Which Of The Following Is Not A Solid

You're staring at a multiple-choice question. Your brain hesitates. Three options look familiar — ice, iron, diamond — and one doesn't belong. Wait, which one isn't a solid?

It happens more often than you'd think. In practice, not because the concept is hard, but because the wording trips people up. Or the options include something sneaky like dry ice (it's solid CO₂, but it sublimates) or mercury (liquid at room temperature) or a colloid that acts* solid-ish but isn't.

Let's clear it up once and for all.

What Is a Solid, Really

A solid isn't just "something hard." That's the trap.

In physics and chemistry, a solid is a state of matter characterized by structural rigidity and resistance to changes in shape or volume. The particles — atoms, molecules, or ions — are packed tightly in a fixed, ordered arrangement. They vibrate in place but don't move past each other.

That's the textbook version. Here's what it means in practice:

• Definite shape — A solid doesn't take the shape of its container. A gold bar stays a gold bar whether it's in a vault, a backpack, or melted down (at which point it stops being a solid).
• Definite volume — It doesn't compress appreciably under normal pressure.
• High density — Particles are close together. Usually denser than the liquid form (water/ice being the famous exception).
• Incompressible — You can't squeeze a solid into a smaller volume without extreme force or phase change.

Solids come in two main flavors:

Crystalline solids

Ordered, repeating structure. Sharp melting points. Examples: salt, diamond, quartz, metals, ice. Break them and they cleave along specific planes.

Amorphous solids

No long-range order. Think of them as frozen liquids. They soften over a temperature range rather than melting sharply. Examples: glass, rubber, many plastics, gels.

Both are solids. Both have definite shape and volume at human timescales. (Yes, glass flows — but on geological timescales. Your window panes aren't thicker at the bottom because of flow. That's a myth.

The Other States — So You Can Spot the Imposter

To answer "which is not a solid," you need to know what the alternatives look like.

Liquids

Definite volume, indefinite* shape. Particles are close but can slide past each other. Takes the shape of its container. Surface tension creates a meniscus. Viscosity varies — honey vs. water — but both are liquids.

Key tell: It pours. (Eventually.)

Gases

Neither definite shape nor definite volume. Particles are molecules in random, high-speed motion. Fills the entire container. Compressible. Expands to occupy available space.

Key tell: It disappears into the air — or fills a balloon, or exerts pressure. That's the part that actually makes a difference.

Plasma

Ionized gas. Electrons stripped from atoms. Conducts electricity, responds to magnetic fields. Stars, lightning, neon signs, fusion reactors. Rare on Earth's surface but most common state in the universe.

Key tell: It glows and reacts to magnets.

Bose-Einstein condensates, fermionic condensates, quark-gluon plasma...

Exotic states. Lab-only (mostly). Not showing up on your multiple-choice test.

Common "Not a Solid" Traps in Multiple Choice Questions

Test writers love certain distractors. Here are the ones that show up again and again.

Mercury (Hg)

Element 80. Silver liquid at room temperature.*
Melting point: −38.83 °C. Boiling point: 356.7 °C.
It's a metal. It conducts electricity. It's dense (13.5 g/cm³). But at 25 °C? Liquid.
If you see "mercury" as an option alongside iron, copper, gold — mercury is the answer.

Bromine (Br₂)

Element 35. Reddish-brown liquid at room temperature.*
The only other element liquid at standard conditions (besides mercury).
Melting point: −7.2 °C. Boiling point: 58.8 °C.
If the list includes iodine (solid), chlorine (gas), fluorine (gas) — bromine is the liquid.

Water / H₂O (at room temperature)

Obvious, right? But sometimes it's disguised: "Which of the following is not a solid at 25 °C and 1 atm?" Options: ice, water, dry ice, iron. Water is the liquid. Ice and dry ice are solids. Iron is solid.

Dry ice (solid CO₂) — wait, this IS a solid*

Don't let the name fool you. Dry ice is solid carbon dioxide. It sublimates at −78.5 °C (goes straight to gas), but at its existing temperature, it's a crystalline solid. Hard, cold, holds its shape.
Not the answer unless the question specifies "at room temperature."

For more on this topic, read our article on how many feet is 82 in or check out how is resource different from gifts of nature.

Ammonia (NH₃), ethanol, acetone, gasoline

Common liquids. If the question is about substances at STP (standard temperature and pressure), these are liquids.

Helium, nitrogen, oxygen, carbon dioxide (at STP)

Gases. CO₂ is a gas at room temp and 1 atm. Only becomes solid (dry ice) under pressure or extreme cold.

Colloids and suspensions

Jelly, gelatin, whipped cream, smoke, fog, paint, blood.
These are mixtures*, not pure states of matter. But they often behave* like solids or liquids depending on timescale.

• Gelatin sets into a gel — semi-solid, but technically a colloid (solid network trapping liquid).
• Whipped cream holds shape briefly — foam (gas in liquid).
• Smoke — solid particles in gas.
• Fog — liquid droplets in gas.

If a question asks "which is not a solid" and includes gelatin or jelly, the answer depends on pedantry level. In a middle-school test? Probably "not a solid.Day to day, " In a materials science context? It's a soft solid / gel. Context matters.

Plasma balls, neon signs, lightning

If "plasma" or "lightning" appears as an option — that's your non-solid. It's ionized gas.

How to Approach the Question Systematically

Don't guess. Use this checklist.

1. Identify the conditions

Is the question explicit about temperature and pressure?

• "At room temperature (25 °C) and 1 atm..."
• "At STP (0 °C, 1 atm)..."
• "Under standard conditions..."

If not stated, assume room temperature and atmospheric pressure — the default for most general chemistry questions.

2. Classify each option

Go through one by one. Write S, L, G, or P next to each.

(H₂O) | Liquid |

3. Watch for "Trick" Elements

If you find yourself stuck, look for these specific outliers that often appear in competitive exams:

• The Mercury Factor: Mercury (Hg) is one of the few metals that is liquid at room temperature. If you see a metal listed alongside iron or gold, check its state immediately.
• The Bromine Factor: Bromine (Br₂) is one of the few non-metals that is liquid at room temperature.
• The Noble Gas Factor: Helium (He) and Neon (Ne) are almost always gases unless the question specifies extreme cryogenic temperatures.
• The Phase Change Trap: Always check if the substance is near its melting or boiling point. A substance like Nitrogen (N₂) is a gas at room temperature, but if the question asks about its state at -200 °C, it is a liquid.

Summary Checklist for Success

When faced with a "Which is not a [State]?" question, follow this mental flow:

1. Check the Environment: Is it room temperature or STP?
2. Scan for Liquids: Look for Mercury, Bromine, or common solvents like water and ethanol.
3. Scan for Gases: Look for Oxygen, Nitrogen, Hydrogen, or CO₂.
4. Identify the "Odd One Out": The substance that does not fit the state described by the question is your answer.

Conclusion

Mastering the states of matter requires more than just memorizing a list of substances; it requires an understanding of how temperature and pressure dictate molecular behavior. Most "trick" questions rely on your tendency to overlook substances that are liquid or gas at room temperature—like mercury or carbon dioxide—because you are focusing on their properties in other environments.

By systematically classifying each option based on the provided conditions and remaining vigilant for common liquid/gas outliers, you can handle even the most pedantic chemistry questions with confidence. Remember: in chemistry, context is everything. Always read the temperature and pressure units before you begin your classification.