Acids, Bases and Salts

What you'll learn

This revision guide covers everything you need to know about acids, bases and salts for your CXC CSEC Integrated Science examination. You will learn to identify properties of acids and bases, understand pH and indicators, perform neutralization reactions, and prepare salts using different methods. These concepts are fundamental to understanding chemical reactions in everyday life, from preserving tamarind balls to treating acidic soils in Caribbean agriculture.

Key terms and definitions

Acid — A substance that produces hydrogen ions (H⁺) when dissolved in water and has a pH less than 7. Acids taste sour, turn blue litmus paper red, and react with metals to produce hydrogen gas.

Base — A substance that accepts hydrogen ions or produces hydroxide ions (OH⁻) in solution. Bases feel slippery, turn red litmus paper blue, and have a pH greater than 7.

Alkali — A soluble base that releases hydroxide ions (OH⁻) when dissolved in water, such as sodium hydroxide or potassium hydroxide.

Neutralization — A chemical reaction between an acid and a base that produces a salt and water. The hydrogen ions from the acid combine with hydroxide ions from the base to form water (H⁺ + OH⁻ → H₂O).

pH scale — A numerical scale from 0 to 14 that measures the acidity or alkalinity of a solution. Values below 7 are acidic, 7 is neutral, and above 7 is alkaline.

Indicator — A substance that changes colour depending on whether it is in acidic or alkaline conditions, used to determine pH.

Salt — An ionic compound formed when the hydrogen ion of an acid is replaced by a metal ion or ammonium ion during a neutralization reaction.

Dilute acid — An acid solution containing a relatively small amount of acid dissolved in a large amount of water, making it less concentrated and less corrosive than concentrated acid.

Core concepts

Properties of acids

Acids share several characteristic properties that allow us to identify them:

Physical properties:

• Sour taste (though you should never taste laboratory acids)
• Turn blue litmus paper red
• pH values between 0 and 6.9
• Conduct electricity when dissolved in water due to presence of ions

Chemical properties:

• React with reactive metals (magnesium, zinc, iron) to produce hydrogen gas and a salt
• React with carbonates and hydrogencarbonates to produce carbon dioxide, water and a salt
• React with bases and alkalis in neutralization reactions to produce salt and water

Common acids you need to know:

• Hydrochloric acid (HCl) — found in stomach acid, used in cleaning
• Sulfuric acid (H₂SO₄) — used in car batteries, fertilizer production
• Nitric acid (HNO₃) — used in fertilizer manufacture
• Ethanoic acid (CH₃COOH) — found in vinegar, used in preserving Caribbean condiments like pepper sauce
• Citric acid — found in citrus fruits like lime, grapefruit and orange grown throughout the Caribbean

Properties of bases and alkalis

Bases neutralize acids, while alkalis are the water-soluble subset of bases.

Physical properties:

• Bitter taste
• Slippery or soapy feel
• Turn red litmus paper blue
• pH values between 7.1 and 14
• Conduct electricity when dissolved in water (alkalis only)

Chemical properties:

• React with acids in neutralization reactions
• React with ammonium salts to produce ammonia gas
• React with fats and oils to make soap (saponification)

Common bases and alkalis:

• Sodium hydroxide (NaOH) — strong alkali used in soap making, drain cleaners
• Calcium hydroxide (Ca(OH)₂) — used to reduce soil acidity in Caribbean farms
• Ammonia solution (NH₃) — weak alkali used in cleaning products
• Magnesium oxide (MgO) — base used in indigestion remedies
• Copper(II) oxide (CuO) — insoluble base

pH scale and indicators

The pH scale measures hydrogen ion concentration in a solution:

pH values:

• 0-3: Strong acid (hydrochloric acid, sulfuric acid)
• 4-6: Weak acid (ethanoic acid, carbonic acid)
• 7: Neutral (pure water, sodium chloride solution)
• 8-10: Weak alkali (sodium hydrogencarbonate solution)
• 11-14: Strong alkali (sodium hydroxide, potassium hydroxide)

Types of indicators:

Litmus paper:

• Blue litmus turns red in acid
• Red litmus turns blue in alkali
• No colour change in neutral solutions
• Does not provide specific pH values

Universal indicator:

• Changes through a range of colours
• Red/orange in acids, green in neutral, blue/purple in alkalis
• Provides approximate pH values using a colour chart
• Available as solution or paper

Natural indicators:

• Red cabbage extract — changes from red in acid to green in alkali
• Hibiscus (sorrel) extract — Caribbean plant that changes colour with pH
• Turmeric — yellow in neutral/acid, red-brown in alkali

Neutralization reactions

Neutralization is the reaction between an acid and a base producing salt and water.

