Matter, elements, compounds and mixtures for NDA

The NDA chemistry syllabus opens with physical and chemical changes, then "elements, mixtures and compounds", the laws of chemical combination (without problems) and the properties of air and water. These produce classification questions: is it a compound or a mixture, a colloid or a solution, which method separates it, which law does this observation show. They are easy marks, but only if your definitions are exact.

Physical and chemical changes have their own detailed guide, physical and chemical changes for NDA. This one covers everything else in that opening block.

States of matter

Plasma, a hot ionised gas found in stars, lightning and glowing neon signs, is often called the fourth state.

Changes of state: melting, freezing, boiling and evaporation, condensation, and sublimation, where a solid turns directly into vapour. Camphor, naphthalene balls, iodine and dry ice (solid carbon dioxide, which sublimes at about −78 °C) are the standard examples. Raising the pressure and lowering the temperature can turn a gas into a liquid, which is why LPG is stored as a liquid in cylinders.

Elements, compounds and mixtures

The classic demonstration uses iron filings and sulphur powder. Simply mixed, a magnet pulls the iron out: that is a mixture. Heated together, they form iron sulphide (Fe + S → FeS), a black solid that a magnet no longer separates: that is a compound.

Elements are metals, non-metals or metalloids (boron, silicon, germanium, arsenic, antimony), which have intermediate properties. Their molecules differ in atomicity: noble gases such as helium and argon are monatomic; hydrogen, oxygen, nitrogen and chlorine are diatomic (H₂, O₂, N₂, Cl₂); ozone is triatomic (O₃); phosphorus (P₄) and sulphur (S₈) are polyatomic.

Solutions, colloids and suspensions

A homogeneous mixture has the same composition throughout (salt solution, air, brass). A heterogeneous mixture does not (sand in water, oil in water). The three kinds of mixture in a liquid are told apart by particle size:

The Tyndall effect is the visible path of a light beam through a colloid, as when sunlight streams through a dense forest canopy or into a dusty room.

Separation methods

The laws of chemical combination

The syllabus asks for these without numerical problems, so learn the statement, the scientist and one example each.

Air and water

Air is a mixture. By volume, dry air is about 78% nitrogen, 21% oxygen and 0.9% argon, with about 0.04% carbon dioxide and traces of other gases, plus variable water vapour. It is a mixture because its composition varies from place to place, its gases can be separated physically, and it shows the properties of each component.

Water is a compound, H₂O, with hydrogen and oxygen in a fixed 1 : 8 ratio by mass and 2 : 1 by volume when it is split by electrolysis. It dissolves more substances than any other common liquid, which is why it is called the universal solvent. Pure water is a very poor conductor of electricity; dissolved salts make tap water conduct. Hard water and its treatment are covered in chemistry in everyday life.

Worked NDA-style MCQs

Q1. Which of these is a compound?
(a) water (b) air (c) brass (d) sea water

Brass is an alloy (a mixture), air and sea water are mixtures. Only water has a fixed composition by chemical combination. Answer: (a).

Q2. The Tyndall effect is shown by:
(a) salt solution (b) sugar solution (c) copper sulphate solution (d) milk

Only milk is a colloid; the others are true solutions. Answer: (d).

Q3. Carbon forms two oxides. In one, 3 g of carbon combines with 4 g of oxygen; in the other, with 8 g. This illustrates the law of:
(a) conservation of mass (b) constant proportions (c) multiple proportions (d) gaseous volumes

The oxygen masses combining with a fixed 3 g of carbon are 4 : 8, that is 1 : 2. These are CO and CO₂. Answer: (c).

Q4. Water from any source contains hydrogen and oxygen in the mass ratio 1 : 8. This illustrates the law of:
(a) conservation of mass (b) constant proportions (c) multiple proportions (d) Avogadro

One compound, one fixed ratio. Answer: (b).

Q5. Kerosene and water are most simply separated by:
(a) filtration (b) sublimation (c) centrifugation (d) a separating funnel

They are immiscible, so they form two layers; the lower (water) layer is run off. Answer: (d).

Practice set

1. Which of these is a homogeneous mixture? (a) air (b) sand in water (c) oil in water (d) chalk in water
2. Fog is a colloid of: (a) solid in gas (b) liquid in gas (c) gas in liquid (d) liquid in liquid
3. Cream is separated from milk by: (a) filtration (b) sublimation (c) centrifugation (d) distillation
4. Oxygen and nitrogen are obtained industrially from air by: (a) filtration (b) crystallisation (c) chromatography (d) fractional distillation of liquid air
5. The law of conservation of mass was given by: (a) Lavoisier (b) Dalton (c) Proust (d) Avogadro
6. Which molecule is triatomic? (a) O₂ (b) N₂ (c) O₃ (d) Ar
7. The different dyes in black ink are separated by: (a) chromatography (b) distillation (c) sublimation (d) a separating funnel
8. Which is not true of a compound? (a) it has a fixed composition (b) its components can be separated by physical means (c) its properties differ from those of its elements (d) it has a definite melting point

Answers:

1. (a). Air has the same composition throughout a sample.
2. (b). Tiny water droplets dispersed in air.
3. (c). Spinning throws the denser milk outward; lighter cream stays near the centre.
4. (d). The gases boil off at different temperatures.
5. (a).
6. (c). Ozone.
7. (a). Each dye moves at a different speed on the paper.
8. (b). Separating a compound needs a chemical reaction.

What to do next

• Draw the element–compound–mixture table from memory and check it.
• Learn the colloid table: one example of each type.
• Match all ten separation methods to an example without looking.
• Say each law of chemical combination in one line, with its scientist.
• Move on to atomic structure and the periodic table.