Types of Solutions

Transcript

0:00Introduction

Hello everyone! Okay, a small puzzle before we start. What do the air you breathe, a bottle of soda water, and a brass key have in common? Strange, but true: all three are solutions! But wait. Isn't a solution always a liquid, like salt in water? Not at all. Today you will see that a solution can be a gas, a liquid, or even a solid. This is the very first idea of the Solutions unit, and everything else in the unit builds on it, so let's begin.

0:31What is a solution?

So, first things first. What exactly is a solution? It is a homogeneous mixture of two or more substances. Homogeneous means uniform: the same all the way through, so every part of it has the same make-up. Okay? Now think. I dissolve one spoon of sugar in a glass of water. Then I make another glass with two spoons of sugar. Are both of them solutions? Yes, both are! That is the second part of the definition: the composition of a solution can vary, within limits. Within limits, because you cannot keep dissolving sugar for ever. But more sugar or less sugar, it is still a solution. Right? Now, the simplest case is a binary solution. Binary means two: a solution made of just two components. Clear? And the two components get names. The solvent is the component present in the larger amount, or the one that decides the physical state of the solution: gas, liquid, or solid. The other component is the solute. So in sugar water, sugar is the solute, and water is the solvent. Got it? Now, a classic trap. In a thick sugar syrup, there is more sugar than water, by mass. Larger amount, so sugar is the solvent, right? No, no, careful! Read the rule again: or the one that decides the physical state. The syrup is a liquid, because of the water. So for a solid dissolved in a liquid, the liquid is the solvent, even when the solid is in larger mass, as in syrup. Okay?

2:06Nine types of binary solutions

Now, the types. In a binary solution, the solute can be a gas, a liquid, or a solid. And the solvent can also be a gas, a liquid, or a solid. Right? So tell me, how many different types of binary solution can we make? Nine, exactly! Three times three. And we group them by the state of the solvent, because the solvent decides the state of the solution. Clear? First, gaseous solutions, where the solvent is a gas. In the brackets, the first letter is the solute and the second is the solvent, so g slash g means gas in gas. Gas in gas: air, a mixture of oxygen and nitrogen. Liquid in gas: chloroform vapour mixed with nitrogen, N two; the small two below is a subscript, two atoms in each molecule. And solid in gas: camphor in nitrogen. With me? Next, liquid solutions, where the solvent is a liquid. These you know well. Gas in liquid: soda water. Liquid in liquid: ethanol in water. Solid in liquid: glucose or salt in water. Okay? Now the surprising group: solid solutions, where the solvent is a solid. Tell me, can a gas really dissolve in a solid? Yes, it can! Hydrogen, H two, dissolves in palladium metal: gas in solid. Liquid in solid: sodium amalgam, an amalgam of mercury with sodium. And solid in solid: copper dissolved in gold. So a solution need not be a liquid at all. Got it?

3:47Topper's trick: like dissolves like

Now, which solute dissolves in which solvent? Here is a quick one. Common salt: will it dissolve better in water, or in benzene? Water, of course! And here is the rule toppers use: like dissolves like. Polar and ionic solutes dissolve in polar solvents; the arrow on screen means dissolve in. So salt and sugar dissolve in water. Okay? And the other way round: non-polar solutes dissolve in non-polar solvents. Naphthalene or iodine dissolves in benzene, or in C C l four, carbon tetrachloride; the subscript four means four chlorine atoms. Clear? Now, why does this rule work? Look at the attractions. Before mixing, solute particles attract each other, and solvent particles attract each other. When the solute dissolves, these attractions are replaced. Right? They are replaced by new solute and solvent attractions. Dissolution is favoured when these new attractions are comparable to, or stronger than, the ones they replace. Like with like, and the new attractions are just as good as the old ones. With me?

4:58Example 1: classify brass, humid air and sodium amalgam

Okay, Example one. Classify each of these. Part a, brass. Part b, humid air. Part c, hydrogen absorbed in palladium. Part d, sodium amalgam. And part e, aerated drinks. Pause the video, and try it first. Done? Okay, let's check. Part a: brass is an alloy, a solid dissolved in a solid. So, solid in solid, s slash s. And there is the brass key from our puzzle! Okay? Part b, humid air: air with water vapour in it. Vapour is a gas, air is a gas, so gas in gas, right? No, no, careful! We classify by the normal state of the pure substance at room temperature, and pure water is a liquid. It is just like chloroform in our table, where chloroform vapour in nitrogen was liquid in gas. So humid air is liquid in gas, l slash g. Got it? Part c, hydrogen absorbed in palladium. Straight from our table: gas in solid, g slash s. Easy, right? Part d, sodium amalgam. Mercury is a liquid, and the amalgam is a solid. So, liquid in solid, l slash s. Clear? And part e, aerated drinks, like the soda water from our puzzle. The gas is carbon dioxide, C O two, dissolved in a liquid. Gas in liquid, g slash l. That's it!

6:31Example 2: why iodine dissolves in CCl4 and in KI

Now Example two, a little harder. In which solvent is iodine more soluble: water, or C C l four? And why does iodine dissolve well in aqueous K I, that is, potassium iodide dissolved in water? Pause and try. Hint: use our topper's trick. Done? Okay, let's check. Iodine is I two, a molecule of two iodine atoms. Is it polar, or non-polar? Non-polar, correct! So, like dissolves like. The double arrow on screen means implies: iodine is far more soluble in non-polar C C l four, where it gives a violet solution. Okay? But now, the twist. K I solution is mostly water, and water is polar. So how can non-polar iodine dissolve well in it? Look at this equation. K I gives the iodide ion, I minus; the little minus at the top is its negative charge. Iodine, I two, plus I minus, gives I three minus. The two half arrows, one each way, mean this is reversible, an equilibrium. With me? Now, I three minus, with the subscript three, is the triiodide ion: three iodine atoms with one negative charge. It is an ion, and polar water molecules crowd around a charged ion and hold it well: it is well hydrated. Ions dissolve in polar water, like dissolves like, once again! Right? So, the answer: C C l four, and in K I, iodine dissolves by forming the triiodide ion. See how one simple rule answered both parts?

8:14Quick revision: nine types and like dissolves like

So, quick revision. One: a solution is a homogeneous mixture, and the solvent is the component in larger amount, or the one that decides the state. Two: the solute and the solvent can each be a gas, a liquid, or a solid, so there are nine types. Three: like dissolves like, polar in polar, non-polar in non-polar. Now go and try the practice problems on this page. You can do them. All the best!