General equation:

Acid + Base → Salt + Water

Specific examples:

1. Acid + Metal oxide:

H₂SO₄ + CuO → CuSO₄ + H₂O
Sulfuric acid + Copper(II) oxide → Copper(II) sulfate + Water

2. Acid + Metal hydroxide:

HCl + NaOH → NaCl + H₂O
Hydrochloric acid + Sodium hydroxide → Sodium chloride + Water

3. Acid + Carbonate:

2HNO₃ + CaCO₃ → Ca(NO₃)₂ + H₂O + CO₂
Nitric acid + Calcium carbonate → Calcium nitrate + Water + Carbon dioxide

Caribbean applications:

• Treating acidic soil with lime (calcium hydroxide) in Jamaican coffee plantations
• Using antacids (bases) to neutralize excess stomach acid
• Treating wasp stings (alkaline) with vinegar (acid)
• Adding lime to acidic ponds in aquaculture operations in Guyana and Trinidad

Reactions of acids

Acid + Reactive metal → Salt + Hydrogen

Example:

Zn + H₂SO₄ → ZnSO₄ + H₂
Zinc + Sulfuric acid → Zinc sulfate + Hydrogen gas

Test for hydrogen: Lit splint produces a "pop" sound

Reactivity order: Magnesium > Zinc > Iron > Copper (no reaction)

Acid + Carbonate → Salt + Water + Carbon dioxide

Example:

CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
Calcium carbonate + Hydrochloric acid → Calcium chloride + Water + Carbon dioxide

Test for carbon dioxide: Limewater turns milky/cloudy

Acid + Base/Alkali → Salt + Water

This is the neutralization reaction covered previously.

Preparation of salts

Salts can be prepared using several methods depending on the solubility of the salt and starting materials.

Method 1: Acid + Insoluble base/metal

Used to prepare soluble salts from insoluble bases or reactive metals.

Steps:

1. Add excess base/metal to acid until no more dissolves
2. Filter to remove excess unreacted solid
3. Heat filtrate to evaporate water
4. Cool to allow crystallization
5. Filter and dry crystals

Example: Preparing copper(II) sulfate from copper(II) oxide and sulfuric acid

Method 2: Acid + Alkali (Titration)

Used when both reactants are soluble.

Steps:

1. Use titration to find exact volumes needed for neutralization
2. Mix the exact volumes without indicator
3. Evaporate water to crystallize the salt
4. Filter and dry crystals

Example: Preparing sodium chloride from hydrochloric acid and sodium hydroxide

Method 3: Precipitation

Used to prepare insoluble salts.

Steps:

1. Mix two soluble salt solutions
2. Insoluble salt forms as precipitate
3. Filter to collect precipitate
4. Wash with distilled water
5. Dry in oven or between filter papers

Example: Preparing lead(II) iodide by mixing lead(II) nitrate solution and potassium iodide solution

Worked examples

Example 1: Identifying acids and bases

Question: A student tested four colourless liquids, P, Q, R and S, using universal indicator. The results are shown below:

• Liquid P: Red colour, pH 2
• Liquid Q: Green colour, pH 7
• Liquid R: Purple colour, pH 13
• Liquid S: Orange colour, pH 5

(a) Which liquid is a strong acid? (1 mark) (b) Which liquid is neutral? (1 mark) (c) Suggest the identity of liquid R. (1 mark) (d) What would happen to red litmus paper in liquid R? (1 mark)

Mark scheme answers: (a) P / Liquid P (pH 2 indicates strong acid) (b) Q / Liquid Q (pH 7 is neutral) (c) Sodium hydroxide / potassium hydroxide / any strong alkali (pH 13 indicates strong alkali) (d) It would turn blue / change from red to blue (alkalis turn red litmus blue)

Example 2: Neutralization reaction

Question: A farmer in St. Vincent notices his soil is too acidic for growing vegetables. He treats the soil with calcium hydroxide.

