Predicting a substance's state from its bonding

Is gallium a solid or a liquid? It is a silvery metal, and a lump of it sits on a desk as a hard, shiny solid. Hold that same lump in your palm and within a few minutes it runs like mercury. Nothing was added to it. Only the temperature changed, by less than twenty degrees. That puddle comes back at the end. First, how to read a melting point at all.

Every pure substance has two temperatures that decide its state. The melting point is where its solid turns to liquid. The boiling point is where its liquid turns to gas. Picture a temperature line, colder on the left and hotter on the right. Mark the melting point on it, then the boiling point further along. Those two points work like gates. Left of the first gate, the substance is a solid. Between the two gates, it is a liquid. Past the second gate, it is a gas. Foundation and Higher students both use this line the same way. To find the state at any temperature, put that temperature on the line as a dot, and read which stretch it lands in. Place the dot every time, rather than comparing two numbers in your head. Here is a table of four substances, A to D. A melts at minus one hundred and one degrees C, and boils at minus thirty-five. Start at twenty-five degrees C, about room temperature. What state is A in? A gas. Room temperature is already past A's second gate, so the dot lands in the last stretch. Now the other three. B melts at eight hundred and one degrees C and boils at one thousand four hundred and thirteen. C melts at one thousand and eighty-five and boils at two thousand five hundred and sixty-two. At twenty-five degrees C, the dots for B and C sit far to the left of both their first gates. B and C are solids at room temperature. That leaves D, which melts at forty-four degrees C and boils at two hundred and eighty. At twenty-five degrees C, is D a solid, a liquid or a gas? A solid. Room temperature sits to the left of D's first gate, so D has not started to melt yet. Forty-four sounds like a low number, and a low number can feel like a liquid. Low is not the test. The test is which side of the gate the dot falls on. Now slide the dot a long way up the line, to one thousand degrees C, with the same four substances. Which of the four is still a solid at one thousand degrees C? Only C, because its first gate is at one thousand and eighty-five. B has melted and is now a liquid. A and D are both gases by then. Here's a problem that runs the other way. Substance E is a liquid at twenty-five degrees C, and a gas at ninety. You are told its state twice, and not one of its own temperatures. What is the tightest thing you can say about E's melting and boiling points? E's melting point is less than twenty-five degrees C, because E has already melted there. Its boiling point is between twenty-five and ninety degrees C, because E has boiled by ninety. One edge case to settle. If the dot lands exactly on a gate, the substance is changing state right there, so at its melting point it can be part solid and part liquid.

The gates are not random numbers. A substance changes state when its particles get enough energy to move away from each other. The stronger the attraction between the particles, the more energy it takes to pull them apart. More energy means a higher temperature before the change of state happens. That turns the table into evidence. A higher melting point tells you a stronger pull holding the particles to each other. Can you rank A to D, from the strongest attraction between particles to the weakest? C, then B, then D, then A. That is the order of their melting points, highest first, so C's particles attract each other most strongly. The table held real substances. A is chlorine and D is a form of phosphorus, both made of small molecules. B is sodium chloride, made of ions, which are charged particles, held in a giant lattice. C is copper, a metal. Be careful what that attraction means. It is the pull between the tiny particles, not how strong or hard the material feels in your hand. Graphite, the grey stuff in a pencil, is soft enough to rub off on paper. Yet it only gives way at over three thousand degrees C, far above the temperature at which iron melts. Why graphite does that has its own video in this chapter.

Two words come next, because every later video in this chapter leans on them: force and bond. A force between particles is an attraction from one particle to its neighbour. When a substance melts or boils, those forces are overcome. Overcome is the verb that goes with a force. A bond is different. A bond sits inside a particle, like the covalent bond holding the two atoms of a chlorine molecule together. Broken is the verb that goes with a bond. For a substance made of small molecules, the forces between the molecules are called intermolecular forces. Inter means between. Melting overcomes those forces, and every bond inside each molecule stays whole. A few giant structures are built differently. In those, what joins one atom to the next is itself a bond, and the video on diamond and silicon dioxide deals with that case. A student wrote this sentence about substance H. H has weak intermolecular forces, so not much energy is needed to break the bonds, and it melts at a low temperature. It sounds fine on a first hearing, and it ends in the right place. What is wrong with the student's sentence about H? The sentence starts with the forces and then switches to breaking the bonds. Those are two different things, and the second one is false, because H's molecules leave whole. The sentence the student meant goes like this. H has weak intermolecular forces, so only a little energy is needed to overcome them, and H melts at a low temperature. One examiner's report describes this slip: "Some answers started well, referring to weak intermolecular forces, but then moved into talking about breaking bonds." The chemistry was right at the start, and one word undid it. The fix is a habit: forces get the verb overcome, bonds get the verb break, and the two never swap. One more, with no numbers at all. F is made of small molecules. G is made of positive and negative ions, held in a giant lattice. There is no table this time, so the only evidence is what each one is built from. Which melts at the higher temperature, F or G, and what is pulled apart in each? G melts higher. In G, the strong attraction between the ions has to be overcome. In F, only the weak forces between the molecules are overcome, and the bonds inside each molecule stay whole.

Time to pull this together with four quick questions, and the last one goes back to that puddle in the palm. What two points split the temperature line, and what state sits between them? The melting point and the boiling point. Between them, the substance is a liquid. Now one you have not seen: ethanol melts at minus one hundred and fourteen degrees C, and boils at seventy-eight. What is it at twenty? A liquid. Twenty degrees sits between ethanol's two gates. Now the language: a small-molecule substance boils. What happens to its forces and bonds? The forces between the molecules are overcome. The covalent bonds inside each molecule are not broken. One more: gallium melts at about thirty degrees C. In a palm at thirty-five, which state is it, and why does so little heat do it? A liquid, because a warm palm is past gallium's first gate. The attraction between gallium's particles is weak enough that the warmth of a hand overcomes it.

A word on the thumbs-up before you go. Press it on this video if you could teach this one to a friend, and you can skip it next time you revise. Any video in this chapter without your thumb is one to return to. If this one is still wobbly, save it, or the playlist, for later. The rest of the chapter keeps using the temperature line, so it comes back round.

Next in the chapter: why ionic compounds melt high and conduct when molten. Hopefully that's next in your feed, but if not, search for ScholaFly C H oh five, oh two.

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.