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Acid or Base?According to the Arrhenius Acid-Base theory, the answer is yes and no. If you rearrange the formula to look like HOH, you can see that it contains a hydrogen (indicating an acid), but also contains a hydroxide OH- (indicating a base).

This makes sense because water is dead center on the pH scale, right between the acidic side and the basic side.

Since we cannot tell by just "looking" at it to determine if its an acid or base, it has to be determined by how it reacts with other acids and bases.

This brings us to the Brønsted-Lowry acid-base theory, which tells us that acids are proton (H+ = H3O+) donors and bases are proton acceptors.

Generally, water will act as an acid (proton donor) when it reacts with a substance that is less acidic than the water. Conversely, water will act as a base when it reacts with a substance that is more acidic than water.

ExamplesHCl + H2O → H3O+ + Cl-

Hydrochloric acid is a strong acid and will ionize completely in water. It is more acidic than water, so the water acts as a base and accepts a proton. Note that the hydronium ion (H3O+) can also be written as a proton (H+).

NaOH + HOH → Na+ + OH-

Sodium hydroxide is a strong base and will ionize completely in water. It is less acidic than water, so the water acts as an acid and donates a proton to form a hydroxide.

Water is naturally neutral. Other things added to water may change it towards an acid or a base. Think of acid rain, or a poison water hole in the desert- acid and base.

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15y ago

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