Vinegar, orange juice, and battery fluid are familiar acids. Different acids have different acid strengths.

Some characteristics of acids are:

• They taste sour.

• They can react with certain metals, such as magnesium and zinc, to produce hydrogen gas.

• They can neutralize bases. This reaction is called aneutralization reaction.

• Acids can change the color of blue litmus to red. Litmus is a kind of pH-sensitive dye. It is therefore used as an acid-base indicator.

The three most important definitions for acids follow:

• Arrhenius definition: Acids produce H⁺ ions in aqueous solutions.

• Bronsted-Lowry definition: Acids are hydrogen-ion donors.

• Lewis definition: Acids are electron pair acceptors.