Common anions — Cl⁻, Br⁻, I⁻, SO₄²⁻, NO₃⁻, CO₃²⁻ — confirmatory tests: Foundations

This page builds — from absolutely nothing — every word and squiggle the parent note uses. If you have never seen a superscript minus sign, start at line one and don't skip. By the end you will be able to read every equation in the parent note without pausing.

1. What an ion is (the invisible suspect)

Ordinary table salt is written $NaCl$ . When you dissolve it in water, it does not stay as tiny "NaCl" lumps. It splits into two separately charged particles.

Look at the figure: the sodium atom hands one electron to chlorine. Now sodium is short one negative, so it's $Na^{+}$ ; chlorine is holding one extra negative, so it's $Cl^{-}$ (the red particle — the anion, the star of this whole chapter).

Figure — Common anions — Cl⁻, Br⁻, I⁻, SO₄²⁻, NO₃⁻, CO₃²⁻ — confirmatory tests

$Na^{+}$ — "sodium plus", a cation.
$Cl^{-}$ — "chloride minus", an anion. The little $-$ means one extra electron.
$SO_{4}^{2 -}$ — the raised $2 -$ means two extra electrons of charge (a bigger negative). The subscript $4$ (small number below) means four oxygen atoms are bolted onto one sulphur.

Cation

A positively charged ion (lost electrons), written with a superscript

+

Anion

A negatively charged ion (gained electrons), written with a superscript

-

What does the subscript 4 in

SO_{4}^{2 -}

mean

Four oxygen atoms are bonded to the central sulphur

Raised small "

^{-}

" vs low full-size "

-

Same minus symbol; raised = a charge label on the particle, low = ordinary negative value

2. The six anions this topic hunts

These are the six "suspects" the parent note fingerprints. Learn to say each one:

Definition The six anions

Symbol	Name	In plain words
$Cl^{-}$	chloride	one chlorine, charge $- 1$
$Br^{-}$	bromide	one bromine, charge $- 1$
$I^{-}$	iodide	one iodine, charge $- 1$
$SO_{4}^{2 -}$	sulphate	sulphur + 4 oxygen, charge $- 2$
$NO_{3}^{-}$	nitrate	nitrogen + 3 oxygen, charge $- 1$
$CO_{3}^{2 -}$	carbonate	carbon + 3 oxygen, charge $- 2$

The first three ( $Cl^{-}, Br^{-}, I^{-}$ ) are a family called the halides — they behave alike because they are all one column of the periodic table, differing only in size. That size difference is the whole secret of section 1 of the parent note, and we build it in §7.

3. Chemical equations: arrows, plus signs, and the falling/rising arrows

A chemical equation is a sentence. Left side = what you start with (reactants); right side = what you end up with (products); the arrow means "turns into".

Example from the parent note: $Ag^{+} + X^{-} \to AgX ↓$ Read it out loud: "a silver cation plus a halide anion turn into solid silver-halide, which drops to the bottom as a precipitate." The $X$ is a placeholder — a stand-in letter meaning "any halide, pick Cl, Br, or I".

What does

↓

mean after a product

It forms a solid precipitate that settles out

What does

↑

mean after a product

It leaves as a gas

What does

⇌

mean

The reaction runs both directions at once and reaches a balance (equilibrium)

What does the state label (aq) mean

Aqueous — the species is dissolved in water as free-floating ions

What do (s), (l), (g) mean

Solid, pure liquid, gas respectively

What does X stand for in AgX

A placeholder for any halide — Cl, Br, or I

4. Precipitate — the solid "clue"

This is the single most-used word in the topic. Picture two clear liquids poured together; suddenly a cloud of solid appears and sinks. That solid is the precipitate.

Why does it form? Some ion-pairs "like" water and stay dissolved; others "grip each other" harder than water can pull them apart, so they clump into a solid. Whether a pair stays dissolved or precipitates is measured by a number called the solubility product, next.

