Intuition The ONE core ideaEvery compound in this topic is built by moving small things — water, CO₂, electrons, or heat — into or out of a cheap starting rock or brine . If you can read a chemical formula as a recipe of atoms and know what "add water", "add electrons", or "heat" each do, then the whole family tree (NaOH, Na₂CO₃, NaHCO₃, CaO, gypsum, PoP) becomes one story instead of twenty facts.
This page assumes nothing . Before you touch the parent note 3.1.9 (parent topic) , we build every symbol it uses, one at a time, each on top of the last.
Definition Formula = a headcount of atomsA formula like N a C l shopping list of atoms glued together . The letters are element abbreviations (Na = sodium, Cl = chlorine). A small number after a letter counts how many of that atom: H 2 O
Look at the first figure. Each coloured ball is one atom; the formula underneath is just the count of the balls.
Intuition Why we need counts at allChemistry is bookkeeping. In any reaction atoms are never created or destroyed — they only get rearranged. So we must count them, or we cannot tell if a recipe is balanced. This single habit is what the parent note's arrows (→
The dot in a formula, e.g. C a S O 4 ⋅ 2 H 2 O
Definition The dot means "loosely attached water"C a S O 4 ⋅ 2 H 2 O C a S O 4 carrying two water molecules." The ⋅ water of crystallisation : water tucked into the crystal, easy to add or bake off. This dot is the whole gypsum → plaster-of-Paris story.
Definition Charge, and the tiny superscript that shows itAtoms contain negative particles called electrons . Lose one and the atom is left positive ; gain one and it is negative . We write the leftover charge as a small raised sign:
N a + lost one electron (net + 1 C l − gained one electron (net − 1 C a 2 + two (net + 2
Intuition Why ions matter hereSalts like N a C l only because + − . Drop N a C l dissociation ) is why brine can conduct electricity and why electrolysis works.
Definition Polyatomic ions — a whole group carrying one chargeSome charged pieces are groups of atoms that travel together:
O H − − 1 base .C O 3 2 − − 2 H C O 3 − H + − 1 S O 4 2 − − 2
Notice H C O 3 − C O 3 2 − plus one H + . That "add a proton" step is exactly how the parent turns carbonate into bicarbonate.
Definition The two opposites
An acid releases H +
A base releases O H − H + N a O H
H + O H − H + + O H − → H 2 O neutralisation .
Intuition Why "acidic oxide" appears in the parentGases like C O 2 C O 2 + H 2 O → C O 2 acidic oxide . When the parent writes "base neutralises acidic oxide" for 2 N a O H + C O 2 → N a 2 C O 3 + H 2 O N a O H C O 2
Mnemonic Strong vs weak, mild vs harshN a O H strong, harsh base (soap-maker). N a H C O 3 mild, edible base (baking soda, antacid). Same family, gentler cousin — because bicarbonate only holds one loose O H −
Definition What each arrow means
→ ⇌ equilibrium " — the reaction runs both ways and settles into a balance. (See Le Chatelier's Principle .)Δ heat Δ apply heat ".↑ leaves as a gas .↓ falls out as a solid (a precipitate ).
Intuition Why up-arrows and down-arrows drive the whole topicIn an equilibrium, if a product escapes (gas ↑ drops out (solid ↓ secret engine of both the Solvay process (N a H C O 3 ↓ C O 2 ↑ Le Chatelier's Principle .
Definition Coefficients vs subscriptsIn 2 N a H C O 3 → N a 2 C O 3 + H 2 O + C O 2
the small 3 N a H C O 3 subscript (counts atoms within one molecule),
the big 2 in front is a coefficient (counts whole molecules ).
Coefficients are the only thing we may change to balance. Count each atom on both sides — they must match, because atoms are conserved.
Definition The two rods and what they doPush electricity through a liquid full of ions and you force a reaction. The two rods are:
Cathode = the rod electrons flow into the liquid from; it gives electrons → things there get reduced (gain e − Anode = the rod electrons flow out to ; things there get oxidised (lose e −
Mnemonic: Red Cat (Reduction at Cathode), An Ox (Oxidation at Anode).
2 H 2 O + 2 e − → H 2 + 2 O H − N a + + e − grabs them most easily wins. Water grabs electrons more readily than N a + + 1 H 2 O H − O H − N a + N a O H electrode potentials — see Electrolysis and Electrode Potentials . The e −
Definition A salt reactingwith water
Hydrolysis = a dissolved ion pulling water apart. The anion of a weak acid grabs an H + O H − basic :
C O 3 2 − + H 2 O ⇌ H C O 3 − + O H − O H − Salt Hydrolysis and pH .
Balancing conserves atoms
Ions plus and minus charges
Electrolysis Red Cat An Ox
Reaction arrows gas up solid down
Escape or precipitate drives equilibrium
Hydrolysis makes solutions basic
Dot means water of crystallisation
Gypsum to Plaster of Paris
Read this map top-to-bottom: atom-counting and charge are the roots; from charge grow dissociation, electrolysis, and acid-base behaviour; the arrow rules and the water-dot feed the two great engines (Solvay/lime and PoP).
Test yourself — cover the right side. If any answer is shaky, re-read that section before the parent note.
What does the small number after a letter in a formula count (e.g. the 2 in H 2 O The number of that atom inside one molecule (a subscript).
What does the dot mean in C a S O 4 ⋅ 2 H 2 O Water of crystallisation — loosely held water tucked in the crystal, addable/removable.
What is N a + A sodium atom that lost one electron, leaving a net charge of + 1
Difference between C O 3 2 − H C O 3 − Bicarbonate is carbonate plus one extra H + − 2 − 1
What is a base, in one line? Something that releases O H − H +
Why is C O 2 It dissolves in water to form a weak acid, so a base neutralises it.
What does ⇌ → Two-way equilibrium (settles into balance) vs one-way "turns into".
What do the ↑ ↓ Product leaves as gas (↑ ↓
Why does a product escaping or precipitating push a reaction forward? It can't return to reverse the reaction, so the equilibrium keeps shifting to replace it (Le Chatelier).
"Red Cat, An Ox" — decode it. Reduction happens at the Cathode; Oxidation happens at the Anode.
In the chlor-alkali cathode, why is water reduced instead of N a + Water grabs electrons more easily than N a + N a +
Why is a N a 2 C O 3 The C O 3 2 − O H −
Coefficient vs subscript? Coefficient (big, in front) counts whole molecules; subscript (small, inside) counts atoms within one molecule.
