An acid that dissociates completely into ions is termed a strong acid. If it does not dissociate completely in the solution it is a weak acid. Acid solutions contain hydrogen ions. Higher the concentration of hydrogen ions, lower the pH, and vice versa. Sulphuric acid is a strong acid and vinegar is a weak acid. Strong acids can be fully ionized but weak acids can be ionized partly in the solution. At the same concentration, strong acids have a higher concentration of hydrogen ions compared to weak acids.