What tool can be used to measure acidity?

Litmus paper is a well-established tool for measuring acidity, functioning as a simple pH indicator. It contains dyes that change color in response to the concentration of hydrogen ions in a solution, signifying whether the solution is acidic, neutral, or basic. When dipped into a liquid, blue litmus paper will turn red in acidic conditions, while red litmus paper will turn blue in basic conditions. This qualitative method provides immediate visual feedback about the acidity or alkalinity of the solution tested, making it widely used in educational and experimental settings.

In contrast, a Bunsen burner is primarily used for heating substances and does not measure properties like acidity. A thermometer is designed to measure temperature and would not provide any information regarding the pH level of a solution. Lastly, a refractometer measures the refractive index of fluids, which can indicate concentration of solutes but does not directly assess acidity levels. This distinct functionality is what makes litmus paper the correct and relevant choice for measuring acidity.