States of Matter and Gas Behaviour – NDA Chemistry Notes

Matter exists in different physical forms: solid, liquid, and gas. You see this every day. Water can be ice (solid), liquid water, or steam (gas). The state of a substance depends on temperature and pressure.

Understanding states of matter means understanding why substances behave the way they do: why glass has no fixed melting point, why dry ice does not melt to a liquid, why mercury does not stick to glass, and why a desiccant in a medicine bottle keeps it dry.

NDA tests this chapter through glass properties questions, dry ice identification, sublimation questions, boiling vs evaporation distinction, when liquid boils, and what desiccant means. These appear regularly as direct questions, NOT-correct statement traps, and process classification questions.

1. States of Matter

Matter exists in five states. For NDA, three are essential. Two are advanced and rarely tested.

Three main states:

State	Shape	Volume	Particle arrangement	Example
Solid	Fixed	Fixed	Closely packed, vibrate in place	Ice, iron, salt
Liquid	No fixed shape (takes container shape)	Fixed	Close together but can flow	Water, mercury, alcohol
Gas	No fixed shape (fills container)	No fixed volume	Far apart, move freely	Air, steam, CO₂

Two additional states (rare):

• Plasma: extremely hot ionised gas (found in Sun and stars, lightning)
• Bose-Einstein Condensate: extremely cold state (near absolute zero)

2. Changes of State

When matter absorbs or releases heat, it can change from one state to another.

Change	From	To	Heat	Example
Melting	Solid	Liquid	Absorbed	Ice → Water
Freezing	Liquid	Solid	Released	Water → Ice
Evaporation	Liquid	Gas	Absorbed	Water → Steam (at surface)
Boiling	Liquid	Gas	Absorbed	Water → Steam (throughout)
Condensation	Gas	Liquid	Released	Steam → Water
Sublimation	Solid	Gas (directly)	Absorbed	Camphor, Iodine, Dry ice
Deposition	Gas	Solid (directly)	Released	Frost formation

All changes of state are physical changes. No new substance is formed. The chemical composition does not change.

Cooling of water vapour to room temperature = condensation = physical change. [NDA 2016-II]

Sublimation of camphor = physical change. [NDA 2024-II, NDA 2016-II]

3. Sublimation

Sublimation is when a substance converts directly from solid to gas without passing through the liquid state. On cooling, the gas converts directly back to solid.

Substances that sublime (must memorise):

• Camphor: used in worship and medicines
• Iodine (I₂): purple-black solid → purple vapour
• Naphthalene: mothballs
• Ammonium chloride (NH₄Cl): separating technique in chemistry
• Dry ice (solid CO₂): sublimes at −78°C
• Anthracene: polycyclic aromatic compound
• Benzoic acid

Memory trick: CABIN (Camphor, Ammonium chloride, Benzoic acid, Iodine, Naphthalene).

Anthracene can be separated from a mixture of anthracene and salt by sublimation. Anthracene sublimes. Salt does not. [NDA 2021-II]

A mixture of sodium chloride and naphthalene can be separated by sublimation. [NDA 2013-II]

Sublimation of iodine is a physical change. No new substance is formed.

Sublimation of camphor is a physical change, NOT a chemical change. [NDA 2024-II]

4. Boiling vs Evaporation

Both involve liquid converting to gas, but they are different processes.

Evaporation:

• Happens at the surface of the liquid
• Happens at ANY temperature (not just boiling point)
• Slow process
• Cooling effect (evaporation takes heat from surroundings)
• Examples: wet clothes drying, sweat cooling the body

Boiling:

• Happens throughout the bulk of the liquid (not just surface)
• Happens only at a specific temperature, the boiling point
• Requires sustained heat supply
• Rapid and vigorous

Boiling is a bulk phenomenon. Evaporation is a surface phenomenon. [NDA 2019-II]

When does a liquid boil? A liquid boils when its vapour pressure equals the atmospheric pressure. [NDA 2023-II]

This is why:

• Water boils at 100°C at sea level (atmospheric pressure = 1 atm)
• Water boils below 100°C at high altitude (atmospheric pressure is lower)
• Water boils above 100°C in a pressure cooker (pressure is higher inside)

5. Glass

Glass is one of the most frequently tested topics in this section of NDA.

What is glass? Glass is an amorphous solid, also described as a supercooled liquid or vitrified liquid. It does NOT have a regular crystalline structure.

Key properties of glass:

• Glass does NOT have a fixed melting point. It softens gradually over a range of temperatures. [NDA 2006-I, NDA 2013-II, NDA 2017-I, NDA 2019-II]
• Glass is amorphous. It has no regular repeating atomic arrangement.
• Glass is NOT a true compound: its composition can vary.
• Glass is NOT a crystalline solid.

