Acids and Alkalis Ions

What happens when an acid dissolves?

Water does more than spread an acid through a liquid. When an acid dissolves in water, it forms hydrogen ions, written $H^+$. These ions are present throughout the solution rather than remaining attached to the acid molecules. That is why $H^+$ is the ion to recognise in an aqueous acid, whatever the acid is.

For example, hydrochloric acid in water contains $H^+$ and chloride ions, $Cl^-$. The chloride ion belongs to this particular acid; the hydrogen ion is the feature shared by aqueous acids. The word aqueous, often shown as $(aq)$ in an equation, means dissolved in water.

What makes a solution an alkali?

An alkali is a soluble base. Its solution contains hydroxide ions, written $OH^-$. For example, sodium hydroxide dissolves in water to give sodium ions, , and hydroxide ions, . The sodium ions identify this particular alkali, while the hydroxide ions are what you should look for in an alkaline solution.

Notice that $H^+$ and $OH^-$ are different ions: $H^+$ has a positive charge, whereas $$ has a negative charge and contains both oxygen and hydrogen. Recognising which ions are present is the starting point for understanding how and alkalis react.

Ions in water

• Acid dissolved in water: forms $H^+$ ions.
• Alkali dissolved in water: solution contains ions.