1. Acids have a sour taste (e.g., citric acid in lemons and acetic acid in vinegar).
2. They are corrosive in nature and can destroy metals and skin tissues on contact.
3. Most acids are soluble in water and conduct electricity due to ionization.
4. Acids turn blue litmus paper red but have no effect on red litmus paper.
5. They have pH values less than 7 depending on their concentration and strength.

Acids exhibit several characteristic chemical reactions with different classes of compounds. Some of the important chemical reactions of acids are described below:

1. Reaction with Metals: Acids react with active metals such as zinc and magnesium to produce a salt and hydrogen gas.

$$ \text{Zn} + \text{H}_2\text{SO}_4 \rightarrow \text{ZnSO}_4 + \text{H}_2 $$ $$ \text{Mg} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2 $$

2. Reaction with Bases (Neutralization Reaction): Acids react with bases or alkalis to form salt and water only — this is called neutralization.

$$ \text{HCl} + \text{NaOH} \rightarrow \text{NaCl} + \text{H}_2\text{O} $$ $$ \text{H}_2\text{SO}_4 + 2\text{KOH} \rightarrow \text{K}_2\text{SO}_4 + 2\text{H}_2\text{O} $$

3. Reaction with Carbonates and Trioxocarbonates (CO₃²⁻): Acids react with carbonates or trioxocarbonates to produce a salt, carbon dioxide gas, and water.

$$ 2\text{HCl} + \text{Na}_2\text{CO}_3 \rightarrow 2\text{NaCl} + \text{H}_2\text{O} + \text{CO}_2 $$ $$ \text{H}_2\text{SO}_4 + \text{CaCO}_3 \rightarrow \text{CaSO}_4 + \text{H}_2\text{O} + \text{CO}_2 $$

4. Reaction with Metal Oxides: Metal oxides are basic in nature and react with acids to form a salt and water.

$$ \text{CuO} + 2\text{HCl} \rightarrow \text{CuCl}_2 + \text{H}_2\text{O} $$ $$ \text{Fe}_2\text{O}_3 + 6\text{HCl} \rightarrow 2\text{FeCl}_3 + 3\text{H}_2\text{O} $$

Acids can be prepared by several methods depending on the type of acid desired. The following are common methods of preparation:

1. Direct Combination of Elements: Acids can be formed when hydrogen reacts directly with certain non-metals such as chlorine.
   $$ \text{H}_2 + \text{Cl}_2 \rightarrow 2\text{HCl} $$

   This method involves the direct union of elements to form a simple acid like hydrogen chloride gas.

2. By Dissolving Acid Anhydrides in Water: Acid anhydrides are non-metal oxides that react with water to form corresponding acids.
   $$ \text{SO}_3 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_4 $$ $$ \text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{CO}_3 $$

   Acid anhydrides are oxides that form acids when combined with water. For example, sulfur trioxide is the anhydride of sulfuric acid.

3. By Reaction with Salts: Strong acids can be obtained when certain salts react with concentrated acids.
   $$ \text{NaCl} + \text{H}_2\text{SO}_4 \rightarrow \text{NaHSO}_4 + \text{HCl} $$ $$ 2\text{KNO}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{K}_2\text{SO}_4 + 2\text{HNO}_3 $$

   In this method, the stronger acid displaces a weaker acid from its salt.

4. By Displacing a Weak Acid with a Stronger Acid: A stronger acid can displace a weaker acid from its salt solution. However, the weak acid formed is usually unstable and decomposes immediately into other products such as water and carbon dioxide.
   $$ \text{Na}_2\text{CO}_3 + 2\text{HCl} \rightarrow 2\text{NaCl} + \text{H}_2\text{CO}_3 $$ $$ \text{This decomposes into;} $$ $$ \text{H}_2\text{CO}_3 \rightarrow \text{H}_2\text{O} + \text{CO}_2 $$
   $$ \text{CaCO}_3 + 2\text{HNO}_3 \rightarrow \text{Ca(NO}_3)_2 + \text{H}_2\text{CO}_3 $$ $$ \text{This decomposes into;} $$ $$ \text{H}_2\text{CO}_3 \rightarrow \text{H}_2\text{O} + \text{CO}_2 $$

   This method is used in the laboratory to produce weak and unstable acids such as carbonic acid (H₂CO₃) or sulfurous acid (H₂SO₃), which decompose immediately after formation.

Acids have numerous applications in industries, laboratories, and domestic activities. Below are some important uses of acids:

1. Manufacturing of Fertilizers: Acids such as nitric acid (HNO₃) and sulfuric acid (H₂SO₄) are used in producing fertilizers like ammonium nitrate and superphosphate.
2. Cleaning and Pickling of Metals: Dilute hydrochloric acid (HCl) is used to remove rust and scale from metals before electroplating or painting.
3. Manufacture of Explosives: Nitric acid is used in making explosives such as trinitrotoluene (TNT).
4. Production of Dyes and Plastics: Acids like sulfuric and acetic acids are used in synthesizing dyes, plastics, and synthetic fibers.
5. Food Preservation: Organic acids such as citric acid and acetic acid are used as food preservatives and flavoring agents.
6. Cleaning Agents: Hydrochloric acid and sulfuric acid are used in domestic and industrial cleaning products.
7. Battery Acid: Sulfuric acid is used as the electrolyte in lead–acid batteries for vehicles.
8. Treatment of Water: Acids help adjust the pH of water in industrial and municipal water treatment plants.
9. Laboratory Reagents: Acids like HCl, H₂SO₄, and HNO₃ are commonly used in laboratories for titrations and other chemical analyses.
10. Production of Pharmaceuticals: Acids such as acetylsalicylic acid (aspirin) and citric acid are important ingredients in medicines.
11. Use of Boric Acid: Boric acid (H₃BO₃) is used as an antiseptic, eye wash, and in the manufacture of glass, ceramics, and flame retardants.
12. Use of Tartaric Acid: Tartaric acid (C₄H₆O₆) is used in baking powders, food processing, and as a stabilizer in wine production.