Ions in acids and alkalis

Definition

Aqueous solution

A solution in which water is the solvent.

When an acid dissolves in water, it forms an aqueous solution that contains hydrogen ions, $H+(aq)\text{H}^{+}\text{(aq)}$ .
All acids produce hydrogen ions in water, which is why different acids share the same reactions with metals, bases and carbonates.
Each acid also releases a negative ion, so hydrochloric acid gives $Cl−\text{Cl}^{-}$ , sulfuric acid gives $SO42−\text{SO}_4^{2-}$ and nitric acid gives $NO3−\text{NO}_3^{-}$ .
When an alkali dissolves in water, its aqueous solution contains hydroxide ions, $OH−(aq)\text{OH}^{-}\text{(aq)}$ .
Sodium hydroxide solution, for example, contains $Na+\text{Na}^{+}$ ions and $OH−\text{OH}^{-}$ ions.
The other ions, such as $Cl−\text{Cl}^{-}$ and $Na+\text{Na}^{+}$ , do not make a solution acidic or alkaline.
In pure water the tiny numbers of $H+\text{H}^{+}$ and $OH−\text{OH}^{-}$ ions are equal, so water is neutral.
An acidic solution has more $H+\text{H}^{+}$ ions than $OH−\text{OH}^{-}$ ions, while an alkaline solution has more $OH−\text{OH}^{-}$ ions than $H+\text{H}^{+}$ ions.
The state symbol (aq) shows that an ion or substance is dissolved in water.

The pH scale

Definition

pH scale

A numbered scale, usually running from 0 to 14, that shows how acidic or alkaline an aqueous solution is.

Definition

Neutral solution

An aqueous solution that is neither acidic nor alkaline and has a pH of 7.

The pH scale usually runs from 0 to 14 and measures how acidic or alkaline an aqueous solution is.
A solution with a pH below 7 is acidic.
A solution with a pH of exactly 7 is neutral.
A solution with a pH above 7 is alkaline.
The lower the pH, the higher the hydrogen ion concentration, so the more acidic the solution.
The higher the pH, the lower the hydrogen ion concentration, so the more alkaline the solution.
A solution at pH 1 is therefore more acidic than one at pH 4, and a solution at pH 13 is more alkaline than one at pH 9.

Universal indicator

Definition

Universal indicator

A mixture of indicators that shows a different colour at each pH, so the colour can be matched to a pH value.

Universal indicator is a mixture of several indicators, so it shows a whole range of colours instead of just two.
Each colour matches a range of pH values on a printed colour chart.
1. pH 0 to 2: red, showing a strongly acidic solution.
2. pH 3 to 6: orange to yellow, showing a slightly acidic solution.
3. pH 7: green, showing a neutral solution.
4. pH 8 to 10: blue, showing a slightly alkaline solution.
5. pH 11 to 14: dark blue to purple, showing a strongly alkaline solution.
Universal indicator is often used as paper, and a drop of the solution is touched onto the paper with a glass rod.
Litmus, methyl orange and thymolphthalein only show whether a solution is acidic or alkaline.
Universal indicator also shows how acidic or alkaline a solution is, so it can be used to compare the acidity of two solutions.

Example

Ranking everyday solutions by pH

Lemon juice turns universal indicator paper red at about pH 2, so it has a high hydrogen ion concentration and is strongly acidic.
Vinegar turns the paper orange at about pH 3, so it is acidic but less so than lemon juice.
Pure water turns the paper green at pH 7, so it is neutral.
Seawater turns the paper green-blue at about pH 8, so it is slightly alkaline.
Oven cleaner turns the paper purple at about pH 13, so it is strongly alkaline.
The order from most acidic to most alkaline is lemon juice, vinegar, water, seawater, oven cleaner.

Practical

Measuring pH with universal indicator paper

Aim: to measure and compare the pH of several aqueous solutions using universal indicator paper.
Apparatus: white tile, universal indicator paper cut into small squares, glass rod, beaker of distilled water, paper towel, pH colour chart, small labelled samples of dilute hydrochloric acid, vinegar, distilled water, sodium chloride solution and dilute sodium hydroxide solution, and eye protection.
Method:
- Lay one square of universal indicator paper on the white tile for each solution, because the white background makes the colour easier to judge.
- Dip a clean glass rod into the first solution and touch it onto its square of paper, instead of dipping the paper into the sample, so the sample is not contaminated with dye.
- Compare the colour with the chart within a few seconds, before the paper dries and the colour changes.
- Record the colour and the pH in a table with the column headings "solution", "colour" and "pH".
- Rinse the rod in distilled water and wipe it dry before testing the next solution, so no solution is carried over into the next sample.
Results: hydrochloric acid gives red at about pH 1, vinegar gives orange at about pH 3, distilled water and sodium chloride solution give green at pH 7, and sodium hydroxide gives purple at about pH 13.
Conclusion: the solutions are ranked from most acidic to most alkaline by placing their pH values in order.
Limitations: the paper gives only whole-number pH values, and judging the colour by eye is less precise than reading a digital pH meter.
Safety: wear eye protection, wash any splash off the skin with plenty of water, and take extra care with sodium hydroxide, which damages the eyes even when dilute.

