What type of investigation could the students conduct to determine the correct order for the liquids?

Use the pH color scale. It shows acidity level of both acids and bases. You dip one of the pH strips into whatever liquid you choose, and then compare the new color of the pH strip to the colors on the scale. This will tell you the level of acidity. pH scale can be found below. As you can see, the brighter colors are more acidic than the darker and duller colors.