Properties of Acids and Bases and the pH Scale
Lemon juice makes your mouth pucker. Soapy water feels slippery. Both are everyday chemicals, but they sit at opposite ends of a scale that scientists use to describe thousands of liquids. When Jayden finds a stream near his school that tests differently from the one next door, how can he tell what is going on? By the end of this page, you will be able to read that scale.
Acids and bases: two families of substances
Chemists sort many water solutions into two big families: acids and bases. They have opposite properties.
| Property | Acids | Bases |
|---|---|---|
| Taste (in foods) | sour | bitter |
| Feel | can sting or burn | slippery, soapy |
| Blue litmus paper | turns red | stays blue |
| Red litmus paper | stays red | turns blue |
| Reaction with some metals (such as magnesium or zinc) | produces hydrogen gas | usually no reaction |
| Examples | lemon juice, vinegar, soda | baking soda solution, soap, ammonia cleaner |
Litmus paper is an indicator: a material that changes color depending on whether a solution is acidic or basic. The trick is to remember that only one color change happens for each family.
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…
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.
| Acids | Bases |
|---|---|
(Note: never taste substances in a science lab. These descriptions come from common foods and household products.)
The pH scale
Saying "acid" or "base" is not enough. Is lemon juice more acidic than milk? To compare, scientists use the pH scale, which runs from 0 to 14.
Why 7? Pure water is neither acidic nor basic, so it is placed in the middle. The farther a number is from 7, the stronger the acid or base.
Here are two ways to measure pH. pH paper or universal indicator changes to a different color for each pH, and you compare it to a color chart. Red cabbage juice also changes color (red in acids, green or yellow in bases). A pH meter gives a number directly.
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?…
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…
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.
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…
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.
How strong is it? Comparing pH values
Two acids are not always alike. A liquid with pH 1 is much more acidic than one with pH 6, even though both are below 7.
The reason is that the scale is built from the amount of hydrogen ions (H⁺) in the water: each step down has ten times as many. That is why a small change in pH can matter a lot to fish and plants.
More acidic solutions react more strongly with metals. For acids of the same type, a piece of magnesium bubbles hydrogen gas faster in a pH 1 solution than in a pH 6 solution.
Jayden measures the pH of three samples from a stream near his school and records the data below. | Sample | pH | |---|---| | A (upstream) | 7.0 | | B (near a factory) |…
Jayden measures the pH of three samples from a stream near his school and records the data below.
| Sample | pH |
|---|---|
| A (upstream) | 7.0 |
| B (near a factory) | 4.5 |
| C (downstream) | 6.0 |
(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?
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…
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. | City | pH of…
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.
| City | pH of rainwater |
|---|---|
| Denver | 5.6 |
| Pittsburgh | 4.4 |
| Houston | 5.0 |
(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.
Mixing acids and bases
When an acid and a base meet, they partly cancel each other. Adding a base to an acid makes the pH rise toward 7, passes through 7, and keeps going up if you add more base. This is called neutralization. Antacid tablets use it: they are bases that reduce stomach acid.
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,…
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.
| Amount of X added (mL) | pH of mixture |
|---|---|
| 0 | 2.5 |
| 10 | 3.8 |
| 20 | 5.5 |
| 30 | 7.0 |
| 40 | 9.2 |
(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.
Back to the start: Jayden's factory sample has a pH of 4.5, so it is acidic, and much more acidic than the upstream water at pH 7. Now you can read the number and the story it tells.