Welcome to Acids, Bases and Salts!
Have you ever tasted the sharp sourness of a lemon, or felt the sting of a bee or wasp? If so, you have already experienced the chemistry of acids and bases in real life! In this chapter, we are going to explore what acids and alkalis actually are, how we measure them using the pH scale, and what happens when they react together to make salts. Don't worry if science has felt difficult before – we will break everything down into clear, easy-to-follow steps.
1. Acids, Alkalis, and the pH Scale
What is an Acid?
Acids are substances that have a sharp or sour taste when found in food, but in the laboratory, strong acids are corrosive (they can burn skin and eat away materials).
• Everyday examples of acids: Lemon juice (contains citric acid), vinegar (ethanoic acid), fizzy drinks (carbonic acid).
• Common lab acids: Hydrochloric acid (\(\text{HCl}\)), Sulfuric acid (\(\text{H}_2\text{SO}_4\)), and Nitric acid (\(\text{HNO}_3\)).
What is an Alkali?
An alkali is a chemical that feels soapy on the skin and can cancel out (neutralise) an acid. Just like strong acids, strong alkalis are also corrosive and hazardous.
• Everyday examples of alkalis: Bleach, oven cleaner, soap, toothpaste, indigestion tablets.
• Common lab alkalis: Sodium hydroxide (\(\text{NaOH}\)), Potassium hydroxide (\(\text{KOH}\)), and Calcium hydroxide (\(\text{Ca(OH)}_2\)).
The pH Scale
The pH scale is a continuous scale that runs from \(0\) to \(14\). It measures how acidic or alkaline a solution is:
• pH \(0\) to \(6\): Acidic (lower numbers mean a stronger acid. \(\text{pH } 0-2\) is strongly acidic, \(\text{pH } 3-6\) is weakly acidic).
• pH \(7\): Neutral (pure water is neutral – neither acidic nor alkaline).
• pH \(8\) to \(14\): Alkaline (higher numbers mean a stronger alkali. \(\text{pH } 8-11\) is weakly alkaline, \(\text{pH } 12-14\) is strongly alkaline).
How Do We Test pH? (Indicators)
An indicator is a special chemical dye that changes colour depending on whether it is placed in an acid or an alkali.
1. Universal Indicator
Universal indicator gives a wide spectrum of colours and can tell us the approximate pH number:
• Strong Acid (\(\text{pH } 0-2\)): Red
• Weak Acid (\(\text{pH } 3-6\)): Orange / Yellow
• Neutral (\(\text{pH } 7\)): Green
• Weak Alkali (\(\text{pH } 8-11\)): Blue
• Strong Alkali (\(\text{pH } 12-14\)): Purple
2. Litmus Paper
Litmus paper is a quick test to see if a chemical is an acid or an alkali, but it does not tell you the exact pH value:
• Red litmus paper stays red in acid, but turns blue in alkali.
• Blue litmus paper turns red in acid, but stays blue in alkali.
• In a neutral solution, red litmus stays red and blue litmus stays blue.
Memory Trick: Think Blue for Base / Alkali and Red for Radical Acid!
Key Takeaway: The pH scale goes from \(0\) (strong acid, red) to \(7\) (neutral, green) to \(14\) (strong alkali, purple).
2. Bases vs. Alkalis: What is the Difference?
This is a common question in exams, but the difference is simple!
• A base is any substance that reacts with an acid to neutralise it (such as metal oxides, metal hydroxides, and metal carbonates).
• An alkali is simply a soluble base (a base that dissolves in water).
Analogy: Think of bases as "shoes" and alkalis as "trainers". All trainers are shoes, but not all shoes are trainers! In the same way, all alkalis are bases, but not all bases are alkalis (because many bases cannot dissolve in water).
3. Reactions of Acids and Making Salts
When acids react, they form new compounds called salts. A salt is formed when the hydrogen in an acid is replaced by a metal.
How to Name a Salt
A salt gets its name in two parts:
1. First name: Comes from the metal (e.g., Sodium, Copper, Magnesium).
2. Last name: Comes from the acid used:
• Hydrochloric acid produces chloride salts.
• Sulfuric acid produces sulfate salts.
• Nitric acid produces nitrate salts.
Example: Copper oxide + Hydrochloric acid \(\rightarrow\) Copper chloride + Water.
