Combined Science (9204) | Acids, bases and salts

🔬 Combined Science (9204) Chemistry: Acids, Bases, and Salts

Hello future chemist! This chapter is fundamental to understanding how chemistry works in the real world—from keeping your stomach healthy to cleaning your home. Don't worry if some parts seem complex; we will break down acids, bases, and the super-important neutralisation reaction into easy-to-manage steps. By the end, you'll be able to predict the products of almost any acid reaction!

Section 1: The Definitions - What Makes an Acid an Acid?

Acids, Bases, and Alkalis: The Essentials

The key difference between these three concepts lies in the ions they release when dissolved in water.

1. Acids

• Definition: Acids are substances that produce hydrogen ions, \(H^+\), when dissolved in water.
• The concentration of these \(H^+\) ions determines how strong the acid is.
• Example Acids: Hydrochloric acid (\(HCl\)), Sulfuric acid (\(H_2SO_4\)), Nitric acid (\(HNO_3\)).
• Real World Example: The citric acid in lemons or the acid in your stomach (hydrochloric acid) for digestion.

2. Bases and Alkalis

All alkalis are bases, but not all bases are alkalis. Confusing? Let's clarify:

• Bases: A base is simply a substance (often a metal oxide or hydroxide) that can neutralise an acid.
• Alkalis: Alkalis are soluble bases—meaning they dissolve in water.
• Definition: Alkalis produce hydroxide ions, \(OH^-\), when dissolved in water.
• Example Alkalis: Sodium hydroxide (\(NaOH\)), Potassium hydroxide (\(KOH\)).
• Analogy: Think of a base as any car, and an alkali as a car that can float (a boat). The boat is a type of car, but not all cars are boats!

The pH Scale and Indicators

The pH scale is used to measure the acidity or alkalinity of a substance. It runs from 0 to 14.

• pH 0 to 6: Acidic (0 is strongly acidic, 6 is weakly acidic).
• pH 7: Neutral (Pure water is pH 7).
• pH 8 to 14: Alkaline (8 is weakly alkaline, 14 is strongly alkaline).

To determine the pH of a substance, we use indicators:

• Litmus Paper:
  • Turns Red in Acid.
  • Turns Blue in Alkali.
• Universal Indicator: This is a mixture of dyes that gives a whole range of colours, allowing you to estimate the exact pH value.
  • Strong Acid: Red
  • Weak Acid: Orange/Yellow
  • Neutral: Green
  • Alkali: Blue/Purple

Section 2: Reactions of Acids (The Four Key Reactions)

Acids are very reactive! They participate in four main types of reaction you must know. In every reaction, an acid produces a salt. The name of the salt depends on the acid used:

• Hydrochloric Acid makes Chlorides (e.g., Sodium Chloride).
• Sulfuric Acid makes Sulfates (e.g., Magnesium Sulfate).
• Nitric Acid makes Nitrates (e.g., Potassium Nitrate).

2.1 Reaction with Metals

When an acid reacts with a reactive metal (like magnesium, zinc, or iron), it produces a salt and hydrogen gas.

Acid + Metal \(\rightarrow\) Salt + Hydrogen

Example:

\(2HCl(aq) + Mg(s) \rightarrow MgCl_2(aq) + H_2(g)\)

• Gas Test: Hydrogen gas is tested using a lighted splint. It produces a characteristic squeaky pop sound.
• Common Mistake: Very unreactive metals (like gold or copper) will NOT react with dilute acids.

2.2 Reaction with Metal Oxides (Bases)

This is the simplest type of neutralisation reaction.

Acid + Metal Oxide \(\rightarrow\) Salt + Water

Example:

\(H_2SO_4(aq) + CuO(s) \rightarrow CuSO_4(aq) + H_2O(l)\)

Did you know? Metal oxides are often used to coat metals (like car bodies) to stop them from reacting with the environment, protecting them from corrosion.

2.3 Reaction with Metal Hydroxides (Alkalis - Neutralisation)

This is the classic definition of neutralisation: the reaction between an acid and an alkali (soluble base).

Acid + Alkali \(\rightarrow\) Salt + Water

Example:

\(HCl(aq) + NaOH(aq) \rightarrow NaCl(aq) + H_2O(l)\)

The Ionic Equation:
In solution, the key reaction is between the hydrogen ion from the acid and the hydroxide ion from the alkali:

\(H^+(aq) + OH^-(aq) \rightarrow H_2O(l)\)

When neutralisation is complete, the resulting salt solution will have a pH of 7 (neutral).

2.4 Reaction with Metal Carbonates

This reaction is special because it produces three products, including carbon dioxide gas.

Acid + Metal Carbonate \(\rightarrow\) Salt + Water + Carbon Dioxide

Example:

\(2HNO_3(aq) + CaCO_3(s) \rightarrow Ca(NO_3)_2(aq) + H_2O(l) + CO_2(g)\)

• Gas Test: Carbon dioxide gas is tested by bubbling it through limewater (calcium hydroxide solution). Limewater turns cloudy (milky) if \(CO_2\) is present.
• Real World Example: When you use acid to clean limescale (which is mostly calcium carbonate) off a kettle, you see bubbling because of the \(CO_2\) being released.

Section 3: Preparing Soluble Salts

Salts are incredibly useful, and scientists often need to produce a pure sample of a specific salt. The method you choose depends entirely on whether the base/alkali you use is soluble or insoluble.

Method 1: Making Soluble Salts using an Insoluble Base (Excess Method)

This method is used when one reactant (the base/metal oxide) is insoluble in water. You add the base in excess to ensure all the acid is neutralised.

Step-by-Step Process:

1. Warm the Acid: Gently heat the dilute acid (e.g., sulfuric acid) in a beaker.
2. Add Excess Base: Add the insoluble metal oxide (the base) a little at a time, stirring constantly, until no more dissolves (i.e., some solid powder remains visible at the bottom). This ensures that all the acid has reacted.
3. Filter: Filter the mixture to remove the unreacted, excess solid base. The resulting liquid (the filtrate) is a pure solution of the soluble salt.
4. Evaporate/Crystallise: Gently heat the salt solution to evaporate most of the water (leaving a saturated solution). Leave the concentrated solution to cool slowly. As it cools, the salt will form pure crystals.
5. Dry: Pat the crystals dry with filter paper.

Method 2: Making Soluble Salts using an Alkali (Titration)

This method is used when both the acid and the alkali are soluble (e.g., reacting \(HCl\) and \(NaOH\)). We cannot use the "excess method" because we cannot easily filter out the excess soluble reactant, and we would contaminate the salt. We must use exactly the right amounts—this is done using titration.

Titration Process:

1. Measure Accurate Volumes: Using a pipette, accurately measure a known volume of the alkali solution into a conical flask. Add a few drops of a suitable indicator (like phenolphthalein or methyl orange).
2. Titrate: Slowly add the acid from a burette into the flask, swirling constantly.
3. Endpoint: Stop adding the acid exactly when the indicator changes colour (the endpoint), indicating neutralisation has occurred. Record the volume of acid used (this is called the titre).
4. Repeat without Indicator: To obtain a pure salt, the reaction must be repeated using the exact same volumes determined in the first step, but this time without the indicator (as the indicator would contaminate the final salt).
5. Crystallise: Evaporate the neutral salt solution and allow it to cool and crystallise, as in Method 1.

Great work! You now have a solid understanding of acids, bases, and the reactions that form salts. Remember to practice writing those balanced chemical equations!