Acids, bases and pH

In a nutshell

Acids release $$H^+$$ ions and bases release $$OH^-$$ions when dissolved in water. Indicators can determine acidity / alkalinity. Acids and bases react to undergo a neutralisation reaction.

Equations

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​​Acids

Acids form acidic solutions in water. Acids release $$H^+$$ ions into solution. Acids have a pH below $$7$$​.

Example

​​$$hydrogen\ bromide(g)\ \rightarrow\ hydrogen\ ion(aq)\ +\ bromide\ ion(aq)\\ \ \\HBr(g)\ \rightarrow\ H^+(aq)\ +\ Br^-(aq)$$​​

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Bases

A base which can dissolve in water is known as an alkali and has a pH greater than $$7$$​. Alkaline solutions release $$OH^-$$​ ions.

Example

​​$$sodium\ hydroxide(s)\ \rightarrow\ sodium\ ion(aq)\ +\ hydroxide\ ion(aq)\\ \ \\NaOH(s) \rightarrow Na^+(aq)\ +\ OH^-(aq)$$

Neutral solutions

A neutral solution has a pH of $$7$$​ and is neither acidic nor alkaline.

Example

An example of a neutral solution is pure distilled water (pH $$7$$​).

pH scale

The pH scale allows you to determine the acidity / alkalinity of a particular solution, its values can range from $$0-14$$​. It relies on determining the $$H^+$$​ ion concentration in solution. More $$H^+$$​ ions in solution results in a lower value on the pH scale. 

Indicators

These can be added with small samples of solution to give a colour response and be used to determine whether a solution is acidic or alkaline and in some cases even determine the pH. 

Example

An example of some common indicators include phenolphthalein, methyl orange and universal indicator. Digital pH meters can also be used to give an accurate pH reading when placed into solution.

Acids and bases neutralise each other

​$$acid\ +\ base\ \rightarrow\ salt\ +\ water\\ \ \\ H^+(aq)\ +\ OH^-(aq)\ \rightarrow\ H_{2}O(l)$$​

Neutralisation reactions lead to the formation of neutral solutions when completed. Indicators can be used to measure their progress.