Final answer:
An acid is a substance that increases the concentration of H+ ions in water, while a base increases OH− ion concentration. The Brønsted-Lowry definition expands this to acids being proton donors and bases as proton acceptors. The pH scale is used to measure the level of acidity or basicity.
Step-by-step explanation:
An acid is a substance that, when dissolved in water, increases the concentration of hydrogen ions (H+), which are often represented as hydronium ions (H3O+). In contrast, a base is a substance that increases the concentration of hydroxide ions (OH−) when dissolved in water. These definitions represent the Arrhenius concept of acids and bases, which is one of the classical theories.
Beyond Arrhenius theory, the Brønsted-Lowry definition states that acids are proton donors and bases are proton acceptors, which allows for the explanation of acid-base reactions in non-aqueous solutions as well. Acids and bases also have unique properties: acids have a sour taste, can turn blue litmus paper red, can react with metals to produce hydrogen gas, and can dissolve carbonates with the evolution of carbon dioxide. Bases, on the other hand, have a bitter taste, feel slippery and can turn red litmus paper blue.
The acid-base character can be measured using the pH scale, which quantifies the acidity or basicity of a solution. Neutral solutions have a pH of 7, acidic solutions have a pH less than 7, while basic solutions have a pH greater than 7.