Reaction 1: Acid + Metal
When an acid reacts with a reactive metal, it produces a salt and hydrogen gas bubbles:
Acid + Metal \(\rightarrow\) Salt + Hydrogen
Example:
\(\text{Hydrochloric acid} + \text{Magnesium} \rightarrow \text{Magnesium chloride} + \text{Hydrogen}\)
\(\text{2HCl} + \text{Mg} \rightarrow \text{MgCl}_2 + \text{H}_2\)
Gas Test for Hydrogen (\(\text{H}_2\)):
Place a lighted splint near the mouth of the test tube. If hydrogen is present, it will burn with a distinctive squeaky pop sound.
Reaction 2: Acid + Base / Alkali (Neutralisation)
When an acid reacts with a metal oxide or metal hydroxide, they cancel each other out to produce a salt and pure water:
Acid + Base \(\rightarrow\) Salt + Water
Example:
\(\text{Sulfuric acid} + \text{Copper(II) oxide} \rightarrow \text{Copper(II) sulfate} + \text{Water}\)
\(\text{H}_2\text{SO}_4 + \text{CuO} \rightarrow \text{CuSO}_4 + \text{H}_2\text{O}\)
Everyday Neutralisation:
• Indigestion remedies: Antacid tablets contain mild bases to neutralise excess stomach acid (\(\text{HCl}\)).
• Treating stings: Bee stings are acidic and can be neutralised with baking soda (mildly alkaline). Wasp stings are alkaline and can be neutralised with vinegar (mildly acidic).
Reaction 3: Acid + Metal Carbonate
Metal carbonates contain carbon and oxygen, so in addition to salt and water, they also produce carbon dioxide gas (fizzing/effervescence):
Acid + Metal Carbonate \(\rightarrow\) Salt + Water + Carbon Dioxide
Example:
\(\text{Hydrochloric acid} + \text{Calcium carbonate} \rightarrow \text{Calcium chloride} + \text{Water} + \text{Carbon dioxide}\)
\(\text{2HCl} + \text{CaCO}_3 \rightarrow \text{CaCl}_2 + \text{H}_2\text{O} + \text{CO}_2\)
Gas Test for Carbon Dioxide (\(\text{CO}_2\)):
Bubble the gas through clear limewater. If carbon dioxide is present, the limewater turns milky / cloudy white.
Key Takeaway:
• Acid + Metal \(\rightarrow\) Salt + Hydrogen
• Acid + Base \(\rightarrow\) Salt + Water
• Acid + Carbonate \(\rightarrow\) Salt + Water + Carbon Dioxide
4. Required Practical: Making Pure, Dry Crystals of a Soluble Salt
A classic exam question asks you to describe how to prepare pure, dry copper(II) sulfate crystals from sulfuric acid and insoluble copper(II) oxide powder.
Step-by-Step Method:
Step 1: Reacting the mixture
• Measure a fixed volume of dilute sulfuric acid into a beaker and warm it gently using a Bunsen burner (heating speeds up the reaction).
• Add copper(II) oxide powder a little at a time and stir with a glass rod.
• Keep adding copper oxide until it is in excess (meaning there is unreacted black powder left at the bottom). Adding excess ensures that all the acid has reacted.
Step 2: Filtration
• Filter the warm mixture using a funnel and filter paper.
• The unreacted copper oxide powder remains on the filter paper (residue), leaving a clear blue solution of copper sulfate in the evaporating dish below (filtrate).
Step 3: Evaporation and Crystallisation
• Heat the copper sulfate solution gently over a water bath or directly with a Bunsen burner to evaporate off some of the water until the crystallisation point is reached (crystals begin to form on a cold glass rod).
• Turn off the heat and leave the concentrated solution in a warm, dry place to cool and crystallise slowly over several days.
Step 4: Drying
• Filter off the crystals and pat them dry carefully between two sheets of clean filter paper (or leave them to dry in a warm desiccator/oven).
5. Quick Summary and Common Exam Traps
Quick Review of Key Terms:
• Acid: A substance with a pH less than \(7\).
• Alkali: A soluble base with a pH greater than \(7\).
• Base: A substance that neutralises an acid (metal oxide, hydroxide, carbonate).
• Salt: A compound formed when hydrogen in an acid is replaced by a metal.
• Neutralisation: A reaction between an acid and a base forming salt and water.
Mistakes to Avoid in Your Exam:
• Don't mix up gas tests: Remember that hydrogen pops with a lighted splint, while carbon dioxide turns limewater cloudy.
• Don't forget why we add excess base: When preparing a salt, we always add excess base to ensure all the acid is completely used up, so our crystals won't be contaminated with acid.
• Watch the salt names: Hydrochloric acid always makes chloride, Sulfuric acid makes sulfate, and Nitric acid makes nitrate.