Answer:
An acid is a substance that produces hydrogen ions, H+(aq), when dissolved in water.
The higher the concentration of hydrogen ions in the solution, the lower the pH. (Higher tier)
An alkali is a substance that produces hydroxide ions, OH-(aq), when dissolved in water.
Step-by-step explanation:
For example, hydrochloric acid ionises completely into hydrogen and chloride ions:
HCl(aq) → H+(aq) + Cl-(aq)
Strong acids include:
hydrochloric acid
sulfuric acid
nitric acid