Describe the characteristic properties of acids and bases, distinguish strong from weak acids and bases, and identify common examples (MA STE supporting content, properties of acids and bases).

What this topic is asking

Beyond the pH scale, a Massachusetts high school chemistry course expects you to know the characteristic properties that identify acids and bases, the difference between strong and weak, and the typical reactions of acids. This page rounds out the acid-base module with the qualitative chemistry that often appears alongside the calculations.

Properties of acids

The properties come from the hydrogen ions an acid releases in water. Two reactions are especially useful to recognize:

• Acid with a reactive metal gives a salt and hydrogen gas: for example, $\text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2$. The hydrogen can be tested with a lighted splint (a squeaky pop).
• Acid with a carbonate gives a salt, water, and carbon dioxide: for example, $2\text{HCl} + \text{CaCO}_3 \rightarrow \text{CaCl}_2 + \text{H}_2\text{O} + \text{CO}_2$. The carbon dioxide turns limewater milky.

Safety note: never taste laboratory chemicals; "sour taste" describes household acids like vinegar and lemon juice.

Properties of bases

The slippery feel of a base comes from its reaction with oils on the skin. Bases that dissolve in water (alkalis) release hydroxide ions. Common household bases include baking soda, soap, and ammonia-based cleaners. The defining chemical behavior of a base is neutralizing an acid, the reaction developed in neutralization and titration.

Strong versus weak

This is a frequent point of confusion. Strength is about how fully a substance ionizes:

• A strong acid such as hydrochloric acid breaks apart completely: nearly every molecule releases its hydrogen ion.
• A weak acid such as acetic acid (in vinegar) ionizes only slightly: most molecules stay intact, so fewer hydrogen ions form.

Strength is not the same as concentration, which is how much acid is dissolved (its molarity). A dilute strong acid can have a higher pH than a concentrated weak acid. Keeping these two ideas separate, building on acids, bases and the pH scale, is essential.

Try this

Q1. Name the gas given off when hydrochloric acid reacts with magnesium metal. [1]

• Cue. Hydrogen gas.

Q2. A concentrated weak acid has a higher pH than a dilute strong acid. Explain how this is possible. [2]

• Cue. The weak acid only partly ionizes, so even when concentrated it releases relatively few hydrogen ions; the strong acid ionizes fully, so even dilute it can give a higher hydrogen ion concentration and lower pH.