This might be a dumb question, but..why is water a liquid? Aren't the elements that make it up normally gases? Is it temperature?
Actually, that’s not a dumb question at all. It’s a very fundamental question, and so the answer is something people often take for granted.
First and foremost, chemical compounds do not necessarily share the properties of the elements they’re made of. This is an important principle of chemistry, because it explains the vast diversity of chemical substances. The human body is made up of many, many different types molecules, but it’s mostly made of just four elements: Oxygen, hydrogen, nitrogen, and carbon. You can combine these four elements into all sorts of different combinations and proportions, and each combination has different properties depending on how the atoms of each element are arranged.
Water is made up of two hydrogen atoms, and one oxygen atom. This happens because oxygen atoms have a lot of electrons in their outermost electron shell, while hydrogen atoms have very few. Atoms can become more stable when they have a completely full outermost shell. They can do this by giving up or acquiring electrons to become ions. Another solution is to share electrons by forming chemical bonds. This is something hydrogen and oxygen do very well.
Hydrogen is the simplest element. It’s outermost electron shell is its only shell, and it only has one electron in it. It needs just on more to have a full shell. Left to themselves, hydrogen atoms will solve this problem by simply sharing electrons with one another, forming molecules with two hydrogen atoms apiece. We can call this molecule “dihydrogen” or “diatomic hydrogen”, or “molecular hydrogen”, but usually it’s just called hydrogen, since this is the form we usually deal with when we think about the element.
Since the two hydrogen atoms are sharing their electrons, they can both have a full outermost electron shell at the same time. This form is more stable, but only if the chemical bond between them remains in tact. But it takes some energy to break the bond. Without that energy, the hydrogen atoms aren’t likely to separate on their own.
What this means is that a hydrogen molecule isn’t particularly reactive. It just sort of floats around without much interest in interacting with anything else. This includes other hydrogen molecules. Because each hydrogen atom only has one electron apiece, they can only form one chemical bond at a time. So there’s not much point in switching partners. The molecules of hydrogen will just sort of float around and bounce off one another without doing anything.
This is why hydrogen is a gas. You can condense hydrogen into a liquid, but only at a very low temperature,
-252.9°C. At that temperature, the molecules become so low in energy that they can’t bounce around as fast, and it becomes easier for them to stick together. But they’re not tightly bound together, so they can still flow around each other. That’s a liquid.
By contrast, iron is a solid at room temperature because its atoms are much more willing to cling together and form rigid crystalline structures. These structures don’t start to come loose until the iron is very hot, which is why iron has a very high melting point (1,538°C), and an even higher boiling point
The same principle that explains hydrogen’s low boiling point also apply to oxygen. Oxygen atoms have six electrons in their outermost shell, but they need eight to be completely full. So they pair off, much like hydrogen atoms, except they share two electrons apiece, forming two chemical bonds, or a double bond.
Now it is possible for oxygen atoms to bond with multiple atoms at one time, this generally doesn’t work when it’s just oxygen atoms involved. Ozone is a molecule of three oxygen atoms, and while they’re able to make it work, the arrangement isn’t very stable, so it doesn’t take much to pull it apart. This is what makes ozone so reactive, and why it’s toxic enough that large cities will declare “ozone alerts” when there’s too much of it in the atmosphere.
Generally speaking, oxygen prefers a two-atom molecular form, because each atom in the pair is too greedy for electrons to let its partner try to share with someone else at the same time. So oxygen molecules behave similarly to hydrogen molecules, bouncing around without much interaction. So oxygen is a gas too, until you cool it down to
-183°C. It’s boiling point isn’t quite as cold as hydrogen’s, but it’s still far colder than anything on Earth outside of a laboratory. So we think of hydrogen and oxygen as gases.
However, while the molecules of these gases don’t interact much with themselves, they do react violently with one another. This is because oxygen is greedy for electrons, while hydrogen is anxious to share electrons. Their diatomic molecules make a decent arrangement, but an even better arrangement can be had by combining together to form a new molecule: water.
Since oxygen can form two chemical bonds at a time, and hydrogen can only form one, the resulting molecule has two hydrogen atoms sharing electrons with the one oxygen atom. But the water molecule has an interesting shape, and this is a consequence of the atoms it’s made up of.
