Ions in water

"What makes an acid an acid?" Glucose and alcohol both contain hydrogen, but they are not acids. Test them with a bulb in a circuit: the bulb glows in dilute hydrochloric acid, but not in glucose or alcohol solution. Acids make ions in water, and ions carry the current.

In water, an acid gives hydrogen ions, H⁺. A hydrogen ion can't exist alone: it joins a water molecule and becomes the hydronium ion, H₃O⁺.

HCl + H₂O → H₃O⁺ + Cl⁻

That is why H⁺ is written as H⁺(aq) or H₃O⁺.

No water, no acid behaviour. Dry hydrogen chloride gas does not change the colour of dry blue litmus. Touch it with moist litmus and it turns red. The water is what lets the acid form its ions.

Bases give hydroxide ions, OH⁻, in water:

NaOH → Na⁺ + OH⁻

Bases that dissolve in water are called alkalis. All alkalis are bases, but not all bases are alkalis.

Dissolving a concentrated acid or base in water gives out a lot of heat. Pour water into concentrated acid and the heat can make the mixture splash out and burn you, or crack the glass. So: always add the acid slowly to water, stirring, never water to the acid.

Adding water is dilution. The same ions are now spread through more water, so the concentration of H₃O⁺ (or OH⁻) per unit volume goes down.