An acid is defined as the substance which looses hydrogen ion or hydronium ions when dissolved in water.
A base is defined as the substance which looses hydroxide ions when dissolved in water.
If the solution has higher hydronium ion concentration as compared to the hydroxide ion concentration, then the pH will be low and the solution will be acidic.
If the solution has low hydronium ion concentration as compared to the hydroxide ion concentration then the pH will be high and the solution will be basic.