Glass is a non-crystalline amorphous solid. [NDA 2017-I]

Glass does NOT have a definite melting point. This is why it is not considered a true compound. [NDA 2013-II]

Glass is a vitrified liquid, meaning it is a liquid that has been cooled so slowly that it became rigid without crystallising. [NDA 2011-I]

Silica is the major component of glass. Sand (SiO₂) is the primary raw material. [NDA 2011-II, NDA 2012-I, NDA 2023-I]

Raw materials for glass making:

• Sand (SiO₂): source of silica
• Soda ash (Na₂CO₃): sodium oxide source
• Limestone (CaCO₃): calcium oxide source

NOT used in glass making: Gypsum (CaSO₄·2H₂O). [NDA 2021-I]

Types of glass:

Type	Also called	Special property	Use
Soda glass	Soda-lime glass	Ordinary, cheap, softens easily	Windows, bottles, everyday glassware
Pyrex glass	Borosilicate glass	Contains boron, resistant to heat and thermal shock	Lab glassware, cooking
Flint glass	Lead glass	Contains lead, high refractive index, sparkles	Optical lenses, crystal glassware

Pyrex (borosilicate) glass is harder than soda glass, NOT the other way around. [NDA 2019-II] The statement “Soda glass is harder than Pyrex glass” is NOT correct.

Boron is present in Pyrex glass. [NDA 2019-II]

Optical glass used in spectacles is made from flint glass. [NDA 2014-II]

Colours of glass from metal oxides:

Metal oxide added	Colour produced
Chromium oxide (Cr₂O₃)	Green
Cobalt oxide (CoO)	Blue
Cuprous oxide (Cu₂O)	Red
Manganese dioxide (MnO₂)	Violet/Purple
Uranium oxide	Yellow

Chromium oxide is used in paints and glass to produce green colour. [NDA 2013-II]

6. Dry Ice

Dry ice is solid carbon dioxide (CO₂). [NDA 2018-II, NDA 2019-II, NDA 2025-I]

It is called “dry” ice because it does NOT melt to a liquid at normal atmospheric pressure. It sublimes directly from solid to gas at −78°C.

Uses of dry ice:

• Refrigerant for transporting food and medicines (much colder than regular ice)
• Creating fog effects in theatre and film
• Preserving biological samples
• Cooling in industrial processes

Dry ice sublimes at −78°C, producing CO₂ gas. This is a physical change.

Dry ice is solid CO₂, NOT solid water, NOT liquid nitrogen, NOT liquid ammonia. [NDA 2018-II, NDA 2019-II, NDA 2025-I]

7. Desiccant

A desiccant is a substance that absorbs moisture from its surroundings. It is used to keep products dry.

Best desiccant = Silica gel (SiO₂·xH₂O)

Silica gel is the white or blue granular substance found in small packets inside medicine bottles, shoe boxes, and electronics packaging. It absorbs water vapour very efficiently.

The best example of a desiccant is silica gel. [NDA 2020 I & II]

Polystyrene, sodium chloride, and sodium carbonate are NOT desiccants.

8. LPG, CNG and Biogas

These were covered in detail in Chapter 11 and Chapter 18. Key facts relevant to states of matter:

LPG (Liquefied Petroleum Gas):

• Main components: Butane (C₄H₁₀) and Propane (C₃H₈)
• Stored as a liquid under moderate pressure
• Thioethanol (ethanethiol) is added to give LPG a foul smell for leak detection. Pure LPG is odourless [NDA 2016-II]

CNG (Compressed Natural Gas):

• Main component: Methane (CH₄)
• Stored as compressed gas, cannot be liquefied at room temperature (critical temperature is too low at −82°C)

Biogas:

• Main component: Methane (CH₄), 55–65%
• Produced by anaerobic decomposition of organic waste (cow dung, crop residue)
• Renewable fuel [NDA 2013-II, NDA 2019-II]

9. Purest Natural Water

Rain water is the purest natural source of water. [NDA 2025-II]

Rain water is formed by natural evaporation and condensation, a natural distillation process. It does not carry dissolved salts from the earth. River water, well water, and sea water all contain dissolved minerals.

10. Mercury and Surface Tension

Surface tension is the force at the surface of a liquid that makes it behave like a stretched elastic sheet.

Most liquids (water, alcohol) wet the walls of a glass container. Their surface tension causes them to climb up the walls slightly (called capillary rise / meniscus curves upward).

Mercury is different. Mercury does NOT wet glass. Its surface tension acts in the opposite direction: the meniscus curves downward. Mercury is repelled by glass walls.

Mercury does not wet the walls of the glass vessel in which it is kept. [NDA 2015-I]

11. Viscosity

Viscosity is a measure of how resistant a fluid is to flowing. High viscosity = thick, slow-flowing liquid. Low viscosity = thin, fast-flowing liquid.

• Honey has high viscosity.
• Water has low viscosity.
• Glass has extremely high viscosity, so high it behaves as a solid.

Heavy water (D₂O) has higher viscosity than ordinary water (H₂O).

12. Molecular Crystalline Solids

Some solids exist as molecular crystals: their units are molecules held together by weak intermolecular forces (van der Waals forces or hydrogen bonds).

These solids are generally:

• Soft
• Low melting point
• Poor conductors of electricity

Examples of molecular crystalline solids: Iodine (I₂), Sulphur (S₈), Ice (H₂O), Naphthalene, Dry ice (CO₂).

Iodine, Sulphur, and H₂O can exist as molecular crystalline solids. [NDA 2008-II]

13. Fractional Distillation

When two liquids have different but close boiling points, they can be separated by fractional distillation.

The liquid with the lower boiling point evaporates first. Its vapour rises and condenses at a lower level in a fractionating column.

Examples:

• Separating petroleum into petrol, diesel, kerosene, fractional distillation of crude oil [NDA 2022-I]
• Separating kerosene and petrol mixture [NDA 2023-II]
• Separating liquid air into nitrogen (bp −196°C) and oxygen (bp −183°C)