Acids, pH and Indicator Use Explained

Chemical changes • Reactions of acids

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What does a pH value greater than 7 indicate?

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The solution is alkaline and has a lower hydrogen ion concentration than neutral water.

Key concepts

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The pH scale and its meaning

The pH scale runs approximately from 0 to 14 and gives a numerical measure of how acidic or alkaline a solution is. A solution with pH less than 7 is acidic, pH 7 is neutral in pure water at standard conditions, and pH greater than 7 is alkaline. A change of one pH unit corresponds to a tenfold change in hydrogen ion concentration. pH is defined by the mathematical relation pH = −log10[H+], so a decrease in pH indicates a proportional increase in [H+]. Extreme pH values outside 0–14 are possible in very concentrated solutions, but typical classroom scale uses 0–14 as the useful range.

Identifying acidic and alkaline solutions using pH

Acidic solutions show pH values below 7 and contain higher concentrations of hydrogen ions compared with neutral water. Alkaline solutions show pH values above 7 and contain lower hydrogen ion concentrations and higher hydroxide ion concentration. The numerical pH value gives relative acidity: lower numbers indicate stronger acidity in terms of higher [H+]. Relative acidity comparison requires consideration of both pH difference and concentration because identical pH implies identical [H+] regardless of acid identity.

Universal indicator and wide-range indicators

A universal indicator is a mixture of several indicators that produces a range of colours across most of the pH scale, allowing an approximate pH to be read from a colour chart. Universal indicator paper or solution gives a single-colour match that corresponds to an estimated pH value. Single indicators (for example, methyl orange, phenolphthalein, litmus) change colour over a narrow pH interval and indicate whether a solution lies above or below that interval. Universal indicators give an approximate numerical pH; single indicators give a binary or limited-range indication useful for titrations and simple tests.

Practical use of indicators and limitations

Universal indicator shows an approximate pH quickly using a colour chart, making it suitable for general testing of unknown solutions. Colour perception and light conditions can affect readings, and mixtures or coloured solutions can obscure indicator colours. Single indicators give precise switching ranges but only indicate that a solution passes through that narrow pH interval. Quantitative pH measurement requires a pH meter for accurate numerical pH; indicators only estimate pH and work best within their specified ranges.

Neutrality and relative acidity in terms of hydrogen ion concentration (HT)

Neutrality corresponds to equal concentrations of hydrogen ions and hydroxide ions in pure water; this usually gives pH 7 at standard conditions. Relative acidity compares [H+] values: a lower pH corresponds to a higher [H+]. Because pH is logarithmic, a solution at pH 3 has ten times more hydrogen ions than a solution at pH 4 and one hundred times more than at pH 5. The logarithmic relationship means small pH differences represent large changes in [H+].

Dilute versus concentrated acids (HT)

Dilute and concentrated describe the amount of acid substance dissolved per unit volume of solution. A concentrated acid contains a large number of acid molecules (and therefore potential H+ donors) per volume; a dilute acid contains relatively few acid molecules per volume. Concentration does not determine whether an acid is strong or weak; a concentrated weak acid can have a relatively low [H+] compared with a dilute strong acid depending on dissociation. Safety and reactivity depend on both concentration and acid strength.

Weak versus strong acids (HT)

Strong acids ionise almost completely in water, so nearly every acid molecule releases an H+ ion; examples include hydrochloric acid and sulfuric acid. Weak acids only partially ionise, so only a fraction of molecules release H+; examples include ethanoic (acetic) acid and carbonic acid. Strength refers to the extent of ionisation and affects [H+] at a given concentration. A strong acid at low concentration can produce a similar [H+] to a weak acid at higher concentration, so both strength and concentration determine the resulting pH.

Key notes

Important points to keep in mind

pH < 7 indicates acid; pH = 7 is neutral (standard conditions); pH > 7 indicates alkali.

pH = −log10[H+]; a 1-unit drop in pH equals a tenfold increase in [H+].

Universal indicator gives an approximate numerical pH by colour matching to a chart.

Single indicators work over narrow pH ranges and are useful for detecting specific pH transitions.

Dilute vs concentrated refers to amount of acid per volume, not strength of ionisation.

Weak vs strong refers to extent of ionisation in water, not concentration.

Indicator readings can be unreliable in strongly coloured solutions or poor lighting.

Accurate pH measurement requires a calibrated pH meter; indicators are for estimation.

Neutrality depends on temperature and solution properties; pH 7 applies to pure water at standard conditions.

Compare acidity by pH differences: each unit represents a tenfold change in [H+].