Solids, liquids and gases, melting, condensation and sublimation, elements, compounds and mixtures, air and sea water, solutions and colloids, and how mixtures are separated.
Salt pans at Marakkanam, Tamil Nadu. Sea water is let into shallow pans and the Sun evaporates the water. Common salt is left behind. Sea water is a mixture; this is the oldest way to separate it. Image: Dey.sandip, Wikimedia Commons · CC BY 3.0
Anything that has mass and takes up space is matter — air, water, a stone, our body. Matter is made of tiny particles that keep moving. How close the particles are decides whether it is a solid, a liquid or a gas. Exams ask about changes of state, which things are mixtures, and the right way to separate them.
On this page you will learn:
The three states of matter, and why ice floats
Melting, boiling, condensation and sublimation
Elements, compounds and mixtures: air, sea water, iodised salt
Solutions, colloids and suspensions
Separation methods; physical and chemical changes
78%
Nitrogen in air
By volume; asked in PSSSB Patwari 2023
0 °C
Ice melts
= 273 K
100 °C
Water boils
= 373 K, at normal air pressure
States of matter
Rule in one line: solids keep shape and volume, liquids keep only volume, gases keep neither.
SOLID, LIQUID AND GAS
Solid
Liquid
Gas
Shape
Fixed
Of the container
Fills container
Volume
Fixed
Fixed
Not fixed
Can be pressed
Almost not
Very little
Easily
Particles
Packed tight
Loose, slide
Far apart, fast
Gases can be pressed into a small space. That is why CNG and LPG fit into cylinders. Gases also spread (diffuse) fastest; solids slowest.
KEY TERM
Water expands when it freezes
Most liquids shrink when they become solid. Water is different. When it freezes, its particles lock into an open pattern with gaps. So ice takes more space than the same water. Ice is lighter (less dense) than water and floats. Water pipes can burst on very cold nights for this reason.
Water cools→Freezes at 0 °C→Ice takes more space→Ice floats
QUICK FACT
Besides solid, liquid and gas, matter can be plasma — the glowing gas inside stars and neon signs — or a Bose–Einstein condensate, a very cold state named after the Indian scientist Satyendra Nath Bose and Albert Einstein.
QUICK CHECK
PYQ · PPSC OHE 2016 — Which word completes the sentence: Water __________ when it freezes?
Tap an option to answer
Answer: A — Expands
Ice takes about 9 percent more space than the water it came from. That is why ice floats.
Changing state
Rule in one line: heating or cooling (and pressure) moves matter from one state to another.
Change
From → to
Example
Melting
Solid → liquid
Ice melts at 0 °C
Freezing
Liquid → solid
Water turns to ice
Boiling
Liquid → gas, all through the liquid
Water boils at 100 °C
Evaporation
Liquid → gas, only at the surface, at any temperature
Wet clothes dry
Condensation
Gas → liquid
Dew; drops outside a cold glass
Sublimation
Solid → gas directly
Camphor, naphthalene balls, dry ice
Deposition
Gas → solid directly
Frost on very cold nights
Changes of state. To change °C into kelvin, add 273: 27 °C = 300 K.
Dry ice is solid carbon dioxide. It turns straight into gas without melting — this is sublimation. The white fog is water vapour of the air condensing in the cold gas. Image: Sarathtly, Wikimedia Commons · Public domain
WHY EVAPORATION COOLS
Evaporating particles take heat from their surroundings. So the place they leave becomes cooler. Sweat cools the body, and water in an earthen pot (ghara) stays cool. Evaporation is faster with a larger surface, higher temperature, dry air and wind.
QUICK CHECK
PYQ · PPSC SEVE 2016 — When a gas turns into a liquid, the process is called:
Tap an option to answer
Answer: A — condensation
Gas to liquid is condensation, as when dew forms. Evaporation is the reverse; sublimation is solid to gas.
Elements, compounds and mixtures
Rule in one line: a compound has a fixed make-up and new properties; a mixture does not.
SORTING MATTER
Matter
Anything with mass that takes up space
↓
Pure substances
Elements (iron, gold, sulphur, diamond) and compounds (water, salt)
↓
Mixtures
Uniform: air, sea water, brass. Not uniform: soil, sand in water
COMPOUND VS MIXTURE
Compound
Mixture
Make-up
Fixed ratio
Any ratio
Properties
New ones
Parts keep theirs
Separated by
Chemical means only
Physical methods
Example
Water (H2O)
Air, iodised salt
Iron filings and sulphur powder make a mixture — a magnet pulls the iron out. Heated together they form iron sulphide, a compound that a magnet cannot separate.
KEY TERM
Iodised salt
Common salt (sodium chloride) with a very small amount of potassium iodide (or potassium iodate) mixed in. The two do not react; they are only mixed. So iodised salt is a mixture. The iodine prevents goitre.
QUICK CHECK
PYQ · PPSC OHE 2016 — Which of the following is a mixture?
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Answer: A — Air
Air is a mixture of nitrogen, oxygen, argon, carbon dioxide and other gases. Water is a compound; gold and sulphur are elements.
QUICK CHECK
PYQ · PPSC OHE 2016 — Which of the following is not a mixture?
Tap an option to answer
Answer: D — Diamond
Diamond is pure carbon, an element. Air is a mixture of gases, sea water has salts dissolved in water, and soil is a mixture of many things.
