in organic chemistry.
3. Properties of Acids
Acids show certain characteristic properties that help us identify them in laboratories and everyday life.
3.1. Physical Properties
- Acids generally taste sour (never taste acids in labs).
- They turn blue litmus paper red.
- They are corrosive in nature.
- Most acids dissolve in water.
3.2. Chemical Properties
- Reaction with Metals: Acids react with metals like zinc or magnesium to produce hydrogen gas.
- Reaction with Bases: Acids neutralise bases to form salt and water.
- Reaction with Carbonates: Acids react with carbonates to release carbon dioxide.
3.2.1. Reaction with Metals
General equation:
\( \text{Acid} + \text{Metal} \rightarrow \text{Salt} + \text{H}_2 \uparrow \)
Example:
\( 2HCl + Zn \rightarrow ZnCl_2 + H_2 \uparrow \)
3.2.2. Reaction with Bases
Neutralisation reaction:
\( \text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water} \)
3.2.3. Reaction with Carbonates
General reaction:
\( \text{Acid} + \text{Carbonate} \rightarrow \text{Salt} + \text{CO}_2 + \text{H}_2O \)
4. Common Examples of Acids
Acids may be natural (found in food) or laboratory-prepared. Some commonly known acids include:
- Hydrochloric Acid (HCl): Found in the stomach.
- Sulphuric Acid (H2SO4): Industrial chemical.
- Acetic Acid (CH3COOH): Found in vinegar.
- Citric Acid: Found in citrus fruits.
- Lactic Acid: Formed in muscles after heavy exercise.
5. Classification of Acids
Acids can be classified in different ways based on their strength, source, and composition.
5.1. Based on Strength
- Strong Acids: Completely dissociate in water (e.g., HCl, HNO3).
- Weak Acids: Partially dissociate in water (e.g., acetic acid).
5.2. Based on Source
- Organic Acids: Obtained from plants/animals (citric acid, acetic acid).
- Mineral Acids: Prepared in the laboratory (HCl, H2SO4).
5.3. Based on Basicity
- Monobasic acids: Release one \( H^+ \) ion (HCl).
- Dibasic acids: Release two \( H^+ \) ions (H2SO4).
- Tribasic acids: Release three \( H^+ \) ions (H3PO4).
6. Uses of Acids in Daily Life
Acids play an important role in industries, household processes, medicines, and even inside living organisms.
- Vinegar (acetic acid) is used in cooking.
- Citrus fruits contain citric acid which gives them a sour taste.
- Hydrochloric acid helps in digestion in the stomach.
- Sulphuric acid is used in batteries and fertilizers.
7. Safety Precautions While Handling Acids
- Wear gloves and goggles in the laboratory.
- Never taste or directly smell acids.
- Always add acid to water, not water to acid.
- Store acids in properly labelled containers.