Hydrogen and oxygen are elements. What that really means is that when an atom, the fundamental unit that makes up everything, has 1 proton (a positively charged sub-atomic particle) it is a hydrogen atom, and when it has 8 protons, it is an oxygen atom. Hydrogen gas is two hydrogen atoms bonded together, H₂. Oxygen gas is two oxygen atoms bonded together, O₂. Water, is one oxygen atom bonded to two different hydrogen atoms, H₂O. When atoms are bonded together, (H₂, O₂, H₂O etc.) we call the total structure a molecule. Hydrogen atoms and oxygen atoms are found in many different molecules, and these different molecules make up, solids, liquids, and gasses. Whether a material is a solid, liquid, or gas depends on how these molecules (made up of atoms) interact with each other. If there is strong interaction the molecules like to stick close together, and they are solids. In liquids the interactions are a little less strong, and in gases, the molecules are not attracted to each other as much at all.

Hydrogen and oxygen are words we use to refer to two different things: molecules and elements/atoms. Hydrogen the molecule (Hydrogen gas) has two Hydrogen atoms bonded together to form H₂. Same deal for oxygen (gas = O₂). When two oxygen atoms bond to form a gas, the molecule itself is non-polar which means the electrons don't tend to cluster on one side of the molecule making it slightly negatively charged in one area and positive in the other. Since O₂ is non-polar, you have very weak interactions between O₂ molecules (they're mostly just bumping into one another like billiard balls). This is why you have to cool O₂ gas very cold to get it to condense into a liquid. Same basic idea with H₂. However! When you replace an oxygen from O₂ with two hydrogen atoms, the large, strongly electronegative oxygen has almost all of the valence electrons clustered around it, making it slightly negative and the hydrogens slightly positive. When two water molecules come together, the slightly positive Hs are attracted to the negative O and they cluster together. This is (basically) why water is liquid at room temperature. It's like throwing a bunch of bar magnets in a box (but they're weak magnets and jiggle so they aren't a solid).

Water isnt just a liquid, but in fact water can exist in three different states: solid, liquid, or gas. Water existing as a gas is called water vapor, water as a liquid we generally just refer to as water, and water as a solid is called ice. The changes from a solid to liquid to a gas or a gas to a liquid to a solid are called phase changes. When substances such as water change phase, its physical properties change, but not its chemical properties (water is always H₂0 - 2 hydrogen atoms and 1 oxygen atom). The specific phase of a substance depends on temperature and pressure - at higher temperature and lower pressure substances will exist as a gas, and at lower temperature and higher pressure substances will exist as a solid.

Gas to Liquid: Water change from a gas (water vapor) to a liquid through the process of condensation. The gas cools and loses energy which forces the particles - hydrogen and oxygen in the case of water - to change state from a gas to a liquid.

Liquid to Solid: When water as a liquid turns to water as a solid (ice) it goes through the process of freezing. The liquid cools below the freezing point and loses energy. The hydrogen and oxygen particles are forced to change state yet again, this time from a liquid to a solid.

Solid to Gas: When water changes from a solid (ice) to a gas (water vapor) it goes through the process of sublimation.

Liquid to Gas: When water changes from liquid water to water vapor (has) it goes through the process of evaporation. The liquid water gains enough energy to overcome all of the attracting forces that keep it in a liquid state so that it evaporates into a gas.

Solid to Liquid: When water as a solid (ice) changes to liquid water it goes through the process of melting. When a solid gains enough energy it can overcome the strong attracting forces that keeps the particles bonded tightly together and it forms looser bonds and melts into a liquid.

Gas to Solid: Sometimes water vapor may directly change into solid water (ice) through a process called frost formation. Darcy, Earth and Life Sciences

Great question. Here's another great example: sodium chloride is table salt. It is fairly innocent. However, elemental sodium is a metal and spontaneously bursts into flames when it touches water, and chlorine gas will kill most living things. Weird? You bet.

We'll come back to that later, but let's start with water. Although oxygen and hydrogen, individually, are gases, those are not to be confused with the atoms of oxygen and hydrogen. Oxygen gas and hydrogen gas are both formed from pairs of oxygen or hydrogen atoms. (Let's call them dioxygen and dihydrogen, to prevent confusion).

