Bases are chemical substances that can neutralize acids. Many common bases that dissolve in water are called alkalis.

Classification: Soluble alkalis include Sodium Hydroxide (NaOH), Potassium Hydroxide (KOH), and Calcium Hydroxide (Ca(OH)₂). Insoluble bases, which do not dissolve in water, include Copper Hydroxide (Cu(OH)₂) and Iron (III) Hydroxide (Fe(OH)₃).

Effect on Indicators: Bases cause specific color changes with chemical indicators. For example, they turn purple litmus solution blue and colorless phenolphthalein solution red or pink.

Neutralization Reaction: A key chemical property of bases is their reaction with acids to form a salt and water. This is called neutralization. For example: NaOH + HCl → NaCl + H₂O.

Bases also react with acidic non-metallic oxides (e.g., CO₂ + 2NaOH → Na₂CO₃ + H₂O) and can react with solutions of certain salts to form new bases and new salts via precipitation reactions.