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. 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 Celcius, whereas dioxygen boils at Celcius, and hydrogen boils at 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 Celcius , and doesn't boil until 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.
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. The interactions between hydrogen and oxygen on different moleculesare called 'hydrogen bonds. For example, hydrogen bonds hold the 2 DNA strands together inthe double helix in all our chromosomes. Hydrogen fuel cells are used for backup power generation, powering spacecraft and remote facilities, and in hydrogen cars.
You might wonder why hydrogen and oxygen make water H 2 O instead of hydrogen peroxide H 2 O 2. The simplest explanation is that it is much more favorable for two hydrogen atoms to react with one oxygen atom than it is to add another oxygen into the mix.
Even though oxygen gas is O 2 , the bond between the atom has to break for oxygen to form bonds with hydrogen to make water. Remember that while the usual oxidation state of oxygen is -2, it actually does display other states. Sometimes hydrogen and oxygen do form hydrogen peroxide, but the molecule is inherently unstable and eventually decomposes into water and oxygen.
Scientists knew oxygen and hydrogen made water long before they understood the molecular basis for chemical reactions. French chemist Antoine Lavoisier even named the element hydrogen for the reaction. Search for:. Related Posts. In biological reactions, however, equilibrium is rarely obtained because the concentrations of the reactants or products or both are constantly changing, often with a product of one reaction being a reactant for another.
To return to the example of excess hydrogen ions in the blood, the formation of carbonic acid will be the major direction of the reaction. However, the carbonic acid can also leave the body as carbon dioxide gas via exhalation instead of being converted back to bicarbonate ion, thus driving the reaction to the right by the chemical law known as law of mass action.
These reactions are important for maintaining the homeostasis of our blood. The outer electron shell dictates how readily and what type of chemical bonds a particular atom will form. The formation of compounds is often visually outlined in chemical equations which show the reactants participating in chemical reactions to form products. Improve this page Learn More. Skip to main content. Module 2: Chemistry of Life. Search for:. Chemical Reactions and Molecules Learning Outcomes Identify the components of simple chemical reactions.
In Summary: Chemical Reactions and Molecules The outer electron shell dictates how readily and what type of chemical bonds a particular atom will form. Try It. Did you have an idea for improving this content?
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