5. Solubility, "insoluble", the mole, and $K_{s p}$

Why the "both ways at once" arrow. Even in a jar of solid $AgCl$ sitting in water, two things happen non-stop: a few ions leave the solid and dissolve, and a few dissolved ions rejoin the solid. When the two rates match, nothing appears to change — this steady balance is equilibrium, and that is exactly why we write the dissolving with the two-headed arrow: $AgCl (s) ⇌ Ag^{+} (aq) + Cl^{-} (aq)$

Intuition Where the

$K_{s p}$ formula comes from — the law of mass action There is a general rule of equilibrium called the law of mass action: at balance, if you multiply the concentrations of the products together, each raised to the power of how many of it the equation makes, you always get the same fixed number for that reaction. Why multiply, and why those powers? Because the rate of the rejoining reaction depends on ions meeting simultaneously — needing 2 of an ion means its crowding matters twice over, i.e. squared. Balancing that against the forward rate leaves this product-of-powers as a constant.

Now the key subtlety: pure solids and pure liquids are left out of the expression. A block of solid $AgCl$ has a fixed density — its "crowding" cannot change no matter how much solid sits there — so it contributes a constant that we simply fold into $K_{s p}$ . That is why only the dissolved (aq) ions appear.

Look at the parent note's table: $K_{s p}$ of $AgCl$ is $1.8 \times 1 0^{- 10}$ , but $AgI$ is $8.5 \times 1 0^{- 17}$ .

What is one mole

A fixed count of

6.02 \times 1 0^{23}

particles — like a "dozen" but for atoms/ions

Why count in moles at all

Reactions happen particle-by-particle, so we compare equal-sized bundles of particles

What do the square brackets

[Ag^{+}]

mean

The molarity — moles of silver ion per litre of solution

Why is molarity (per-litre crowding) the basis of

K_{s p}

Equilibrium depends on how often ions collide, and crowding-per-volume sets that collision rate

What is the law of mass action

At equilibrium, the product of product concentrations, each raised to its stoichiometric count, is a fixed constant

Why are pure solids left out of

K_{s p}

A pure solid has fixed density — its "crowding" cannot change — so it is a constant folded into

K_{s p}

; only (aq) ions vary

General

K_{s p}

for

M_{a} X_{b}

K_{s p} = [M^{+}]^{a} [X^{-}]^{b}

— each dissolved ion's molarity raised to its count

K_{s p}

expression for

Ag_{2} CrO_{4}

[Ag^{+}]^{2} [CrO_{4}^{2 -}]

(silver has power 2 because two appear)

A small

K_{s p}

means the salt is

Very insoluble (forms a strong precipitate)

Does

K_{s p}

change with temperature

Yes — it is fixed only at a given temperature and usually rises as temperature increases

Which is more insoluble, AgCl (

1 0^{- 10}

) or AgI (

1 0^{- 17}

)

AgI — its

K_{s p}

is far smaller

6. Acids, $H^{+}$ , and "acidify"

Many tests say "acidify first". You must know what an acid is as a particle.

Why does the topic care? Because some impostor precipitates (like $BaCO_{3}$ ) react with $H^{+}$ and dissolve away, while the true target ( $BaSO_{4}$ ) ignores acid. Acid is used as a filter to remove fakes. See Acid–Base Reactions of Salts.

$CO_{3}^{2 -} + 2 H^{+} \to H_{2} O + CO_{2} ↑$ Carbonate meets two hydrogen ions, and out comes water plus escaping carbon-dioxide gas — that's the "fizz" of section 4.

What does "acidify" mean

Add acid so the solution is full of

H^{+}

ions

What is

H^{+}

A hydrogen ion — a hydrogen atom that lost its electron, pure positive charge

Difference between dil. and conc.

dil. = watered-down/gentle; conc. = strong/undiluted

7. Size, polarisability, and "covalent character" (why Cl→Br→I differ)

The three halides sit in one periodic-table column. Going down — $Cl^{-} \to Br^{-} \to I^{-}$ — the ion gets bigger because it has more electron shells.

