Chemistry

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Matter, separation methods and atomic structure

⏱ 5 min read🧩 6 question types🎯 18 practice Q

The idea in one minute

Matter is anything with mass that takes up space. This lesson covers its states, how to separate mixtures, and what an atom is made of. It ends with the mole and two gas laws. Most questions are fact recall, plus one or two small calculations.

States of matter

Matter is anything that has mass and takes up space. It has five states.

Solid, liquid and gas are the three everyday states.
Plasma is the fourth state. It is ionised gas, found in the Sun, stars, lightning and neon signs.
Bose-Einstein condensate is the fifth state. Matter is cooled almost to absolute zero. S.N. Bose and Einstein predicted it.

Sublimation is a solid turning straight into vapour. Five substances are the exam favourites: camphor, naphthalene balls, iodine, ammonium chloride and dry ice (solid CO₂).

Physical and chemical change

A physical change alters only the form. It is reversible, and no new substance forms. Melting ice and dissolving sugar are examples.

A chemical change makes a new substance. Burning, rusting, curdling of milk, cooking and digestion are examples.

Tip: Ask one question: is a new substance made? Yes means chemical. No means physical.

Separating mixtures

A mixture keeps the properties of its parts. Match the method to the job.

Method	Use
Chromatography	Dyes in ink, pigments, drugs in blood
Distillation	Liquids with very different boiling points; pure water
Fractional distillation	Petroleum parts, liquefied air
Centrifugation	Cream from milk, blood tests
Sublimation	Salt mixed with camphor or ammonium chloride
Crystallisation	Pure salt or copper sulphate from a solution
Separating funnel	Immiscible liquids such as oil and water
Filtration, evaporation, magnet	Insoluble solid, dissolved solid, iron filings from sand

Solutions and colloids

A solution (sugar in water) is clear and never settles. A suspension (muddy water) is cloudy and settles down.

A colloid lies in between. Its particles are too small to see. They still scatter a light beam. This is the Tyndall effect. The particles also zig-zag constantly. This is Brownian motion.

Milk is an emulsion (liquid in liquid).
Fog and clouds are an aerosol (liquid in gas).
Smoke is a solid in gas.
Shaving cream is a foam. Jelly is a gel.

Watch: A true solution does not show the Tyndall effect. Only colloids and suspensions do.

Inside the atom

Particle	Found by	Charge
Electron	J.J. Thomson (1897, cathode rays)	-1
Proton	Goldstein (canal rays); identified by Rutherford	+1
Neutron	James Chadwick (1932)	0

The atomic models came in this order.

Dalton: atoms cannot be split.
Thomson: plum-pudding model.
Rutherford: gold-foil experiment showed a tiny, dense nucleus.
Bohr: electrons move in fixed orbits (shells).

Atomic number, isotopes and shells

Atomic number Z is the number of protons. Mass number A is protons plus neutrons.

Isotopes: same Z, different A. Hydrogen has protium (¹H), deuterium (²H) and tritium (³H). Tritium is radioactive.
Chlorine is ³⁵Cl and ³⁷Cl. Its average mass is 35.5. Carbon-14 is used to date old remains.
Isobars: same A, different Z. Argon-40 and calcium-40 are an example.
Isotones: same number of neutrons.

Shell n holds at most $2n^{2}$ electrons. So K holds 2, L holds 8 and M holds 18. The outer electrons are the valence electrons.

The mole and the gas laws

One mole holds $6.022 \times 10^{23}$ particles. This is Avogadro's number. One mole weighs its molar mass in grams. So 18 g of water is 1 mole, and 44 g of CO₂ is 1 mole.

Boyle's law: at fixed temperature, $P_1V_1 = P_2V_2$ .
Charles's law: at fixed pressure, volume rises with kelvin temperature.

Rule: In Charles's law, always change °C to kelvin first. Add 273.

Question types you will see

Each type: how to recognise it, the method step by step, and one question to try.

Type 1very common3 practice Q

Physical vs chemical change; sublimation and states

How to spot it:

The question asks 'which is a chemical change', 'which sublimes', the 4th or 5th state, or 'dry ice is'.

Method

Ask: is a new substance formed?
Yes: chemical (burning, rusting, curdling, cooking, digestion). No: physical.
For sublimation, pick from camphor, naphthalene, iodine, ammonium chloride, dry ice.
For states: plasma is ionised gas; BEC is near absolute zero.

Why it works:

The two change types differ only in whether a new substance appears.

Try this

Burning of a candle involves which change?

Show solution

Melting of wax is physical.
Burning of the vapour is chemical.

Answer

Both physical and chemical changes

Type 2very common3 practice Q

Separation method and mixture

How to spot it:

The question asks 'which method separates cream from milk, petroleum parts, or dyes in ink', or gives a match-the-column.

Method

Find the property that differs: boiling point, density, solubility, magnetism.
Close boiling points: fractional distillation. Different densities: centrifugation.
Immiscible liquids: separating funnel. Dyes: chromatography.
Cream from milk is always centrifugation.

Why it works:

Each method uses one physical property that differs between the parts.

Try this

The components of petroleum are separated by which method?

Show solution

The parts boil only a few degrees apart.
A fractionating column is needed.

Answer

Fractional distillation

Type 3common2 practice Q

Colloids, Tyndall effect and Brownian motion

How to spot it:

The question says 'milk is an example of', 'scattering of light by particles is called', or 'which shows the Tyndall effect'.

