A strong acid is completely ionised in aqueous solution, so every acid particle splits to release a hydrogen ion. Hydrochloric, nitric and sulfuric acids are strong acids. A weak acid is only partially ionised in aqueous solution, so most of its particles stay as they are. Ethanoic, citric and carbonic acids are weak acids.
Do not mix up strength with concentration. Strong and weak describe the degree of ionisation. Dilute and concentrated describe the amount of acid dissolved in a given volume of water. A concentrated solution has more acid in each unit of volume. A weak acid can be concentrated, and a strong acid can be dilute.
For solutions of the same concentration, the stronger the acid, the lower the pH, because a strong acid makes more hydrogen ions. As the pH decreases by one unit, the hydrogen ion concentration increases by a factor of 10. So a solution of pH 2 has 10 times the hydrogen ion concentration of one with pH 3, and 100 times that of one with pH 4.
