Chemistry 5070/12 — October/November 2013

Walkthrough

When liquid bromine is placed on a table, it evaporates to form bromine gas. The bromine particles then mix with the air particles by diffusion.

• Option A is correct: the bromine particles do collide with air particles. These collisions change the direction of the particles and help them spread through the room.
• Option B is correct: gas particles move in a random way. The path of any one particle is unpredictable because of collisions.
• Option C is correct: gases have no fixed shape or volume, so the bromine particles spread out to occupy all the space available.
• Option D is incorrect: "vibrate from side to side" describes particles in a solid vibrating about fixed positions. Gas particles do not stay in fixed positions; they move freely through the space. Since the question asks what is NOT part of the explanation, D is the answer.

Key Takeaways

• Diffusion is the spreading of particles from a region of higher concentration to a region of lower concentration.
• Gas particles move rapidly in random directions and collide with other particles.
• A gas always spreads to fill the whole container or room.
• Vibrating in a fixed position is a property of particles in a solid, not a gas.

Common Mistakes

• Choosing A because a student thinks collisions are not needed. Collisions are actually an important part of explaining diffusion because they cause the random motion.
• Choosing C because a student thinks gases have a fixed volume. Gases expand to fill all available space.
• Misreading the question: it asks for what is NOT part of the explanation. Always check whether the question wants the true statement or the false statement.

Things to Be Careful About

• The word "not" is the key word in this question. Underline it if needed.
• Remember the three states: solids vibrate about fixed positions, liquids slide past each other, gases move freely and rapidly.
• Diffusion is faster for lighter gas particles, but this question does not compare different gases, so that idea is not needed here.