Acids and Alkalis

In water, acids form positively charged hydrogen ions (H⁺), which makes the solution acidic. On the other hand, alkalis form negatively charged hydroxide ions (OH⁻) in water, which makes the solution alkaline. This can be seen in examples such as:

  • Hydrochloric acid (HCl) dissociates in water to form H⁺ and Cl⁻ ions, making the solution acidic
  • Sodium hydroxide (NaOH) dissociates in water to form Na⁺ and OH ions, making the solution alkaline
diagram comparing the dissociation of hydrogen chloride and sodium hydroxide in water. On the left, hydrogen chloride breaks down into hydrogen ions and chloride ions in an acidic solution. On the right, sodium hydroxide separates into sodium ions and hydroxide ions in a basic solution. Both reactions are depicted with the respective molecules above beakers of water, showing the ions formed dispersed in the solution below.

The pH Scale

The pH Scale is a numerical scale that measures how acidic or alkaline a solution is.

  • Acidic solutions have a pH lower than 7
  • Alkaline solutions have a pH greater than 7
  • Neutral solutions have a pH equal to 7
A colourful pH scale diagram depicting common substances and their acidity or alkalinity. From left to right, the scale starts with battery at pH 1, showing it as highly acidic in red, and progresses through stomach acid, lemon, soda, tomato, coffee, milk, and water which is neutral at pH 7 in green. Continuing towards alkaline in blue are blood, egg white, stomach tablets, ammonia solution, soap, bleach, and ending with drain cleaner at pH 14. The progression is marked with visual representations of each substance above its corresponding pH value, with the terms "Acidic", "Neutral", and "Alkaline" labelled along the bottom axis.

The pH scale is based on the concentration of hydrogen (H⁺) and hydroxide (OH⁻) ions. The higher the concentration of hydrogen ions, the lower the pH level of the solution. So, a pH level of 1 would indicate the highest concentration of hydrogen ions.

In contrast, solutions with higher concentrations of hydroxide ions have higher pH levels. The highest pH level is 14, which indicates the highest concentration of hydroxide ions.

The pH of a solution can be measured using an indicator or a pH meter.

To recap:

  • Acidic solutions have a higher concentration of hydrogen ions than hydroxide ions
  • Alkaline solutions have a higher concentration of hydroxide ions than hydrogen ions
  • Neutral solutions have an equal concentration of hydrogen and hydroxide ions


Indicators are substances that are used to test whether a solution is acidic or alkaline. Two common indicators are universal indicator and litmus paper.

Universal indicator

Universal indicator is an acid-alkali indicator that gives a more precise measurement of pH. It produces a range of colours that can be compared to a colour chart to determine the pH of a solution.

  • On the left side of the scale, below pH 7, are acidic solutions. These solutions will range in colour from dark red (strong acid) to yellow (weak acid).
  • In the middle, at pH 7, you get neutral solutions, which is indicated by a colour change to green. This is where you find pure water.
  • On the right side of the scale, above pH 7, are alkaline solutions. These solutions will range in colour from dark green (weak alkali) to purple (strong alkali).

When performing acid-base reactions, we can use universal indicators to determine when the reaction has reached a neutral state. If neutralisation has occurred, the solution should change to a green colour.”

Litmus paper

Litmus paper is a strip of paper that has been treated with a mixture of dyes. There are two types of litmus paper:

  • Blue litmus paper – This turns red in acidic solutions
  • Red litmus paper – This turns blue in alkaline solutions

diagram illustrating the litmus test. The header states "Litmus paper is usually RED or BLUE". Below, two beakers are shown. The first depicts an acidic solution where blue litmus paper turns to a red colour, while the second shows an alkaline solution turning red litmus paper to a blue colour. On the right, there are two sections: "Benefit", indicating the litmus test is a quick and simple method, and "Limitation", noting it cannot provide an exact measure of pH value and that other substances in the solution might interfere with the litmus paper's colour change.

Litmus paper is a quick and easy way to test the pH of a solution, but it is not as precise as universal indicator.

pH meter

A pH meter is a more precise and accurate way to determine the acidity or basicity of a solution compared to using a universal indicator. When placed in a solution, the pH meter provides a numerical value indicating the solution’s acidity or basicity.

For example, if the reading is 2, the solution is acidic. The numerical data provided by the pH meter is more accurate than the colour-based method of using a universal indicator.