See, we might assume the molecule exists in a straight line: H-O-H. But this isn’t how it works. Oxygen atoms have six outermost electrons, and electrons repel one another, so when you have six off them in a valence shell together, they prefer to space themselves out. The best way to do that is a sort of tetrahedral formation. Imagine the oxygen atom like a big balll, and a pair of electrons sticks out at the very top. Then towards the bottom three more pairs are arranged like the legs on a stool. The advantage of this is that each electron pair is as far apart from one another as possible, including the electrons used to form bonds with hydrogen.
So when all is said and done water molecules have this bent structure, with an angle of about 105 degrees. And this creates an odd distribution of electrons in the overall structure.
See, you’ve got these other six electrons on one side of the oxygen atom, and on the other you have these hydrogens sharing relatively few electrons. This lopsided electron distribution leads to a phenomenon called a dipole moment, where each water molecule sort of has a little positive charge on the hydrogen side, and a little negative charge on the oxygen side. Because of this, the atoms in a water molecule will be drawn to other water molecules.
The intermolecular attraction between hydrogen and oxygen isn’t as strong as a chemical bond. Otherwise every water molecule would just fall apart as the atoms all swapped dance partners over and over. But the attraction is there. Chemists call this a hydrogen bond, since hydrogen atoms have a tendency to do this in molecules where they’re stuck to electron greedy species like oxygen. This attraction between the molecules makes it tougher to force them apart. In other words, it’s tougher to get water to behave like a gas, which means it has a higher boiling point: 100°C.
And that’s a temperature above what we usually see on earth, so we tend to think of water as a liquid.
Note that water has very different properties from oxygen and hydrogen, but we can explain those differences because of the properties of oxygen and hydrogen. Oxygen’s greed for electrons and hydrogen’s unique one-electron structure can tell us a lot about why water does what it does. For example, the bent molecules and the hydrogen bonding are what cause water to arrange into hexagonal crystals when it solidifies, and this is why snowflakes look the way they do.
Alkanes are one of the simplest of Organic Compounds. They consist of single bonded Carbon and Hydrogen atoms and align themselves linearly unless they’re cyclic or ringed structures. I’m mostly going to be talking about simple linear alkanes in this post.
They’re saturated Hydrocarbons, meaning that they’re saturated or filled with Carbon and Hydrogen atoms. Alkanes can be represented formulaically as CnH2n+2; that is for every n number of carbon atoms there are twice as many hydrogen atoms plus two hydrogen atoms in the molecule. Each carbon atom can contain 3 hydrogen atoms unless its at the start or end of the chain. This simple formula easily allows you to know how many Carbon and Hydrogen atoms in an alkane, granted you know its name or the number of carbons. For example Heptane above has 7 Carbons: Plugging this into the formula we get: C(7)(7*2)+2(H) = 7 C and 16 Hydrogens.
Properties and nomenclature
Alkanes are non-polar molecules and are therefore soluble in non-polar solvents (like attracts like!). The more non-polar a solvent is the greater solubility of an alkane in it. They have low boiling points when compared to polar compounds because they only exhibit weak london dispersion forces between molecules; polar molecules exhibit much stronger dipole-dipole forces or hydrogen bonding. An example of the difference in boiling points between non-polar and polar molecules can be done by comparing Methane (CH4) to Water (H2O); both molecules have similar molecular weights (16 vs 18), but their boiling points are vastly different; Methane boils at -162 degrees C while Water boils at 100 degrees C. That is the power of Dipole-Dipole forces and Hydrogen bonds! In fact in order for an alkane to approach water’s boiling point it needs many carbons in its chain, 7 in fact. Heptane has a boiling point of 98 Degrees C and it has 7 Carbons and many Hydrogen atoms. Low carbon alkanes (Methane - Butane) are gaseous at room temperature; Medium Carbon chains are liquid at room temperature (Pentane to Hexadecane); Heavy Carbon chains are solid at room temperature ( Heptadecane and higher).
Continuous chain alkanes which are linear are named by adding a prefix followed by -ane. These are determined by the amount of Carbon atoms in the molecule. There are unique non-numerical prefixes used from C1 to C4 which are Meth-, Eth-, Prop-, and But-; when you hit 5 carbons it gets easier, all prefixes correspond to greek numerical roots which most people already know; Pent- for 5, Hex- for 6, Hep- for 7, etc. It changes a bit so I’ll list a table below:
Number of Carbons in molecule: Prefix used:
Sources: CHEM 2409 (Organic Chemistry I) notes from BCIT.