Concept of Acids, and Bases

Base

A base is a substance that contains the oxide ion or hydroxide ion and reacts with acid to give salt and water only. Alternatively, a base can be defined as a soluble substance that accepts a proton. A soluble base is called an alkali.

Examples of alkalis

• Calcium hydroxide (lime water): $Ca(OH)_2$
• Magnesium hydroxide (milk of magnesia): $Mg(OH)_2$

Magnesium hydroxide is sparingly soluble, and soluble bases, when in solution, produce $OH^-$ as their only negative ion.

Properties of bases

1. Alkalis have a soapy, slippery feeling.
2. Alkalis have a bitter taste.
3. Alkalis turn red litmus paper to blue.
4. They react with acids and water only.
5. Alkalis yield ammonia when warmed with ammonium salts.

Uses of bases

Bases have various uses, including the preparation of salts, soaps, and neutralization reactions.

Acids

Acids are substances that, when in solution, produce hydrogen ions ($H^+$) as their only positive ions. They are also substances that turn blue litmus paper red and contain hydrogen that can be replaced by a metal.

Types of acids

i. Mineral acids

These acids are derived from substances found in minerals. For example, sulfuric acid ($H_2SO_4$) is derived from minerals.

ii. Organic acids

Organic acids are derived from plants and animals. For example, citric acid from fruits and acetic acid from vinegar.

Strength of acids

Acids are classified as:

1. Strong Acids: These acids ionize completely in aqueous solution.
2. Weak Acids: These acids ionize partially in aqueous solution. All organic acids are weak.

Properties of acids

1. Have a sour taste.
2. Action on indicators: Indicators like litmus, methyl orange, and phenolphthalein change color in acids.
3. Corrosive action: Concentrated acids are corrosive and can burn skin, leaves, and plant stems.
4. Reaction with metals: Metals above hydrogen in the activity series replace hydrogen from acids, producing salt and hydrogen gas.
5. Reaction with metal oxides and hydroxides to form salt and water.
6. Reaction with carbonates and bicarbonates (hydrogen carbonates) to produce salt, water, and carbon dioxide ($CO_2$).
7. Action with sulphates: When acids react with sulphates, they produce $SO_2$, water, and salts.

Basicity of acids

The basicity of an acid refers to the number of hydrogen ions ($H^+$) produced by one mole of an acid in aqueous solution. All organic acids are monobasic.

pH scale

The pH scale is used to describe the strength of an acid or base by indicating the concentration of hydrogen ions. It can be represented as follows:

1-4: Strong Acid

5-6: Weak Acid

7: Neutral

8-9: Weak Alkali

10-14: Strong Alkali

Uses of acids

1. Preparation of Salts: For example, from fertilizers or reactions with bases/metals.
   1. $Ca(OH)_2 + 2HCl \rightarrow CaCl_2 + 2H_2O$
   2. $NH_4OH + H_2SO_4 \rightarrow (NH_4)_2SO_4 + 2H_2O$
2. Preparation of Other Acids: For example, $Pb(NO_3)_2 + H_2SO_4 \rightarrow PbSO_4 + 2HNO_3$
3. Manufacture of Artificial Silk
4. Clearing of Metals: Acids combine with metals to form salts that are easily removed.
5. Necessary for Digestion: Acids are involved in the digestion of proteins in the stomach.