Acids, Bases and Salts: H⁺, pH and Making Salts
Acid Mode: One Ion to Rule Them All 🧪
Introduction
1. Introduction
Okay, acids and bases sounds like a whole vibe shift, but here's the secret the whole chapter is built on: an acid is just anything that dumps hydrogen ions () into water. That's it. Every fizz, every colour change, every pH number traces back to .
And its opposite, an alkali, dumps hydroxide ions (). When the two meet they cancel out into water, which is the move called neutralisation. Lock in those two ions and the reactions, the indicators, the oxides, and even the recipes for making salts all line up. This is your fast refresh, not the full textbook: one key idea per topic, one quick worked example, and the exact move the marker wants. Let's get into acid mode. 🧪
And its opposite, an alkali, dumps hydroxide ions (). When the two meet they cancel out into water, which is the move called neutralisation. Lock in those two ions and the reactions, the indicators, the oxides, and even the recipes for making salts all line up. This is your fast refresh, not the full textbook: one key idea per topic, one quick worked example, and the exact move the marker wants. Let's get into acid mode. 🧪
2. Acids, Bases and the H⁺ Ion
An acid is a substance that releases hydrogen ions, , when it dissolves in water, and literally every acidic thing it does is in action. A base is a metal oxide or hydroxide that cancels an acid; if a base dissolves in water it's called an alkali and it releases ions. So all alkalis are bases, but not all bases are alkalis (copper(II) oxide cancels acid but won't dissolve, so it's a base only). Extended adds a second pair of definitions: is just a proton, so an acid is a proton donor and a base is a proton acceptor. To label a reaction, follow the proton: in , HCl gives its proton away (acid) and water takes it (base).
Key idea🔑 Key idea: Acid = makes in water, a proton donor. Base = a proton acceptor; alkali = a soluble base that makes . They cancel: . ⚡
Worked example
Worked Example: The Ion Every Acid Shares
Worked Example: Spot the H⁺ 🔬
Nitric acid, sulfuric acid and ethanoic acid are all dissolved in water. Which one ion do all three contain that makes them acidic?
- 1An acid is defined by the ion it releases: the hydrogen ion, .
- 2Each makes a different second ion (, , ), but the one they all share is .
So the answer is the hydrogen ion, . When acids are listed together, find what they share. ✅
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3. The Reactions of Acids
Acids do three classic reactions, and once you know the patterns the products write themselves. Acid + metal → salt + hydrogen (the hydrogen pops with a lighted splint). Acid + base → salt + water (that's neutralisation). Acid + carbonate → salt + water + carbon dioxide (the turns limewater milky). The acid even names the salt: hydrochloric → chloride, sulfuric → sulfate, nitric → nitrate.
Key idea🔑 Key idea: metal → salt + hydrogen; base → salt + water; carbonate → salt + water + CO₂. The acid fixes the salt's family name. 🎯
Worked example
Worked Example: Acid Meets Carbonate
Worked Example: What Do You See? 👀
Dilute hydrochloric acid is added to solid magnesium carbonate. State two observations.
- 1Pattern: acid + carbonate → salt + water + carbon dioxide.
- 2The solid gets used up, so it disappears (dissolves), and the makes bubbles (fizzing).
So: the solid disappears and there's effervescence. (An "observation" is what you see, not the gas's name.) 🔥
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4. Indicators and the pH Scale
Indicators are dyes that change colour to tell you acid from alkali: litmus (red in acid, blue in alkali), thymolphthalein (colourless in acid, blue in alkali), methyl orange (red in acid, yellow in alkali). For a proper reading, universal indicator gives a rainbow matched to the pH scale from 0 to 14: 7 is neutral, below 7 is acidic, above 7 is alkaline. More ions = lower pH.
Key idea🔑 Key idea: litmus red/blue, thymolphthalein colourless/blue, methyl orange red/yellow. pH 7 = neutral; lower pH = more acidic. 🎯
Worked example
Worked Example: Methyl Orange in Acid
Worked Example: Name That Colour 🟥
Methyl orange is added to dilute hydrochloric acid. What colour does it turn?
- 1Methyl orange is red in acid, yellow in alkali.
- 2Hydrochloric acid is an acid.
So the methyl orange turns red. (Don't get caught by thymolphthalein, which is colourless in acid.) ✅
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5. Strong vs Weak Acids
Don't mix these two words up. Strength is about how much the acid splits into ions in water, and it is not the same as concentration (how much acid is in there). A strong acid splits completely (HCl, nitric, sulfuric) and we write it with a single forward arrow. A weak acid splits only a little (ethanoic acid) and we write it with a reversible arrow. At the same concentration the weak acid has fewer ions, so it has a higher pH and reacts slower.
Key idea🔑 Key idea: Strong = splits fully (single arrow). Weak = splits a little (reversible arrow). Strength ≠ concentration. ⚡
Worked example
Worked Example: HCl vs Ethanoic Acid
Worked Example: Which Arrow? ➡️
How do the dissociation equations of hydrochloric acid and ethanoic acid differ?
- 1HCl is strong, so it splits fully and uses a single forward arrow:
- 2Ethanoic acid is weak, so it splits partly and uses a reversible arrow:
So strong = single arrow, weak = reversible arrow. Don't swap them, that's the trap. ✅
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Answer
6. Bases and Alkalis
A base is a metal oxide or hydroxide that neutralises an acid. An alkali is just a base that happens to dissolve in water (so it can release into solution). That one word, soluble, is the whole difference. Bases have a signature move too: warm a base with an ammonium salt and it releases ammonia gas, which turns damp red litmus paper blue (that's the test for an ammonium salt).
