Acid, Alkali, and pH scale – chemistry notes

Here you will get the fundamental concepts of acids, alkalies, and pH value (concepts, measurements, & examples).


What is an Acid?

An acid is a substance that produces hydrogen ions, H+, in an aqueous solution. For example, solutions of:
● hydrochloric acid (HCl) contains hydrogen (H+) ions and chloride (Cl) ions
● sulfuric acid (H2SO4) contains hydrogen (H+) ions and sulfate (SO42–) ions
● nitric acid (HNO3) contains hydrogen (H+) ions and nitrate (NO3) ions.

What is an Alkali?

An alkali is a substance that produces hydroxide (OH) ions, in an aqueous solution. For example, solutions of:
● sodium hydroxide (NaOH) contain sodium (Na+) ions and hydroxide (OH) ions
● potassium hydroxide (KOH) contain potassium (K+) ions and hydroxide (OH) ions
● calcium hydroxide (Ca(OH)2) contain calcium (Ca2+) ions and hydroxide (OH) ions.

The pH scale

The pH scale is a measure of how acidic or alkaline a solution is.

A solution with a pH of 7 is neutral, whereas a solution with a pH below 7 is acidic and one with a pH above 7 is alkaline.

The further away from 7 the pH is, the more acidic or alkaline the solution is (Figure 1).

figure 1: pH scale

The scale is often shown as running from 0 to 14, but it does go further in both directions.
For example, it is common for the solutions of acids in school laboratories to have a pH that is less than 0 (typically about −0.3).

Measuring pH using universal indicator solution

The approximate pH of a solution can be measured using a universal indicator solution. A few drops of the indicator are added to the solution. The colour is compared to a colour chart to give the approximate pH of the solution (Figure 2).

figure 2: The colours for universal indicator

Measuring pH using pH probe

A more accurate way of finding the pH of a solution is to use a pH probe (Figure 3). There are different types but the probe is dipped into the solution and the pH shown on the display, often to 1 or 2 decimal places.

figure 3: pH probe

pH is based on the concentration of H+ ions in the solution

The pH of a solution is based on the concentration of H+ ions in the solution.

The higher the concentration of H+ ions the lower the pH.
As the pH decreases by one unit, the concentration of hydrogen ions increases by a factor of 10.

For example, a solution with a pH of 2 has a concentration of H+ ions that is 10 times greater than one with a pH of 3.

A solution with a pH of 1 has a concentration of H+ ions that is 100 times greater than one with a pH of 3.

In a neutral solution, the concentration of H+ ions equals the concentration of OH ions.

Acid, Alkali, and pH scale – chemistry notes
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