Differentiate hydrogen ion from hydroxide ions in all aspect not just the structure.

It all has to do with hydrogen ions (abbreviated with the chemical symbol H⁺). In water (H₂O), a small number of the molecules dissociate (split up). Some of the water molecules lose a hydrogen and become hydroxide ions (OH⁻). The "lost" hydrogen ions join up with water molecules to form hydronium ions (H₃O⁺). For simplicity, hydronium ions are referred to as hydrogen ions H⁺. In pure water, there are an equal number of hydrogen ions and hydroxide ions. The solution is neither acidic or basic.

An acid is a substance that donates hydrogen ions. Because of this, when an acid is dissolved in water, the balance between hydrogen ions and hydroxide ions is shifted. Now there are more hydrogen ions than hydroxide ions in the solution. This kind of solution is acidic.

A base is a substance that accepts hydrogen ions. When a base is dissolved in water, the balance between hydrogen ions and hydroxide ions shifts the opposite way. Because the base "soaks up" hydrogen ions, the result is a solution with more hydroxide ions than hydrogen ions. This kind of solution is alkaline.

Acidity and alkalinity are measured with a logarithmic scale called pH. Here is why: a strongly acidic solution can have one hundred million million, or one hundred trillion (100,000,000,000,000) times more hydrogen ions than a strongly basic solution! The flip side, of course, is that a strongly basic solution can have 100,000,000,000,000 times more hydroxide ions than a strongly acidic solution. Moreover, the hydrogen ion and hydroxide ion concentrations in everyday solutions can vary over that entire range.