Method

Name the dispersed phase and the medium.
Liquid in liquid is an emulsion; liquid in gas is an aerosol; solid in gas is smoke.
Light scattering is the Tyndall effect. Zig-zag motion is Brownian motion.
A true solution never shows the Tyndall effect.

Why it works:

Colloid particles are big enough to scatter light but too small to settle.

Try this

Fog is a colloid in which what is spread in what?

Show solution

Water droplets are liquid.
Air is the gas medium.

Answer

Liquid in gas, an aerosol

Type 4common2 practice Q

Atomic models and discoverers

How to spot it:

The question asks who proposed the plum-pudding, nuclear or orbit model, or which experiment found the nucleus.

Method

Recall the order: Dalton, Thomson, Rutherford, Bohr, Chadwick.
Thomson: electron and plum pudding. Rutherford: nucleus. Bohr: orbits.
In the gold-foil test most alpha particles passed straight through. A few bounced back.

Why it works:

Each scientist added one part to the picture of the atom.

Try this

Rutherford's gold-foil experiment proved what?

Show solution

Most alpha particles went straight through.
A few bounced back.

Answer

The atom has a tiny, dense, positive nucleus

Type 5common3 practice Q

Isotopes, isobars, atomic number and shells

How to spot it:

The question says 'same atomic number, different mass number', asks the hydrogen isotopes, or asks electrons in a shell.

Z=\text{protons},\quad A=\text{protons}+\text{neutrons},\quad \text{shell }n\text{ holds }2n^2

Method

Same Z, different A: isotopes. Same A, different Z: isobars.
Hydrogen isotopes: ¹H, ²H, ³H. Chlorine: ³⁵Cl and ³⁷Cl, average 35.5.
Shell n holds 2n² electrons. Calcium (Z = 20) is 2, 8, 8, 2.

Why it works:

The atomic number sets the element. The mass number only counts the heavier particles.

Try this

How many electrons can the M shell hold at most?

Show solution

M is the third shell, so n = 3.
2n² = 2 × 9 = 18.

Answer

Type 6occasional3 practice Q

Mole and gas-law numericals

How to spot it:

You get a mass and must find moles or molecules, or a Boyle or Charles change of volume, pressure or temperature.

\text{moles}=\frac{\text{mass}}{\text{molar mass}},\quad P_1V_1=P_2V_2\ (\text{at fixed }T),\quad \frac{V_1}{T_1}=\frac{V_2}{T_2}\ (\text{at fixed }P)

Method

Find the molar mass first: H₂O is 18, CO₂ is 44.
Moles = given mass ÷ molar mass.
Boyle: equate the two products P × V.
Charles: convert to kelvin, then equate V ÷ T.

Why it works:

A mole is a counting unit, and the gas laws are simple proportions.

Try this

A gas takes 600 mL at 1 atm. What pressure squeezes it into 400 mL at the same temperature?

Show solution

Boyle: P₁V₁ = P₂V₂.
1 × 600 = P₂ × 400.
P₂ = 600 ÷ 400 = 1.5.

Answer

1.5 atm

Formula sheet

Moles from mass

n = \frac{m}{M}

M = molar mass in g/mol

Number of particles

N = n \times N_A,\quad N_A = 6.022 \times 10^{23}

Avogadro number

Boyle's law

P_1V_1 = P_2V_2

temperature constant

Charles's law

\frac{V_1}{T_1} = \frac{V_2}{T_2}

pressure constant, T in kelvin

Ideal gas equation

PV = nRT

R = 8.314 J mol⁻¹ K⁻¹

Maximum electrons in a shell

2n^{2}

K = 2, L = 8, M = 18

Shortcuts that save time

⚡ Iso-words by the letter

Isotopes share the atomic number (P for protons). Isobars share A, the mass number. Isotones share neutrons.

Example

Deuterium and tritium are what to each other?

Show solution

Both are hydrogen, so atomic number is 1.
Mass numbers differ: 2 and 3.

Answer

Isotopes

⚡ Sublimation list: CNIAD

Camphor, Naphthalene, Iodine, Ammonium chloride, Dry ice. If an option is on this list, it sublimes.

Mistakes to avoid

Where most students lose marks on this subtopic.

Mistake 01

Calling milk a true solution.

Milk is a colloid (an emulsion). It scatters light.

Mistake 02

Saying tritium is stable.

Tritium is the radioactive isotope of hydrogen.

Mistake 03

Treating rusting or curdling as a physical change.

Both make new substances, so both are chemical changes.

Mistake 04

Using Celsius in Charles's law.

Add 273 to get kelvin before you divide.

Mistake 05

Using distillation for petroleum.

Its parts boil close together. Use fractional distillation.

Quick revision

Read this the night before the exam.

Five states: solid, liquid, gas, plasma, Bose-Einstein condensate.
Sublimation: camphor, naphthalene, iodine, ammonium chloride, dry ice.
Chromatography for dyes; fractional distillation for petroleum; centrifuge for cream; funnel for oil and water.
Colloid shows Tyndall effect and Brownian motion. Milk is an emulsion. Fog is an aerosol.
Electron: Thomson. Nucleus: Rutherford. Orbits: Bohr. Neutron: Chadwick.
Shell capacity is 2n². K = 2, L = 8, M = 18.
One mole = 6.022 × 10²³ particles. 18 g of water = 1 mole. PV = nRT.

Practice: 18 questions

Sets of 10, mixed across the question types above. Every answer has a step-by-step explanation.

Topic test · 10 questions

Suggested time 3 min · wrong answers go to your mistake notebook automatically.

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