Acids, Bases and Salts: Reactions, Indicators and Making Salts
H⁺ Energy: The Chemistry of Sour ⚗️
Introduction
1. Introduction
Okay, acids and bases sound intense, but honestly this whole chapter runs on one cheat code: an acid is just something that dumps hydrogen ions () into water. That's it. Every fizz, every colour change, every salt recipe traces back to that one ion.
Bases are the opposite vibe: they cancel acids out. And a base that dissolves in water (an alkali) drops hydroxide ions (). Acid meets alkali, they neutralise, you get water. Lock that in and the reactions, the pH scale, the oxides, and the salt-making recipes all click into place. This is your fast refresh, not the full textbook: one key idea per topic, one worked example, and exactly the move the marker wants. Let's get into it. ⚗️
Bases are the opposite vibe: they cancel acids out. And a base that dissolves in water (an alkali) drops hydroxide ions (). Acid meets alkali, they neutralise, you get water. Lock that in and the reactions, the pH scale, the oxides, and the salt-making recipes all click into place. This is your fast refresh, not the full textbook: one key idea per topic, one worked example, and exactly the move the marker wants. Let's get into it. ⚗️
2. Acids Are All About H⁺
Here's the thing that ties the whole chapter together: every single acid, no matter how fancy its name, releases the same ion in water, the hydrogen ion . Hydrochloric, sulfuric, nitric, all of them. That's why they all behave the same way. Alkalis are the mirror image: they release . When meets , they join up to make plain water, and that's neutralisation.
Key idea🔑 Key idea: Acids release ; alkalis release . They neutralise to water: . 💧
Worked example
Worked Example: The Ion in Every Acid
Worked Example: Spot What They Share 🔎
All acids in water contain the same ion. What's its formula: , , or ?
- 1Acids are defined by one ion: the hydrogen ion. Different acids have different other bits (HCl has , sulfuric has ), but they all share .
- 2So the ion in every acid is . ( is the alkali one, don't get baited.)
The thing they share is always the mark. ✅
---
Answer
3. The Three Reactions of Acids
Acids do three classic reactions, and each one makes a salt plus something extra. With a metal you get salt + hydrogen (it pops with a lighted splint). With a base (a metal oxide or hydroxide) you get salt + water. With a carbonate you get salt + water + carbon dioxide (the turns limewater milky). The acid decides the salt's surname: hydrochloric → chloride, sulfuric → sulfate, nitric → nitrate.
Key idea🔑 Key idea: Acid + metal → salt + hydrogen. Acid + base → salt + water. Acid + carbonate → salt + water + carbon dioxide. 🧂
Worked example
Worked Example: Carbonate Plus Acid
Worked Example: Name the Three Products 🫧
Magnesium carbonate reacts with nitric acid. What are the three products?
- 1Use the pattern: acid + carbonate → salt + water + carbon dioxide.
- 2Metal is magnesium, acid is nitric → the salt is magnesium nitrate. So the three products are magnesium nitrate, carbon dioxide and water.
Carbonates give carbon dioxide, not hydrogen, that's the trap. 🫧
---
Answer
4. Indicators and the pH Scale
Indicators are dyes that change colour to rat out whether something is acid or alkali. Three to memorise: litmus (red in acid, blue in alkali), thymolphthalein (colourless in acid, blue in alkali), and methyl orange (red in acid, yellow in alkali). For an actual number, universal indicator gives you the pH scale from 0 to 14: below 7 acidic, exactly 7 neutral (green), above 7 alkaline. Lower pH = more = more acidic.
Key idea🔑 Key idea: Litmus red/blue, thymolphthalein colourless/blue, methyl orange red/yellow. pH 0–14: under 7 acid, 7 neutral, over 7 alkali. 🌈
Worked example
Worked Example: Two Indicators in Alkali
Worked Example: Pick the Right Column 🎨
Thymolphthalein is colourless in acid and blue in alkali. Methyl orange is red in acid and yellow in alkali. Both are added to sodium hydroxide. What colour is each?