Water is different. You take 1 oxygen atom and 2 hydrogen atoms, and combine them to make water, whose properties are much different. Why? The hydrogen atoms and oxygen share electrons, but they do so unequally. This leads to a slightly unbalanced density of electrons across the molecule. (One side is slightly positive, and the other is slightly negative). This unbalance attracts other water molecules. Because water molecules are attracted to each other, they want to be closer together, and, as you know, molecules in a liquid are much closer together than in a gas. Similarly, because water molecules are strongly attracted to each other, they are harder to break apart, which is why you have to add a lot of heat to turn water into a gas. Water can exist as a gas (this is what happens when you boil water), but it's boiling point is high compared to dioxygen or dihydrogen.

So, let's summarize and compare. Water is an unbalanced molecule, whereas dihydrogen and dioxygen are symmetric, balanced molecules. Water molecules are strongly attracted to each other, so they remain condensed (that is, a liquid, and not a gas) at higher temperatures. Dihydrogen and dioxygen have no strong attractive forces to hold the molecules together, so they exist as gases at room temperature and pressure (that is, in our everyday lives).

In general, we can measure the self-attraction of a substance by its boiling point, and so we expect water to boil at a higher temperature than dioxygen or dihydrogen, which is exactly what we see. At normal atmospheric pressure, water boils at 100 Celcius, whereas dioxygen boils at -183 Celcius, and hydrogen boils at -253 Celcius).

I mentioned salt at the beginning, and it is a more extreme case. In the case of sodium chloride, the atoms do not share electrons, but rather transfer them completely, forming positive and negative ions. These ions would be unstable if they were to be isolated; sodium and chloride ions attract each other very strongly and stay together. This is why table salt exists as a solid, even up to very high temperatures (801 Celcius), and doesn't boil until 1413 Celcius.

As one more point of interest, you know that salt dissolves in water. The reason this happens is because the slightly positive and slightly negative sides of water interact with the positive and negative ions of sodium and chloride, allowing the sodium and chloride to break up.

I like your thoughtful question.

When hydrogen and oxygen combine into a water molecule, the hydrogenatoms and the oxygen atoms share some of their electrons. But theoxygen atom holds the electrons tighter than the hydrogen atom. Thismakes a water molecule where the oxygen atom has a bit of a negativecharge, and the hydrogen atoms have bits of positive charge. So thenthe water molecules are attracted to each other, so that the positivecharge on the hydrogen atoms are near the negative charges on theoxygen atoms.

In hydrogen gas and oxygen gas, the 2 atoms in a molecule share theelectrons equally, so there aren't different charges on the differentsides of the molecules.

When a molecule, like water, has slightly different charges on 2sides, that's called a 'dipole.'

The interactions between hydrogen and oxygen on different moleculesare called 'hydrogen bonds.' Hydrogen bonds are very important inlife. For example, hydrogen bonds hold the 2 DNA strands together inthe double helix in all our chromosomes.

Hydrogen and oxygen are only gasses at the temperature and pressure you're used to dealing with them on - cool them down, and they become liquids, and even freeze and become solids. Heat them up enough and they will become plasmas. The same is true for water, except that at room temperature and sea-level pressure, water can exist as either a liquid or as a gas.

The reason for the (much) higher boiling and freezing points for water is that the oxygen atoms in water accumulate negative electric charge, while the hydrogen atoms accumulate positive electric charge. Because opposite charges attract, the molecules can orient themselves into chains, fit together like building blocks to become crystals, etc., which pure hydrogen or pure oxygen cannot do because they have no charge separation. This means that you really have to heat water up a lot in order to break up the crystal structure that is ice, and heat it up even more to break up the chains that make liquid water. By contrast, it takes very little heat to break up the poorly held-together crystal structure of oxygen or hydrogen - so much so that there is nowhere on Earth cold enough naturally to have these two substances in liquid, let alone solid, form. Go to Pluto, however, and you might find your oxygen turning to a liquid.