Exercises: Properties of Acids and Bases and the pH Scale

The pH scale is used to describe how acidic or basic a solution is. Pure water at room temperature has a pH of 7. Is a solution with a pH of 3 acidic, basic, or neutral? Write one sentence explaining how you decided.

Emma tests four household liquids with pH paper and records the results: lemon juice, pH 2; milk, pH 6.5; baking soda solution, pH 9; ammonia cleaner, pH 11. Classify each liquid as an acid or a base (or weakly acidic or weakly basic), and name the liquid that is the most basic.

Acids and bases have different properties. Copy the table below and sort these properties into the correct column: tastes sour, feels slippery, turns blue litmus paper red, turns red litmus paper blue, reacts with some metals to produce hydrogen gas, tastes bitter.

(Note: never taste substances in a science lab. These descriptions come from common foods and household products.)

Jayden measures the pH of three samples from a stream near his school and records the data below.

(a) Which sample is neutral? (b) Which sample is the most acidic? (c) Compare sample B with sample C: which is more acidic, and how do you know from the pH numbers?

Priya says, "A solution with a pH of 12 is an acid because 12 is a big number." Explain why her claim is incorrect. In your answer, state what a pH of 12 tells you about the solution compared with pure water, and describe one property you would expect it to have.

Marcus has a liquid with a pH of 1 and another with a pH of 6. He says both are acids, so they must behave the same way. Write 3 to 4 sentences that compare the two liquids. State what they have in common, how they differ according to the pH scale, and which one you would expect to react more strongly with a piece of magnesium metal.

A class tests the pH of rainwater collected in three cities. The data are below. Normal, unpolluted rain is slightly acidic, with a pH of about 5.6.

(a) Rank the three samples from most acidic to least acidic. (b) Each step of 1 on the pH scale means a tenfold change in acidity. Pittsburgh's rain is about 1 pH unit lower than Houston's rain in this data set, more precisely 0.6 units lower. Explain whether Pittsburgh's rain is more or less than 10 times as acidic as Houston's. (c) Write a claim, evidence, and reasoning (CER) statement about which city's rain is the most different from normal rain.

Sofia adds a few drops of a clear liquid X to red litmus paper and to blue litmus paper. The red paper turns blue and the blue paper stays blue. She then mixes liquid X, little by little, into a beaker of vinegar (pH about 2.5) and checks the pH after each addition.

(a) Is liquid X an acid or a base? Use the litmus results as evidence. (b) Describe the trend in the data table and explain what is happening to the acidity of the mixture. (c) Between which two amounts of X was the mixture closest to neutral, and what would you predict for the pH if 60 mL of X were added? Justify your prediction.