- 1Sodium hydroxide is an alkali, so read the "in alkali" answers.
- 2Thymolphthalein → blue. Methyl orange → yellow.
Reminder: thymolphthalein is colourless in acid, not red, that's the classic bait. 🎨
---
5. Base vs Alkali
People mix these up constantly. A base is any metal oxide or hydroxide that neutralises an acid. An alkali is just a base that dissolves in water (and then gives ions). So every alkali is a base, but not every base is an alkali, because plenty of bases (like copper(II) oxide) don't dissolve. The deciding factor is literally: does it dissolve in water?
Key idea🔑 Key idea: Every alkali is a base, but only the soluble bases are alkalis. Insoluble bases (like CuO) are bases but not alkalis. 🧠
Worked example
Worked Example: Which Base Is an Alkali
Worked Example: Does It Dissolve? 💦
Which of these bases is also an alkali: sodium hydroxide, copper(II) oxide, iron(III) oxide or magnesium oxide?
- 1Alkali = a base that dissolves in water. So ask which one dissolves.
- 2Sodium hydroxide dissolves to give an alkaline solution → it's the alkali. The other three are insoluble metal oxides → bases only.
Solubility is the whole deciding factor. ✅
---
Answer
6. Sorting Oxides
Oxides split into two camps, and one question decides which: is the element a metal or a non-metal? Metal oxides are basic (they react with acids), like , CaO, CuO. Non-metal oxides are acidic (they react with alkalis), like , , . That's the whole rule: metal → basic, non-metal → acidic.
Key idea🔑 Key idea: Metal oxide = basic. Non-metal oxide = acidic. One question (metal or not?) sorts them. ⚖️
Worked example
Worked Example: Classify Phosphorus Oxide
Worked Example: Check the Element First 🧪
Is phosphorus(V) oxide an acidic or a basic oxide, and why?
- 1Is phosphorus a metal or a non-metal? It's a non-metal.
- 2Non-metal oxides are acidic. So phosphorus(V) oxide is acidic, because phosphorus is a non-metal.
The "why" is the mark: tie it back to metal-vs-non-metal. ✅
---
7. Solubility Rules
Before you can make a salt, you've got to know if it dissolves, because that picks the method. The syllabus gives six rules, each with named exceptions: (a) all sodium, potassium and ammonium salts are soluble; (b) all nitrates are soluble; (c) chlorides are soluble, except lead and silver chlorides; (d) sulfates are soluble, except barium, calcium and lead sulfates; (e) carbonates are insoluble, except sodium, potassium and ammonium carbonates; (f) hydroxides are insoluble, except sodium, potassium and ammonium hydroxides, and calcium hydroxide, which is partially soluble. Check both parts of the name: rules (a) and (b) win over everything.
Key idea🔑 Key idea: Na, K, NH₄ salts and all nitrates: soluble. Chlorides soluble (not Pb, Ag). Sulfates soluble (not Ba, Ca, Pb). Carbonates and hydroxides insoluble (not Na, K, NH₄; calcium hydroxide partially soluble). 🌊
Worked example
Worked Example: The All-Insoluble Set
Worked Example: Eliminate the Soluble Ones ❌
Which set is all insoluble? (a) calcium sulfate, magnesium hydroxide, silver chloride; (b) ammonium carbonate, silver chloride, sodium sulfate; (c) barium sulfate, calcium chloride, magnesium hydroxide; (d) calcium nitrate, magnesium carbonate, lead(II) hydroxide.
- 1Cross off any set with a soluble compound: (b) has ammonium carbonate and sodium sulfate (rule a), (c) has calcium chloride (chlorides are soluble except lead and silver), (d) has calcium nitrate (rule b). All out.
- 2That leaves (a) — calcium sulfate, magnesium hydroxide and silver chloride are all insoluble.