Neutralisation

Definition

Neutralisation

The reaction in which hydrogen ions from an acid combine with hydroxide ions from an alkali to form water.

When an acid reacts with an alkali, the $H+\text{H}^{+}$ ions from the acid react with the $OH−\text{OH}^{-}$ ions from the alkali.
Each $H+\text{H}^{+}$ ion combines with one $OH−\text{OH}^{-}$ ion to form one molecule of water.
The ionic equation for neutralisation is shown below.
$\text{H}^{+}\text{(aq)} + \text{OH}^{-}\text{(aq)} \rightarrow \text{H}_2\text{O(l)}$
The equation is the same for every reaction between an acid and an alkali, because the other ions take no part in the reaction.
These other ions stay dissolved in the solution and make up the salt.
As alkali is added to an acid, $H+\text{H}^{+}$ ions are removed, so the pH rises towards 7.
When exactly enough alkali has been added, all the $H+\text{H}^{+}$ ions have reacted and the solution is neutral at pH 7.
Adding more alkali after this point leaves extra $OH−\text{OH}^{-}$ ions, so the pH rises above 7.
Neutralisation gives out thermal energy, so the temperature of the mixture rises.

Example

Following the ions in a neutralisation

Hydrochloric acid reacts with sodium hydroxide solution to form sodium chloride and water.

\text{HCl(aq)} + \text{NaOH(aq)} \rightarrow \text{NaCl(aq)} + \text{H}_2\text{O(l)}

The acid supplies $H+\text{H}^{+}$ and $Cl−\text{Cl}^{-}$ ions, and the alkali supplies $Na+\text{Na}^{+}$ and $OH−\text{OH}^{-}$ ions.
The $H+\text{H}^{+}$ and $OH−\text{OH}^{-}$ ions join to make water.
The $Na+\text{Na}^{+}$ and $Cl−\text{Cl}^{-}$ ions are left unchanged in the solution as dissolved sodium chloride.

\text{H}^{+}\text{(aq)} + \text{OH}^{-}\text{(aq)} \rightarrow \text{H}_2\text{O(l)}

Common Mistake

Do not write the ions as H and OH, because the charges must always be shown, as in $H+\text{H}^{+}$ and $OH−\text{OH}^{-}$ .
Do not say that a neutral solution contains no ions, because the ions of the salt are still dissolved in it.
Do not think that a higher pH means a stronger acid, because a lower pH means a higher hydrogen ion concentration.

Exam technique

Comparing acidity with pH

Link pH to hydrogen ion concentration in the same sentence, such as "lower pH, so a higher concentration of $H+\text{H}^{+}$ ions".
Name universal indicator when asked how to compare acidity, because litmus cannot show a pH value.
Give both the colour and the pH when a question asks what you would observe.
Include state symbols in the ionic equation for neutralisation when they are asked for.

Self review

Which ion is present in every aqueous solution of an acid?
Which ion is present in every aqueous solution of an alkali?
What colour does universal indicator turn in a neutral solution, and what is its pH?
Which has the higher hydrogen ion concentration, a solution at pH 2 or one at pH 5?
Write the ionic equation for the neutralisation of an acid by an alkali.

Ions in acids and alkalis

Definition

Aqueous solution

A solution in which water is the solvent.

