Defining Bases and Alkalis
An alkali is a specific type of base that is soluble in water. A base is any substance that can neutralize an acid, while an alkali is a base that can dissolve in water to form hydroxide ions (OH⁻). Therefore, the key rule to remember is: all alkalis are bases, but not all bases are alkalis.
Section 2: Key Characteristics
Bases are broadly defined as substances that accept hydrogen ions (protons). They can be either soluble or insoluble in water. Alkalis, being a subset of bases, are always soluble in water. When dissolved, they produce hydroxide ions, which gives alkaline solutions their characteristic slippery feel and the ability to turn red litmus paper blue.
Section 3: A Practical Example
Sodium hydroxide (NaOH) is a classic example of an alkali because it is a base that readily dissolves in water. In contrast, copper(II) oxide (CuO) is a base because it reacts with and neutralizes acids, but it is not an alkali because it is insoluble in water. You can react copper(II) oxide with acid, but you cannot dissolve it in a simple glass of water.
Section 4: Why the Distinction Matters
Understanding this difference is crucial for solution chemistry. When discussing pH changes in aqueous (water-based) solutions, we are typically dealing with alkalis because their solubility allows them to release hydroxide ions and directly impact the pH. The term 'base' is a broader classification that includes solid substances that might only react in non-aqueous environments.