(a) What type of substance is calcium hydroxide? (1 mark) (b) Write a balanced equation for the reaction between calcium hydroxide, Ca(OH)₂, and nitric acid, HNO₃. (3 marks) (c) Name the salt produced in this reaction. (1 mark) (d) Explain why this treatment helps the vegetables grow better. (2 marks)

Mark scheme answers: (a) Base / alkali (1 mark) (b) Ca(OH)₂ + 2HNO₃ → Ca(NO₃)₂ + 2H₂O (3 marks: 1 for correct formula, 1 for products, 1 for balancing) (c) Calcium nitrate (1 mark) (d) The base neutralizes the acid in the soil (1 mark), bringing the pH closer to neutral which is better for plant growth / allows nutrients to be absorbed better (1 mark)

Example 3: Preparing a salt

Question: Describe how you would prepare a pure, dry sample of copper(II) sulfate crystals starting with copper(II) oxide powder and dilute sulfuric acid. (6 marks)

Mark scheme answer:

1. Add excess copper(II) oxide to dilute sulfuric acid (1 mark)
2. Stir/warm gently until no more copper(II) oxide dissolves (1 mark)
3. Filter the mixture to remove excess copper(II) oxide (1 mark)
4. Heat/evaporate the filtrate to reduce the volume of water (1 mark)
5. Leave to cool and crystallize (1 mark)
6. Filter the crystals and dry between filter paper / in a warm oven (1 mark)

Common mistakes and how to avoid them

• Confusing acids and bases: Remember acids turn blue litmus red and have pH below 7, while bases/alkalis turn red litmus blue and have pH above 7. Use the mnemonic "Acid = Add hydrogen ions."

• Forgetting to balance equations: Always check you have the same number of each type of atom on both sides. In neutralization reactions, remember water is H₂O, not H₂O₂.

• Not using excess solid when making salts: When preparing salts using an insoluble base or metal, you must add excess to ensure all the acid reacts. The excess is then filtered off.

• Confusing the gas tests: Hydrogen produces a "pop" with a lit splint, while carbon dioxide turns limewater milky. Don't mix these up in exam answers.

• Writing incomplete salt preparation methods: Include all steps: reaction, filtration, evaporation, crystallization, drying. Missing steps will lose marks.

• Naming salts incorrectly: The salt name comes from the metal (or ammonium) followed by the acid part. Hydrochloric acid gives chlorides, sulfuric acid gives sulfates, nitric acid gives nitrates.

Exam technique for "Acids, Bases and Salts"

• Command words matter: "State" requires a brief answer without explanation (1 mark), "Describe" needs a sequence of steps or observations (multiple marks), "Explain" requires reasons why something happens (usually 2+ marks).

• Chemical equations carry multiple marks: Write correct formulae (1 mark), show correct products (1 mark), and balance the equation (1 mark). Always check your balancing before moving on.

• Practical questions require detail: When describing salt preparation or experiments, include specific apparatus (filter funnel, evaporating basin), observations (colour changes, precipitate formation), and safety precautions where relevant.

• Link chemistry to Caribbean contexts: If questions mention regional examples (treating soil acidity, food preservation, local industries), make sure your answers connect the chemistry to the specific application mentioned.

Quick revision summary

Acids produce H⁺ ions in solution, have pH below 7, turn blue litmus red, and react with metals, carbonates and bases. Bases accept H⁺ ions, have pH above 7, and turn red litmus blue. Neutralization occurs when acids and bases react to produce salt and water. The pH scale ranges from 0 (strong acid) to 14 (strong alkali), with 7 being neutral. Indicators like litmus and universal indicator detect pH changes. Salts are prepared by reacting acids with metals, insoluble bases, or alkalis, followed by crystallization. Understanding these reactions is essential for applications in agriculture, medicine and industry throughout the Caribbean.