When an acid dissolves in water, it forms an aqueous solution that contains hydrogen ions, $H+(aq)\text{H}^{+}\text{(aq)}$ .
All acids produce hydrogen ions in water, which is why different acids share the same reactions with metals, bases and carbonates.
Each acid also releases a negative ion, so hydrochloric acid gives $Cl−\text{Cl}^{-}$ , sulfuric acid gives $SO42−\text{SO}_4^{2-}$ and nitric acid gives $NO3−\text{NO}_3^{-}$ .
When an alkali dissolves in water, its aqueous solution contains hydroxide ions, $OH−(aq)\text{OH}^{-}\text{(aq)}$ .
Sodium hydroxide solution, for example, contains $Na+\text{Na}^{+}$ ions and $OH−\text{OH}^{-}$ ions.
The other ions, such as $Cl−\text{Cl}^{-}$ and $Na+\text{Na}^{+}$ , do not make a solution acidic or alkaline.
In pure water the tiny numbers of $H+\text{H}^{+}$ and $OH−\text{OH}^{-}$ ions are equal, so water is neutral.
An acidic solution has more $H+\text{H}^{+}$ ions than $OH−\text{OH}^{-}$ ions, while an alkaline solution has more $OH−\text{OH}^{-}$ ions than $H+\text{H}^{+}$ ions.
The state symbol (aq) shows that an ion or substance is dissolved in water.

The pH scale

Definition

pH scale

A numbered scale, usually running from 0 to 14, that shows how acidic or alkaline an aqueous solution is.

Definition

Neutral solution

An aqueous solution that is neither acidic nor alkaline and has a pH of 7.

The pH scale usually runs from 0 to 14 and measures how acidic or alkaline an aqueous solution is.
A solution with a pH below 7 is acidic.
A solution with a pH of exactly 7 is neutral.
A solution with a pH above 7 is alkaline.
The lower the pH, the higher the hydrogen ion concentration, so the more acidic the solution.
The higher the pH, the lower the hydrogen ion concentration, so the more alkaline the solution.
A solution at pH 1 is therefore more acidic than one at pH 4, and a solution at pH 13 is more alkaline than one at pH 9.

Universal indicator

Definition

Universal indicator

A mixture of indicators that shows a different colour at each pH, so the colour can be matched to a pH value.

Universal indicator is a mixture of several indicators, so it shows a whole range of colours instead of just two.
Each colour matches a range of pH values on a printed colour chart.
1. pH 0 to 2: red, showing a strongly acidic solution.
2. pH 3 to 6: orange to yellow, showing a slightly acidic solution.
3. pH 7: green, showing a neutral solution.
4. pH 8 to 10: blue, showing a slightly alkaline solution.
5. pH 11 to 14: dark blue to purple, showing a strongly alkaline solution.
Universal indicator is often used as paper, and a drop of the solution is touched onto the paper with a glass rod.
Litmus, methyl orange and thymolphthalein only show whether a solution is acidic or alkaline.
Universal indicator also shows how acidic or alkaline a solution is, so it can be used to compare the acidity of two solutions.

Example

Ranking everyday solutions by pH

Lemon juice turns universal indicator paper red at about pH 2, so it has a high hydrogen ion concentration and is strongly acidic.
Vinegar turns the paper orange at about pH 3, so it is acidic but less so than lemon juice.
Pure water turns the paper green at pH 7, so it is neutral.
Seawater turns the paper green-blue at about pH 8, so it is slightly alkaline.
Oven cleaner turns the paper purple at about pH 13, so it is strongly alkaline.
The order from most acidic to most alkaline is lemon juice, vinegar, water, seawater, oven cleaner.

Practical

Measuring pH with universal indicator paper

Aim: to measure and compare the pH of several aqueous solutions using universal indicator paper.
Apparatus: white tile, universal indicator paper cut into small squares, glass rod, beaker of distilled water, paper towel, pH colour chart, small labelled samples of dilute hydrochloric acid, vinegar, distilled water, sodium chloride solution and dilute sodium hydroxide solution, and eye protection.
Method:
- Lay one square of universal indicator paper on the white tile for each solution, because the white background makes the colour easier to judge.
- Dip a clean glass rod into the first solution and touch it onto its square of paper, instead of dipping the paper into the sample, so the sample is not contaminated with dye.
- Compare the colour with the chart within a few seconds, before the paper dries and the colour changes.
- Record the colour and the pH in a table with the column headings "solution", "colour" and "pH".
- Rinse the rod in distilled water and wipe it dry before testing the next solution, so no solution is carried over into the next sample.
Results: hydrochloric acid gives red at about pH 1, vinegar gives orange at about pH 3, distilled water and sodium chloride solution give green at pH 7, and sodium hydroxide gives purple at about pH 13.
Conclusion: the solutions are ranked from most acidic to most alkaline by placing their pH values in order.
Limitations: the paper gives only whole-number pH values, and judging the colour by eye is less precise than reading a digital pH meter.
Safety: wear eye protection, wash any splash off the skin with plenty of water, and take extra care with sodium hydroxide, which damages the eyes even when dilute.

Neutralisation

Definition

Neutralisation

The reaction in which hydrogen ions from an acid combine with hydroxide ions from an alkali to form water.

