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.

Carlos 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?

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 soccer ball, 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.

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.

A small camping gas canister holds enough gas to fill many large balloons. How is it possible for so much gas to fit in such a small container? Relate this to the empty spaces between the particles of a gas.

A deodorant can reads: "Do not expose to temperatures exceeding $50\ ^\circ\text{C}$". The can is airtight and contains gas under pressure. Explain what happens to the gas particles when the can is heated and why the can might burst.

After a long bike ride on a hot day, Omar notices the tyres are harder than when he set off, without having used the pump and without punctures. Explain, using kinetic-molecular theory, what has happened to the air particles inside the tyres.

Weather balloons that measure atmospheric weather are released half-inflated. As they rise, the external air pressure decreases, and the balloon expands more and more until it bursts. The figure shows the balloon when released and at high altitude.

Explain, thinking about collisions of the particles inside and outside, why the balloon grows as the external pressure decreases.

An empty, tightly sealed plastic bottle is left inside a car in the sun: after a while, it bulges, as if inflated. If you put it in the fridge for a few hours, it appears crumpled, collapsed inwards. Explain both changes using kinetic-molecular theory: what the internal air particles do in each case and what happens to the pressure.

A dented ping-pong ball without holes is placed in hot water and recovers its round shape.

1. a) Explain why the heat from the water helps to pop out the dent: what do the air particles inside do?
2. b) Would this trick work if the ball had a small hole? Justify your answer.

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.

Camila 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?

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?

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.

When walking past a bakery, you smell freshly baked bread, but the smell does not reach you instantly: it takes a few seconds to travel the few meters to you, even though the aroma particles move very fast. What happens to those particles along the way? Think about the air particles that are already present between the bakery and your nose.

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?

In the laboratory, Eric prepares two glasses: one with cold water from the fridge and another with hot water. He drops a drop of food colouring into each, at the same time and without stirring, as shown in the figure.

1. a) Predict what will be observed in each glass after five minutes.
2. b) Explain your prediction using kinetic-molecular theory.
3. c) State a general conclusion relating temperature to the rate of diffusion.

Design an experiment to verify that gas particles are in continuous motion using only a spray air freshener, a stopwatch and a tape measure. State:

1. a) What you would do and what you would measure.
2. b) What result you expect to obtain.
3. c) What conclusion you would reach if the result confirms the theory.

A glass of water left forgotten on a window sill drops in level after a few weeks, even though no one drinks from it. Explain, using kinetic-molecular theory, what happens to the faster particles on the surface of the water. Then, predict what would happen to the water level if the glass were hermetically sealed, and justify your answer.