This same "bigger ion holds its electron more loosely" idea also explains ease of oxidation ( $I^{-} > Br^{-} > Cl^{-}$ ) — coming up in §8. Takeaway: one property, ion size, quietly drives colour, insolubility, and oxidation trends across the halides.

Going down

Cl^{-} \to Br^{-} \to I^{-}

, ion size does what

Increases (more electron shells)

Larger halide ion → its electrons are

More loosely held / more polarisable (softer)

What does "polarisable" (soft) mean

The ion's loosely-held outer electrons are easily pushed around / distorted

More covalent Ag–X bond means the salt is

Less soluble (smaller

K_{s p}

)

Which single ion property drives colour, insolubility and oxidation trends

Ion size (bigger down the group)

8. Oxidation, reduction, and electron swapping (redox)

The layer test and the brown-ring test are about electrons jumping between particles.

In $Cl_{2} + 2 Br^{-} \to 2 Cl^{-} + Br_{2}$ : each $Br^{-}$ loses its extra electron (oxidised) and becomes neutral bromine $Br_{2}$ ; the chlorine grabs those electrons (reduced) and becomes $Cl^{-}$ .

Oxidation is loss or gain of electrons

Loss of electrons

Reduction is loss or gain of electrons

Gain of electrons

Which halide is oxidised most easily and why

I^{-}

— biggest ion, loosest electron, gives it up readiest

9. Complex ions (the coordination "wrapping")

Two products in the parent note look scary: $[Ag(NH_{3})_{2}]^{+}$ and $[Fe(H_{2} O)_{5} NO]^{2 +}$ .

What do the outer square brackets in

[Ag(NH_{3})_{2}]^{+}

enclose

The whole complex ion — central metal plus everything wrapped around it

Colour of the complex

[Fe(H_{2} O)_{5} NO]^{2 +}

Brown (the brown-ring signal)

10. Prerequisite map

Every box on the left must be solid before the tests in the parent topic make sense. Related deep-dives: Group Analysis of Cations uses the exact same precipitation logic for the opposite charge.

Equipment checklist

Reveal each — if you can answer without peeking, you are ready for the parent note.

I can read the charge and atom-count in

SO_{4}^{2 -}

Sulphur + 4 oxygens, net charge

- 2

(superscript = charge, subscript = atom count)

I know what

↓

and

↑

mean in an equation

↓

= solid precipitate forms;

↑

= gas escapes

I know what the

⇌

arrow means

The reaction runs both ways at once and reaches a balance (equilibrium)

I know what the state label (aq) means

Aqueous — dissolved in water as free-floating ions

I can say what a precipitate is

A solid that forms and settles when two dissolved ions combine and cannot stay dissolved

I know what a mole and molarity are

A mole =

6.02 \times 1 0^{23}

particles; molarity = moles of ion per litre

I know what a small

K_{s p}

tells me

The salt is very insoluble; few ions escape into solution

I can write the general

K_{s p}

for

M_{a} X_{b}

K_{s p} = [M^{+}]^{a} [X^{-}]^{b}

— each dissolved ion's molarity to the power of its count

I know why solids are omitted from

K_{s p}

A pure solid has constant density; only variable (aq) concentrations appear

I know whether

K_{s p}

depends on temperature

Yes — it holds only at a fixed temperature and usually rises as temperature increases

I can explain why

I^{-}

is oxidised more easily than

Cl^{-}

I^{-}

is larger, holds its outer electron loosely, so it lets go (loses the electron) more readily

I know what "acidify with dil. HNO₃" does

Fills the solution with

H^{+}

using gentle acid, destroying impostor ions without adding a halide

I can read

[Ag(NH_{3})_{2}]^{+}

A complex ion: one

Ag^{+}

wrapped by two ammonia molecules, overall charge

+ 1

I know oxidation vs reduction

Oxidation = loss of electrons; Reduction = gain of electrons