Key idea🔑 Key idea: Every alkali is a base, but only the soluble bases are alkalis. Base + ammonium salt → ammonia (turns damp red litmus blue). 🎯
Worked example
Worked Example: Base or Alkali?
Worked Example: Does It Dissolve? 💧
Sodium hydroxide is called an alkali, but copper(II) oxide is only a base. Why?
- 1A base neutralises acids; an alkali is a base that dissolves in water.
- 2Sodium hydroxide dissolves (base and alkali); copper(II) oxide doesn't dissolve (base only).
So the difference is solubility. Same family, one dissolves, one doesn't. ✅
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7. Classifying Oxides
Every oxide can be sorted, and the big clue is whether the element is a metal or a non-metal. Metal oxides are basic (they react with acids): , CaO, CuO. Non-metal oxides are acidic (they react with bases, and dissolve to give acids): , , . Across a period the metallic character falls, so the oxides go from basic, through amphoteric, to acidic: in Period 3, sodium and magnesium oxides are basic, aluminium oxide is amphoteric, and phosphorus and sulfur oxides are acidic. The amphoteric oxides react with both, coming up next.
Key idea🔑 Key idea: Metal oxide → basic. Non-metal oxide → acidic. Across a period: basic → amphoteric → acidic. 🎯
Worked example
Worked Example: An Oxide of Phosphorus
Worked Example: Sort It 🧮
An oxide of phosphorus dissolves in water to give pH 1. Classify it.
- 1Phosphorus is a non-metal, so it's a non-metal oxide.
- 2A pH of 1 is strongly acidic, so it's an acidic oxide.
So it's a non-metal, acidic oxide (not a metal oxide, not basic). Non-metal and acidic are the matching pair. ✅
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8. Amphoteric Oxides
Most oxides pick a side, but amphoteric oxides react with both acids and bases to make a salt and water. There are only two you need for 0620: aluminium oxide, , and zinc oxide, ZnO. With an acid they behave like a basic oxide (); with a base such as aqueous sodium hydroxide they behave like an acidic oxide (). To test an oxide, try it with dilute acid and, separately, with aqueous sodium hydroxide: reacts with both = amphoteric.
Key idea🔑 Key idea: Amphoteric = reacts with both acids and bases. The two to memorise: and ZnO. 🎯
Worked example
Worked Example: Find the Amphoteric Oxide
Worked Example: Both Ways 🔁
Which is amphoteric: aluminium oxide, calcium oxide, silicon(IV) oxide or oxygen?
- 1Amphoteric means it reacts with both acids and bases. Aluminium oxide does exactly that.
- 2Calcium oxide is basic (metal), silicon(IV) oxide is acidic (non-metal), and oxygen is an element, not an oxide.
So the answer is aluminium oxide. ✅
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9. Making Salts
To make a salt, first ask: does it dissolve? Use the six solubility rules with their named exceptions. All sodium, potassium and ammonium salts are soluble. All nitrates are soluble. Chlorides are soluble except lead(II) chloride and silver chloride. Sulfates are soluble except barium, calcium and lead(II) sulfates. Carbonates are insoluble except sodium, potassium and ammonium carbonates. Hydroxides are insoluble except sodium, potassium and ammonium hydroxides, and calcium hydroxide is partially soluble. For a soluble salt, react an acid with an excess of an insoluble solid (base, carbonate or metal), filter off the leftover, then crystallise; or if the base is soluble, use titration. For an insoluble salt, mix two solutions so it drops out as a precipitate, then filter, wash and dry.
Key idea🔑 Key idea: Soluble salt → excess solid (filter + crystallise) or titration. Insoluble salt → precipitation (filter, wash, dry). 🎯
Worked example
Worked Example: Making Copper(II) Sulfate
Worked Example: Choose the Solid 🟦
To make copper(II) sulfate, which solid do you react with dilute sulfuric acid: copper metal, copper(II) oxide, sodium chloride or copper(II) sulfate?
- 1The salt needs copper and sulfate, so use sulfuric acid. Copper metal won't react (too unreactive).
- 2Copper(II) oxide is a base that reacts: .
So you use copper(II) oxide (in excess), then filter and crystallise. ✅
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10. Water of Crystallisation
Some salt crystals have water chemically combined inside them: the water of crystallisation, shown after a dot in the formula. A hydrated substance is chemically combined with water; an anhydrous substance contains no water. The two to know: copper(II) sulfate, (hydrated blue, anhydrous white) and cobalt(II) chloride, (hydrated pink, anhydrous blue). Heating drives the water off; adding water back reverses the colour, which is why both anhydrous salts are tests for water. To find how many waters (), compare the moles of dry salt with the moles of water lost.
Key idea🔑 Key idea: Hydrated = water combined in the crystal; anhydrous = none. blue → white; pink → blue. Find from moles of salt vs moles of water. 💧
Worked example
Worked Example: How Many Waters?
Worked Example: Find x 🔢
25.0 g of is heated until all the water leaves, giving 16.0 g of . Find . (: , .)
- 1Moles of dry salt: mol. Water lost g, so mol.
- 2Ratio: .
So the formula is . ✅
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