Introduction to Reactions of Acids and Salts

Welcome to one of the most practical parts of Chemistry! In this chapter, we are going to look at how acids behave when they meet different substances. When an acid reacts, it often produces something called a salt. In everyday life, "salt" usually means the stuff you put on your chips (sodium chloride), but in chemistry, a salt is a whole family of compounds formed during these reactions. Don't worry if the chemical names seem long at first—there is a very simple pattern to naming them that we will master together!

1. What are Acids and Alkalis? (A Quick Refresher)

Before we dive into the reactions, remember these two key definitions:

• Acids are substances that produce hydrogen ions, \(H^+\), in water.
• Alkalis are substances that produce hydroxide ions, \(OH^-\), in water.

(For more on the pH scale and indicators, check out the "Acids, alkalis and the pH scale" chapter!)

2. The Four Key Acid Reactions

There are four main reactions you need to know. A great way to remember these is to look at what is produced in each one.

A. Acid + Metal

When an acid reacts with a metal, it produces a salt and hydrogen gas. General Equation: \(acid + metal \rightarrow salt + hydrogen\)

The Hydrogen Test: How do we know hydrogen is there? We use the "Squeaky Pop" test. If you place a lighted splint into a test tube containing hydrogen, you will hear a distinctive pop sound.

B. Acid + Metal Oxide (Base)

Metal oxides are "bases" (substances that neutralise acids). General Equation: \(acid + metal\ oxide \rightarrow salt + water\)

C. Acid + Metal Hydroxide (Alkali)

This is a classic neutralisation reaction. General Equation: \(acid + metal\ hydroxide \rightarrow salt + water\)

D. Acid + Metal Carbonate

Carbonates are a bit special because they have carbon in them, so they produce an extra product: carbon dioxide. General Equation: \(acid + metal\ carbonate \rightarrow salt + water + carbon\ dioxide\)

The Carbon Dioxide Test: We bubble the gas through limewater. If the gas is carbon dioxide, the limewater will turn cloudy (or milky).

Quick Tip: Naming the Salt
The name of the salt comes in two parts:

1. The first part comes from the metal in the reactant (e.g., Sodium hydroxide \(\rightarrow\) Sodium...).
2. The second part comes from the acid used:
• Hydrochloric acid makes chlorides.
• Sulfuric acid makes sulfates.
• Nitric acid makes nitrates.

Example: Magnesium + Sulfuric Acid \(\rightarrow\) Magnesium Sulfate + Hydrogen.

3. Core Practical: Preparing a Soluble Salt

Core Practical 3.17: Preparation of pure, dry hydrated copper sulfate crystals from copper oxide, using a water bath. In this experiment, we react an acid with an insoluble reactant (the copper oxide powder).

The Step-by-Step Process:

Step 1: Heat the acid. Gently warm the sulfuric acid in a beaker using a water bath. (Heating speeds up the reaction).
Step 2: Add the base in excess. Add copper oxide powder to the acid and stir. Keep adding it until no more will dissolve. This ensures all the acid has been used up.
Step 3: Filtration. Filter the mixture using a funnel and filter paper. This removes the "excess" (unreacted) copper oxide, leaving you with a blue solution of copper sulfate and water.
Step 4: Evaporation. Pour the solution into an evaporating basin and heat it over a water bath to evaporate some of the water until the "crystallisation point" is reached.
Step 5: Crystallisation. Leave the remaining solution to cool down slowly. Beautiful blue copper sulfate crystals will form. Pat them dry with filter paper.

Common Mistake: Don't evaporate all the water with the Bunsen burner! If you heat it too strongly until it's dry, you might dehydrate the crystals or cause them to spit. Slow cooling makes bigger, better crystals.

4. Solubility Rules

To know if a salt will dissolve or form a solid (precipitate), you must learn these rules:

• Common Chlorides: Soluble (except silver and lead).
• Common Sulfates: Soluble (except lead, barium, and calcium).
• Common Carbonates and Hydroxides: Insoluble (except sodium, potassium, and ammonium).

Note: All sodium, potassium, and ammonium salts are soluble!

5. Preparing Insoluble Salts (Precipitation)

What if we want to make a salt that doesn't dissolve in water? We use a precipitation reaction.

How to do it:

1. Pick two soluble salts that contain the ions you need (e.g., to make lead sulfate, you could use lead nitrate and sodium sulfate).
2. Mix the two solutions together. A solid precipitate will form instantly.
3. Filter the mixture. The insoluble salt stays on the filter paper.
4. Wash the solid with distilled water to remove any leftover soluble impurities.
5. Dry the pure salt in a warm oven.

Summary Key Takeaways

• Metal + Acid \(\rightarrow\) Salt + Hydrogen (\(H_2\)).
• Carbonate + Acid \(\rightarrow\) Salt + Water + Carbon Dioxide (\(CO_2\)).
• To make a soluble salt from an insoluble base, add the base in excess, filter, and crystallise.
• To make an insoluble salt, mix two soluble solutions and filter the precipitate.
• Always use limewater to test for \(CO_2\) and a lighted splint for \(H_2\).