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# Acids and bases

## Acid definition

An acid is a substance that releases hydrogen ions ($\ce{H^+}$) in an aqueous solution. Acids tend to dissociate (break up) into $\ce{H^+}$ ions and negatively charged ions.

An $\ce{H^+}$ ion can be thought of as a proton. Therefore, the dissociation of $\ce{H^+}$ is often called deprotonation.

The general equation for the dissociation of acids is as follows.

$$\ce{HA -> H^+ + A^-}$$

$\ce{A^-}$ represents the anion of the acid. $\ce{A^-}$ would be the chloride ion ($\ce{Cl^-}$) in the case of hydrochloric acid.

The sour taste in foods is caused by acids. Lemons and vinegar both contain acid.

The characteristic sour taste of citrus fruits is due to the presence of citric acid.

## Bases definition

A base is a substance that reacts with acids by accepting $\ce{H^+}$ ions.

An alkali is a base that releases hydroxide ions ($\ce{OH^-}$) in an aqueous solution.

$$\ce{MOH -> M^+ + OH^-}$$

The $\ce{OH^-}$ ions neutralise $\ce{H^+}$ ions to form water.

$\ce{M}$ represents the cation of the alkali, and is usually a metal (such as $\ce{Na}$ in sodium hydroxide and $\ce{Mg}$ in magnesium hydroxide).

Not all bases are alkalis. For instance, ammonia is a base that reacts with acids but does not dissociate to give $\ce{OH^-}$ ions.

$$\ce{NH3 + HCl -> NH4Cl}$$

A base has a bitter taste. Soap and other cleaning agents usually contain bases.

## Acidic and basic solutions

Solutions can be either acidic, basic or neutral depending on the ions dissolved in the solution.

In an acidic solution, $\ce{H^+}$ ions are more prevalent than $\ce{OH^-}$ ions.

In a basic solution, $\ce{OH^-}$ ions are more prevalent than $\ce{H^+}$ ions.

In a neutral solution, $\ce{H^+}$ ions and $\ce{OH^-}$ ions are present in equal numbers.

Universal indicator is added to colour the three solutions based on their nature. They contain acidic, neutral and basic solutions respectively.

An acid or base is dilute in solution if the concentration is relatively low.

An acid or base is concentrated in solution if the concentration is relatively high.

A solution containing $\text{0.005 mol/}\cdm$ of $\ce{HCl}$ is dilute. A solution containing $\text{10 mol/}\cdm$ of $\ce{HCl}$ is concentrated.

## Degree of acidity and basicity

The degree to which a solution is acidic or basic depends on the concentration of $\ce{H^+}$ and $\ce{OH^-}$.

These solutions have universal indicator added to them, which colours them according to how acidic or basic they are. The red solution is more acidic than the orange solution. The violet solution is more basic than the blue solution.

A solution containing $\text{0.1 mol/}\cdm$ of $\ce{H^+}$ (pH 1) is more acidic than a solution containing $\text{0.001 mol/}\cdm$ of $\ce{H^+}$ (pH 3).

A solution containing $\text{0.1 mol/}\cdm$ of $\ce{OH^-}$ (pH 13) is more basic than a solution containing $\text{0.001 mol/}\cdm$ of $\ce{OH^-}$ (pH 11).

An acidic solution becomes less acidic as $\ce{OH^-}$ ions are added to it. When $\ce{OH^-}$ ions become equal in number to $\ce{H^+}$ ions, the solution is neutral.

The solution becomes basic when there are more $\ce{OH^-}$ ions than $\ce{H^+}$ ions. The solution becomes more basic as more $\ce{OH^-}$ ions are added.

## pH scale

The pH scale is used to indicate the acidic or basic nature of a solution.

The scale typically ranges from 0 to 14, with 0 being the most acidic reading and 14 being the most basic reading.

The midpoint (7) describes a neutral solution (equal amounts of $\ce{H^+}$ and $\ce{OH^-}$).

pH value Example
2 Lemon juice
5 Black coffee
7 Distilled water
11 Ammonia solution
13 Bleach

A pH scale is a logarithmic measure and is related to the concentration of $\ce{H^+}$ ions. A solution with a pH of 3 has 10 times as many $\ce{H^+}$ ions per unit volume as a solution with a pH of 4.

A solution with a pH of 12 has 100 times more $\ce{OH^-}$ ions per unit volume than a solution with a pH of 10.

## pH indicators

pH indicators are chemicals that have different colours under different pH conditions.

A commonly used pH indicator is litmus paper, which turns blue in basic conditions and red in acidic conditions.

Other indicators are liquid solutions. Three common indicators are universal indicator, methyl orange and phenolphthalein.

Colour of various pH indicators at various pH levels
pH Universal indicator Methyl orange Phenolphthalein
0 to 2 Red Red Colourless
3 to 5 Orange-yellow Orange Colourless
6 Orange-yellow Yellow Colourless
7 Green Yellow Colourless
8 to 11 Blue Yellow Pink
12 to 14 Violet Yellow Colourless

Universal indicator is called universal because it covers a broad pH range. It does not indicate properties other than pH.

## Strong and weak acids

A strong acid dissociates completely into ions when dissolved in water.

Hydrochloric acid $(\ce{HCl})$ and sulfuric acid $(\ce{H2SO4})$ are examples of strong acids.

$$\ce{HCl {(aq)} -> H^+ {(aq)} + Cl^-{(aq)}}$$

A weak acid does not dissociate completely in water. Some of the hydrogen ions remain attached to the molecule.

Acetic acid $(\ce{CH3COOH})$, formic acid $(\ce{HCOOH})$ and nitrous acid $(\ce{HNO2})$ are examples of weak acids.

In weak acids, the molecules often flow back and forth between dissociated and non-dissociated state.

The equations for dissociation are thus written with the $\leftrightharpoons$ symbol.

These arrows indicate that the reaction is reversible and can go in either direction.

$$\ce{CH3COOH {(aq)} <=> H^+ {(aq)} + CH3COO^- {(aq)}}$$

The venom found in bee stings contains formic acid, a weak acid.