The limits of the simple particle model (Higher)

Why does liquid nitrogen poured into a cup boil furiously in a warm room, while water poured into the cup beside it sits still? Draw both liquids with the particle model and the two drawings come out the same: circles close together, touching, sliding past one another. It's tempting to say the nitrogen must have more energy, but it's far colder than the water, at minus one hundred and ninety-six degrees C. Whatever decides it, the drawing has left it out.

This video is Higher tier, and not every course includes this topic at all, so check the screen for whether yours does. If yours leaves it out, the next video for you is Mixtures, and choosing a separation technique.

The particle model draws a substance as particles: regular and touching in a solid, touching but random in a liquid, and far apart in a gas. For describing states and changes of state, that model is the right tool, and it stays the right tool after this video. A limitation of a model is one specific thing the model does not show. That's why every limitation here starts with the same words, the model does not show, followed by one thing it leaves out. The first: the model does not show the forces of attraction between the particles. The phrase is forces of attraction, never bonds. The circles sit side by side with nothing drawn between them, so nothing in the picture pulls on anything. Now a question on that. If the model has no forces, does a liquid have no forces between its particles? It does have them. Real particles pull on one another in a solid, a liquid and a gas, and how hard they pull differs from one substance to another. It's only the model that leaves the forces out. One examiner's report on a Higher paper names the belief that there are no forces between the particles in a liquid as a common misconception. The habit that avoids it is to put the words the model does not show in front of every limitation, so the gap is always the model's. A different Higher report, from another year, says students referred to bonds between particles, where the point was that the model doesn't take the forces of attraction into account. So say the whole phrase every time: forces of attraction between the particles.

The second limitation is about the particles themselves. The model draws every particle as a solid sphere, a hard little ball that never squashes or changes shape. Now a test of those opening words. A student writes: the model shows particles as solid spheres. Is that a limitation yet? Not yet. It only describes the drawing. The limitation is that real particles are not solid spheres, and many of them aren't even round. The third limitation: the model does not show the true size of the particles, or the true space between them. In a real gas, the particles are much further apart for their size than any drawing on a screen can fit in. Now put the model to work on melting. It gets the before and the after right: fixed and vibrating, then random and sliding. Now think about the missing forces. What can't the model explain about melting? What holds the particles in their fixed positions in the first place, so why melting needs energy at all, and why each substance melts at its own temperature. Here's a worked case. A student uses the model to explain solid iodine turning straight into a purple gas when it's warmed. The task is to give one limitation that stops the model explaining why the iodine has to be warmed at all. Pause it there and work it out for yourself. The model does not show the forces of attraction between the particles. Warming gives the iodine particles enough energy to pull away from one another, and a model with no forces in it has nothing for them to pull away from.

Two answers to a limitation question now, and only one of them names a limitation. Answer A says: the particles are not really solid spheres. Answer B says: the model is not accurate because real particles are more complicated. Take your pick, and hold on to your reason before I show you. A is the one that works. It names one specific thing the model leaves out. B is a verdict, not a limitation. Saying a model is not accurate doesn't say what it leaves out, so it doesn't answer the question. Now a repair job. How would you rewrite B so that it names one omission? Put the opening words in front: the model does not show the forces of attraction between the particles. Any one of the three limitations would do. And a limitation doesn't make the model wrong. It still describes states and changes of state well, and it's still the picture to draw.

Now three questions to close on, and the first is about two melting points: ice at nought degrees C, and salt at eight hundred and one degrees C.

Which limitation stops the model explaining that gap? The model does not show the forces of attraction between the particles, and those forces decide where a substance melts.

Now, how else can this sentence end: the model does not show what? That real particles are not solid spheres, or the true size of the particles and the space between them.

Now a final one. Does a limitation mean the particle model is wrong? No. It's right about arrangement and movement, and a limitation is one specific thing it leaves out.

Back to the two cups. Water particles attract one another far more strongly than nitrogen particles do. That's why nitrogen particles escape into a gas with very little energy, even at minus one hundred and ninety-six degrees C, while water needs one hundred degrees C. Two drawings that look the same, and the difference between them is the forces of attraction the model leaves out.

If you could teach this one to a friend, give it a thumbs-up. If not, leave it un-thumbed, and if that felt fast, that's normal.

Next in the chapter is Mixtures, and choosing a separation technique. Hopefully that's next in your feed, but if not, search for ScholaFly C H oh three, oh three.

Every other video is on scholafly.com too, easy to search by video code or by keyword. Whatever you're revising, go there and have a look, or, watch the next video.