The kinetic-molecular theory

When making a soft drink, you use tap water, put in some ice cubes, and at the back of the kitchen, steam comes out of a pot. Name the three states in which water can be found and write an everyday example for each, different from the ones shown here.

Complete the sentence with the missing words: "According to kinetic-molecular theory, all matter is made up of very small $\dots$ that are in continuous $\dots$." Then explain in your own words why we cannot see those particles.

Aitana leaves a stone she collected at the beach on her bedroom shelf. The stone does not change shape or size, wherever it is. The figure shows how particles are arranged inside a solid like this stone.

Explain, using kinetic-molecular theory, why solids have a fixed shape and volume. Think about how their particles are joined together and how they move.

In a solid, particles vibrate almost without changing position; in a liquid, they slide over one another; in a gas, they move freely throughout the container. Match each type of motion to one of these observations and justify the relationship:

1. a) An iron block always keeps its shape.
2. b) Water from a bottle can be poured into a glass.
3. c) Air in a balloon fills its entire interior.

Martina compares two balls: one deflated, which is soft, and another newly inflated, which is hard. Inside the second one there are many more air particles, as shown in the figure.

Explain, thinking about the collisions of particles against the walls of the ball, why the inflated ball is harder.

Iker pours juice from a wide glass into a narrow bottle: the juice changes shape, but the amount of juice remains the same. The figure shows the juice in both containers.

Explain why a liquid has a fixed volume but no fixed shape, thinking about how its particles move.

Paula covers the outlet of a syringe filled with air with her finger and pushes the plunger: the air can be compressed significantly. If she fills the syringe with water and repeats the test, the plunger barely moves. The figure represents the two tests.

What difference is there between the particles of a gas and those of a liquid that explains this result?

Hugo carefully drops an ink droplet into a glass of water without stirring. After a while, all the water has become coloured, as shown in the figure.

What phenomenon has occurred? Explain it using kinetic-molecular theory: what do the ink particles and the water particles do?

When inflating a ball, the air that enters ends up filling all of its interior. It does not matter whether the ball is spherical, like a football, or oval, like a rugby ball: the air always fills it completely. In the figure, its particles are represented inside the ball.

Explain what the particles of a gas do and why gases have neither a fixed shape nor a fixed volume.

A newly inflated air mattress is hard: when you press it with your hand, you feel resistance. Explain, using kinetic-molecular theory, what the air particles do against the inner walls of the mattress and why that explains why we feel this resistance.

Valeria fills a syringe with $10\ \text{mL}$ of air, covers the outlet with her finger and pushes the plunger until the air is reduced to $5\ \text{mL}$.

1. a) Has the number of air particles inside the syringe changed?
2. b) What happens to the number of particle collisions against the walls per second?
3. c) Does the air pressure increase or decrease? Reason all three answers.

Laia puts perfume on her wrist in her bedroom and, after a short while, her brother smells it from the living room without any air current blowing. How did the perfume particles reach her brother's nose? What is this phenomenon called?

If a small amount of gas escapes from a butane cylinder, it can be smelled throughout the kitchen in a few seconds. In contrast, a drop of ink in a glass of water takes several minutes to dye it completely. Why do you think diffusion is much faster in gases than in liquids? Think about the distance between particles and how free they are to move.

At Carla's house, the hot soup that has just been served can be smelled from the hallway, but cold gazpacho is barely noticeable until you bring your nose close to the plate. Explain the difference using kinetic-molecular theory: what effect does temperature have on the particles, and how does this influence the diffusion of the smell?