QUICK CHECK
PYQ · PPSC EO 2018 — Iodised salt is a:
Tap an option to answer
Answer: A — Mixture of potassium iodide and common salt
A small amount of potassium iodide is simply mixed into common salt; no new compound forms. PPSC SSO 2018 asked the same question.
Air: a mixture of gases
Rule in one line: the share of each gas in air is counted by volume.
Gases in dry air, by volume
Nitrogen
78%
Oxygen
21%
Argon
0.93%
Carbon dioxide
0.04%
Air also holds water vapour, dust and tiny amounts of neon, helium and other gases. Because the share can change and the gases keep their own properties, air is a mixture.
QUICK CHECK
PYQ · PSSSB Patwari 2023 — What is the percentage of nitrogen in the atmosphere?
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Answer: B — 78%
About 78 percent of dry air is nitrogen. 21 percent is the share of oxygen.
QUICK CHECK
PYQ · PPSC ICS 2016 — Nitrogen constitutes about 78% of the atmosphere
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Answer: C — By volume
Air's make-up is given by volume. By mass, nitrogen is a little less, about 75 percent. PPSC OHE 2016 asked the same question.
Solutions, colloids and suspensions
Rule in one line: the smaller the particles, the clearer the mixture and the less it settles.
Type
Particles
Behaviour
Examples
Solution
Smallest
Clear; never settles; passes through filter paper
Salt water, soda water, air
Colloid
Medium
Looks uniform; scatters a light beam (Tyndall effect)
Milk, fog, smoke, jelly
Suspension
Largest
Cloudy; settles on standing; can be filtered
Muddy water, chalk in water
In a solution, the part that dissolves is the solute; the part it dissolves in is the solvent. Water dissolves so many things that it is called the universal solvent.
Tyndall effect in fog. Fog is a colloid of tiny water drops in air. The drops scatter the light, so the beams become visible. A true solution, like salt water, does not do this. Image: Domala72, Wikimedia Commons · CC BY 4.0
KEY TERM
Soda water
Soda water (an aerated drink) is carbon dioxide gas dissolved in water under pressure. So it is a gas-in-liquid solution: carbon dioxide is the solute and water is the solvent. When the bottle is opened, the pressure falls and the gas bubbles out.
QUICK CHECK
PYQ · RSMSSB LDC 2024 — The aerated drinks (soda water) constitute: (a) A mixture of gases (b) Gas in liquid solution (c) Liquid in gas solution (d) Carbon dioxide as solute and water as solvent (e) Carbon dioxide as solvent and water as solute.
Tap an option to answer
Answer: B — b and d only
Carbon dioxide gas is dissolved in liquid water (b). The gas is the solute and water the solvent (d).
Separating mixtures
Rule in one line: pick the method that uses a difference between the parts.
Method
Uses the difference in
Example
Evaporation
One part turns to vapour
Salt from sea water
Distillation
Boiling points far apart
Pure water from salt water
Fractional distillation
Boiling points close together
Gases of air; parts of petroleum
Separating funnel
Liquids that do not mix
Oil from water
Centrifugation
Density, by fast spinning
Cream from milk; blood tests
Chromatography
How far each part travels
Colours in ink
Sublimation
One part sublimes
Ammonium chloride from salt
Crystallisation
Solubility on cooling
Pure copper sulphate, alum, sugar
Magnet
Magnetic property
Iron filings from sand
Winnowing
Weight, using wind
Husk from grain
Separation methods. Alum (phitkari) makes mud settle faster before the clear water is poured off (decanting).
Distillation. The liquid in the flask (2) is heated and turns to vapour. The vapour cools in the condenser (5) and drips into the receiving flask (8) as pure liquid. This is evaporation + condensation. Image: Fadingblitz, Wikimedia Commons · CC0
Two lab methods
Separating funnel: oil floats on water; open the tap to drain the water out first
Water expands when it freezes, so ice takes more space than the same mass of water. Ice is therefore less dense than water and floats.
What is condensation?
The change of a gas into a liquid on cooling, such as dew forming or drops appearing outside a cold glass.
What is sublimation? Give examples.
The direct change of a solid into a gas without becoming a liquid. Camphor, naphthalene balls, ammonium chloride and dry ice sublime.
Is air a compound or a mixture?
A mixture. Its gases are not joined chemically, keep their own properties, and their share can vary. They can be separated by fractional distillation of liquid air.
How much nitrogen is there in air?
About 78 percent by volume. Oxygen is about 21 percent, argon about 0.93 percent and carbon dioxide about 0.04 percent.
Is iodised salt a compound?
No. It is common salt with a small amount of potassium iodide mixed in, so it is a mixture.
What is the difference between a compound and a mixture?
A compound has a fixed make-up and new properties, and can be split only by chemical means. A mixture can have any make-up, its parts keep their properties, and it can be separated by physical methods.
What kind of solution is soda water?
A solution of a gas in a liquid: carbon dioxide (the solute) is dissolved in water (the solvent) under pressure.
What is the Tyndall effect?
The scattering of a beam of light by the particles of a colloid, which makes the beam visible — as in fog, or sunlight entering a dusty room.
How is common salt obtained from sea water?
By evaporation. Sea water is kept in shallow salt pans; the Sun evaporates the water and the salt is left behind.
Now sit the questions they actually set
Previous-year PPSC questions on this topic, free to read with the worked answer.
