phases of matter. In practice, it is pretty difficult to observe the exact melting point of a substance.
What molecular properties lead to lower melting points?
Most of the trends we talked about for raising boiling points hold true for melting points for most of the same reasons. There is one major exception, though. The more branched, and therefore compact, a molecule is, the higher its melting point, because, in general, compact molecules will pack better in a crystal lattice. The better packed a lattice is, the more stable it is, and the more heat (energy) it takes to break up that lattice and melt the solid.
How do the boiling point and melting point of a substance change as a solute/impurity is added?
The addition of a solute typically raises the boiling point and lowers the melting point of a substance. These effects are appropriately named “boiling point elevation” and “melting point depression.”
Boiling points are raised when a nonvolatile solute (like NaCl) is added to a solution because the solute lowers the vapor pressure of the solution. That is a somewhat circular explanation though. It is important to know that this change in boiling point does not depend on what you add to the liquid, so there are no specific interactions going on here (like forming hydrogen bonds, etc.). As long as the solute has lower vapor pressure (remember we said it was nonvolatile, so its vapor pressure is essentially zero), this effect will be present. It is perfectly correct to think about it as just lowering the vapor pressure of the mixture (if you add something with very low vapor pressure, the average vapor pressure of whatever liquid you’re adding it to will go down).
Melting (or freezing) points are usually lowered when a solute is added to a liquid. The best explanation for this effect is based on entropy (see “Physical and Theoretical Chemistry”). When a molecule of solvent moves from the liquid to the solid phase (freezes), the amount of liquid solvent (i.e., its volume) is reduced. This means the same amount of solute is in a smaller space, which reduces their entropy (or raises their energy). This raising of energy means that you have to take even more energy out of the system for each molecule that joins the solid phase. Less energy means lower temperature, so adding a solute lowers the freezing point. An alternative way to look at this is that any impurity will disrupt the crystal lattice, raising its energy, relative to the liquid phase. This also contributes to lowering the freezing point as solutes are added.
How is the concentration of a solution defined?
The concentration of a substance is the amount of that substance in a solution divided by the volume of the solution. Chemists typically use molar concentration (moles of material/volume).
What properties influence solubility?
The most significant properties are intermolecular forces and temperature. If there are favorable interactions between the solute and the solvent, solubility will be higher. This is actually a balance of the interactions of the solute with the solvent and the stability of the solute in the solid phase. Temperature also influences solubility, and for most substances, solubility increases as the temperature of the solvent rises.
Why does putting salt on the road help to melt snow?
Like we discussed, when a solute is added to a solution, its freezing point is lowered. When salt is placed on snow, it begins to dissolve into any small amount of water on the ice with which it is in immediate contact. This lowers the freezing point of the surrounding water/ice, causing it to melt into the water. This process continues until the salt is completely dissolved.
What are the basic units of length?
Almost every length scale used in science is based on the meter. Chemistry frequently deals with very small lengths, so while you’re probably familiar with millimeters (10−3 m), it’s hard to have an intuitive sense about just how small a nanometer (10−9 m) is. There’s another length scale commonly used when talking about chemical bonds—the Ångström. An Ångström (Å) is one ten-billionth of a meter (10−10 m). The lengths of chemical bonds vary depending on the elements and other factors, but are usually around 1–2 Å.
How much space does an atom occupy?
The atomic radius of the smallest atom, hydrogen, is 53 × 10−12 meters, so it is about 10−10 meters in size. The atomic radius of the largest atom, cesium, is about 270 × 10−12 meters, or roughly five times that of hydrogen. These are all very, very small sizes!
How much space does the nucleus take up?
The nucleus of an atom takes up a very, very small fraction of the total space occupied by the atom. The diameter of a nucleus is on the order of 100,000 times smaller than that of a whole atom.
How long are chemical bonds?
Chemical bonds are typically about 2 atomic radii in length, since they are formed from two atoms joined together. These distances are on the order of 10−10 meters.
What are the basic units of pressure?
Unlike units of length and temperature, pressure is reported in at least six common units. The pascal (abbreviated Pa) is the official standard unit, but bar, millimeters of mercury (mmHg), standard atmospheres (atm), torr, and pounds per square inch (psi) are all used in different areas.
How do planes stay in the air?
Airplanes are very heavy, so the force required to balance gravity and keep them in the air must be large. The engines propel the airplane forward, but we need to understand what gives the upward push, or lift, necessary to keep the plane in the air. This lift comes from the shape of the wings, which are typically curved on the top and flat on the bottom. This design requires air to flow more rapidly over the top of the wings than over the bottom, which creates a lower air pressure above the wing than below. The lower air pressure above the wing is what lifts the plane off the ground and keeps it in the air. This is commonly referred to as the Bernoulli principle. If you blow across the top of a sheet of paper, you will see it lift into the air for the same reason.
What makes oil more slippery than water?
The purpose of lubricants, like motor oil, is to reduce the friction between surfaces so that parts last longer and less energy can be expended in the process of moving them. The key to a good lubricant is that the characteristic length scale for the formation of a thin film of the lubricant must be much smaller than the characteristic length scale of movement in the application. Basically, oils are good lubricants because they can form very thin films that persist even when the parts they serve to lubricate are constantly in motion. This ability to form thin films typically correlates with other properties that are easier to recognize. For example, good lubricants often have a high boiling point, low freezing point, high viscosity, and are stable toward chemical oxidation and changes in temperature.
What prevents all of the air from escaping Earth’s atmosphere?
Gravity! Every molecule on Earth is pulled toward the planet by gravity, even the very lightest gas molecules. To overcome a planet’s gravitational pull, an object, be it a space ship or