When an acid reacts with an alkali, the $H+\text{H}^{+}$ ions from the acid react with the $OH−\text{OH}^{-}$ ions from the alkali.
Each $H+\text{H}^{+}$ ion combines with one $OH−\text{OH}^{-}$ ion to form one molecule of water.
The ionic equation for neutralisation is shown below.
$\text{H}^{+}\text{(aq)} + \text{OH}^{-}\text{(aq)} \rightarrow \text{H}_2\text{O(l)}$
The equation is the same for every reaction between an acid and an alkali, because the other ions take no part in the reaction.
These other ions stay dissolved in the solution and make up the salt.
As alkali is added to an acid, $H+\text{H}^{+}$ ions are removed, so the pH rises towards 7.
When exactly enough alkali has been added, all the $H+\text{H}^{+}$ ions have reacted and the solution is neutral at pH 7.
Adding more alkali after this point leaves extra $OH−\text{OH}^{-}$ ions, so the pH rises above 7.
Neutralisation gives out thermal energy, so the temperature of the mixture rises.

Example

Following the ions in a neutralisation

Hydrochloric acid reacts with sodium hydroxide solution to form sodium chloride and water.

\text{HCl(aq)} + \text{NaOH(aq)} \rightarrow \text{NaCl(aq)} + \text{H}_2\text{O(l)}

The acid supplies $H+\text{H}^{+}$ and $Cl−\text{Cl}^{-}$ ions, and the alkali supplies $Na+\text{Na}^{+}$ and $OH−\text{OH}^{-}$ ions.
The $H+\text{H}^{+}$ and $OH−\text{OH}^{-}$ ions join to make water.
The $Na+\text{Na}^{+}$ and $Cl−\text{Cl}^{-}$ ions are left unchanged in the solution as dissolved sodium chloride.

\text{H}^{+}\text{(aq)} + \text{OH}^{-}\text{(aq)} \rightarrow \text{H}_2\text{O(l)}

Common Mistake

Do not write the ions as H and OH, because the charges must always be shown, as in $H+\text{H}^{+}$ and $OH−\text{OH}^{-}$ .
Do not say that a neutral solution contains no ions, because the ions of the salt are still dissolved in it.
Do not think that a higher pH means a stronger acid, because a lower pH means a higher hydrogen ion concentration.

Exam technique

Comparing acidity with pH

Link pH to hydrogen ion concentration in the same sentence, such as "lower pH, so a higher concentration of $H+\text{H}^{+}$ ions".
Name universal indicator when asked how to compare acidity, because litmus cannot show a pH value.
Give both the colour and the pH when a question asks what you would observe.
Include state symbols in the ionic equation for neutralisation when they are asked for.

Self review

Which ion is present in every aqueous solution of an acid?
Which ion is present in every aqueous solution of an alkali?
What colour does universal indicator turn in a neutral solution, and what is its pH?
Which has the higher hydrogen ion concentration, a solution at pH 2 or one at pH 5?
Write the ionic equation for the neutralisation of an acid by an alkali.

7.1.3 Acidity, pH and neutralisation

Ions in acids and alkalis

The pH scale

Universal indicator

Ranking everyday solutions by pH

Measuring pH with universal indicator paper

Neutralisation

Following the ions in a neutralisation

Comparing acidity with pH

Review 7.1.3 Acidity, pH and neutralisation by teaching Genie

7.1.3 Acidity, pH and neutralisation

Ions in acids and alkalis

The pH scale

Universal indicator

Ranking everyday solutions by pH

Measuring pH with universal indicator paper

Neutralisation

Following the ions in a neutralisation

Comparing acidity with pH

Review 7.1.3 Acidity, pH and neutralisation by teaching Genie

7.1.3 Acidity, pH and neutralisation

Ions in acids and alkalis

The pH scale

Universal indicator

Ranking everyday solutions by pH

Measuring pH with universal indicator paper

Neutralisation

Following the ions in a neutralisation

Comparing acidity with pH

Review 7.1.3 Acidity, pH and neutralisation by teaching Genie

7.1.3 Acidity, pH and neutralisation Revision Guide

7.1.3 Acidity, pH and neutralisation

Ions in acids and alkalis

The pH scale

Universal indicator

Ranking everyday solutions by pH

Measuring pH with universal indicator paper

Neutralisation

Following the ions in a neutralisation

Comparing acidity with pH

Review 7.1.3 Acidity, pH and neutralisation by teaching Genie

7.1.3 Acidity, pH and neutralisation Revision Guide