Identify an example of plasma as a state of matter.

Correct — A. A neon sign bulb contains plasma. A high voltage across the low-pressure gas strips electrons from the atoms, and the resulting mixture of free electrons and positive ions is what glows.

That ionised, electrically conducting gas is the fourth state of matter — the state stars are made of, and the state a lightning stroke leaves along its channel.

The remaining options are ordinary solids, liquids and gases, with no ionisation anywhere in them. Option (a).

Why the others are wrong

• (b)
  Freon — Freon is a refrigerant, a chlorofluorocarbon that cycles between liquid and gas inside a cooling system. It is never ionised in that use.
• (c)
  Blood — Blood plasma is the straw-coloured liquid part of blood, and the shared word is the entire trap. It is a liquid, not a state of matter.
• (d)
  Dry ice — Dry ice is solid carbon dioxide, known for subliming straight to gas without melting. That is a change between two ordinary states, not plasma.

Matter is taught in three familiar states — solid, liquid and gas — separated by how tightly the particles are held.

Plasma is the fourth. Heat a gas far enough, or drive a strong electric field through it, and electrons break free of their atoms. What is left is a gas of charged particles that conducts electricity and responds to magnetic fields.

Textbooks add a fifth, the Bose-Einstein condensate, formed near absolute zero, where atoms lose their separate identity and behave as a single quantum entity.

Everyday plasma is easier to find than it sounds: neon signs, fluorescent tubes, welding arcs, lightning and the Sun.

Key facts

• Plasma is an ionised gas of free electrons and positive ions, and it conducts electricity.
• Plasma is described as the fourth state of matter and the Bose-Einstein condensate as the fifth.
• Neon signs and fluorescent tubes glow because the gas inside them is ionised into plasma.
• Dry ice is solid carbon dioxide and sublimes directly into gas.

Common traps

• Blood plasma and the plasma state of matter share a name and nothing else.
• Dry ice tests sublimation, which is still a solid-to-gas change, not plasma.
• A neon tube holds plasma only while current is flowing through it.

No directly related past PYQ was found.