Final answer:
When an unknown substance causes the pH of a solution to increase, it is best described as a base, since bases raise the pH by increasing the concentration of hydroxide ions in the solution.
Step-by-step explanation:
When an unknown substance is added to a solution and the pH increases, it indicates that the substance has a basic or alkaline characteristic.
According to the Arrhenius definition, a base is a substance that increases the amount of hydroxide ions (OH-) in an aqueous solution, which contribute to a higher pH level. Therefore, the substance added is best described as a (4) base.
The pH scale is used to measure the acidity or basicity of a solution, with 7 being neutral. A pH lower than 7 is acidic, while a pH higher than 7 is basic. The inclusion of a base in a solution will reduce the concentration of hydrogen ions (H+), increasing the pH and making the solution more alkaline.