Eliminate the soluble ones and the answer falls out. ❌
---
Answer
8. Making Soluble Salts
For a salt that dissolves, you react an acid with the right thing, get rid of any leftover, then crystallise. If the other reactant is an insoluble base, carbonate or metal, use the excess solid method: pile in extra solid so all the acid is used up, filter off the leftover, then evaporate to the point of crystallisation. If the other reactant is a soluble alkali, you can't filter an excess, so you titrate to find the exact volume, then redo it with no indicator and crystallise.
Key idea🔑 Key idea: Insoluble reactant → excess solid, filter, crystallise. Soluble alkali → titrate, repeat with no indicator, crystallise. 🧫
Worked example
Worked Example: Making Copper(II) Sulfate
Worked Example: Why Copper Metal Won't Do 🟦
Copper is less reactive than hydrogen, so it doesn't react with dilute acids. Which pair of reactants makes copper(II) sulfate?
- 1Copper metal is a no-go. Use a copper carbonate (or copper oxide) with sulfuric acid (acid + metal carbonate):
- 2Add it in excess, filter off the leftover, evaporate to the point of crystallisation, cool, and dry the blue crystals.
Excess insoluble solid = all the acid used up, easy to filter out. 🟦
---
9. The Titration Route
An alkali such as sodium hydroxide dissolves, so any excess would stay in solution with the salt and you couldn't filter it off (same for soluble carbonates like sodium or potassium carbonate). So you find the exact volumes that react by titration: pipette 25.0 cm³ of alkali into a conical flask, add a few drops of indicator, and add acid from a burette until the colour just changes (the end-point). Volume added = final reading − starting reading. Then repeat with the same volumes and no indicator, heat to the point of crystallisation, cool, filter off the crystals and dry them. This is how sodium, potassium and ammonium salts are made.
Key idea🔑 Key idea: Soluble alkali or carbonate → titrate to find the exact volumes, repeat without indicator, evaporate to the point of crystallisation, cool, filter, dry. 🥅
Worked example
Worked Example: Potassium Sulfate by Titration
Worked Example: Same Volumes, No Indicator 🧼
25.0 cm³ of aqueous potassium carbonate reacts exactly with 14.6 cm³ of dilute sulfuric acid, using methyl orange. Why can't you just add excess potassium carbonate and filter, and how do you now get pure, dry potassium sulfate crystals?
- 1Potassium carbonate is soluble, so any excess would stay dissolved and couldn't be filtered off.
- 2Mix 25.0 cm³ of the carbonate with 14.6 cm³ of acid again, with no indicator (it would colour the crystals); warm until crystals start to form, cool, then filter and dry the crystals.
Key phrases: "same volumes without indicator" and "evaporate to the point of crystallisation". 🧼
---
10. Hydrated vs Anhydrous
Learn two definitions word for word. A hydrated substance is a substance that is chemically combined with water. An anhydrous substance is a substance containing no water. Don't mix them up with aqueous, which just means dissolved in water. Hydrated copper(II) sulfate is blue; heating drives the water off and leaves white anhydrous copper(II) sulfate. Add water to the white powder and it turns blue again. That colour flip is actually the test for water.
Key idea🔑 Key idea: Hydrated = chemically combined with water (blue copper(II) sulfate). Anhydrous = containing no water (white copper(II) sulfate). Aqueous = dissolved in water. 💎
Worked example
Worked Example: The Three Forms of Copper Sulfate
Worked Example: Match Each Word 🔵
Copper(II) sulfate exists as aqueous, hydrated and anhydrous. Match each word to its meaning.
- 1Aqueous = dissolved in water (a solution).
- 2Hydrated = a solid chemically combined with water → blue crystals. Anhydrous = a solid containing no water → white powder.
So: aqueous = dissolved, hydrated = solid + water, anhydrous = dry solid. "Hydrated" means water inside a solid crystal, not just floating in solution. 🔵
---
Practice this in the app
Unlock the full chapter: practice questions, flashcards, mock papers and notes, free.
Continue revising