The pH scale describes the acidic or basic nature of a solution. The values of pH typically range from zero, which is very acidic, to 14, which is very basic. Water has a neutral pH of seven. HCl is known as hydrochloric acid when it is in an aqueous solution. Hydrochloric acid has a pH of zero in a one molar solution. As you add HCl to water, it will lower the pH of water.
The Components of Water
Water's chemical formula is H2O. This means that it has two atoms of hydrogen for every one atom of oxygen. When a hydrogen ion is introduced into water, it will combine with a water molecule to form an H3O+ ion. The pH scale is based on the concentration of H3O+ ions in a solution. In water, the concentration of H3O+ is balanced by an equal concentration of OH- ions. Hence, its pH is neutral. An acidic solution has a higher concentration H3O+ ions than OH- ions. A basic solution has a higher concentration of OH- ions than H3O+ ions.
What Happens When HCl is Added?
When HCl is added to water, it donates an H+ ion to the water, leaving a Cl- ion in solution. This increases the H+ activity of the solution without increasing the concentration of OH- ions in the solution. Hence, the pH of water becomes more acidic; as you add an increasing amount of HCl, the pH gets lower.
How Do You Detect Changes in pH?
Indicators are chemicals that change color once a solution's pH changes. There are a number of different indicators, each with a specific pH range. If you are monitoring the change in pH as you add HCl to water, you should use an indicator that changes color in a pH range below seven. Thymol blue, methyl orange and methyl red all change color in a pH range below seven.
How is pH Calculated?
Because the concentration of H3O+ ions can change drastically with small additions to a solution, this is not a straightforward way of communicating pH. Instead, we look at the exponential nature of that concentration using the logarithmic function. The pH of a solution is defined as the negative base ten logarithm of the H3O+ concentration of a solution. This separates values on the pH scale by powers of ten.
- Ableimages/Digital Vision/Getty Images