Why water makes a good solvent, and what kinds of molecules dissolve best in it.
Has life ever given you lemons? If so, you've no doubt followed the old adage and made lemonade - involving, of course, a lot of sugar! If you've stirred sugar into lemonade (or tea, or any other water-based drink) and watched it dissolve, then you've already seen the solvent properties of water in action. A solvent is simply a substance that can dissolve other molecules and compounds, which are known as solutes. A homogeneous mixture of solvent and solute is called a solution, and much of life’s chemistry takes place in aqueous solutions, or solutions with water as the solvent.
Because of its polarity and ability to form hydrogen bonds, water makes an excellent solvent, meaning that it can dissolve many different kinds of molecules. Most of the chemical reactions important to life take place in a watery environment inside of cells, and water's capacity to dissolve a wide variety of molecules is key in allowing these chemical reactions to take place.
Solvent properties of water
Thanks to its ability to dissolve a wide range of solutes, water is sometimes called the "universal solvent." However, this name isn't entirely accurate, since there are some substances (such as oils) that don't dissolve well in water. Generally speaking, water is good at dissolving ions and polar molecules, but poor at dissolving nonpolar molecules. (A polar molecule is one that's neutral, or uncharged, but has an asymmetric internal distribution of charge, leading to partially positive and partially negative regions.)
Water interacts differently with charged and polar substances than with nonpolar substances because of the polarity of its own molecules. Water molecules are polar, with partial positive charges on the hydrogens, a partial negative charge on the oxygen, and a bent overall structure. The unequal charge distribution in a water molecule reflects the greater electronegativity, or electron-greediness, of oxygen relative to hydrogen: the shared electrons of the O-H bonds spend more time with the O atom than with the Hs. In the image below, the partial positive and partial negative charges on a water molecule are represented by the symbols δ and δ, respectively.
Because of its polarity, water can form electrostatic interactions (charge-based attractions) with other polar molecules and ions. The polar molecules and ions interact with the partially positive and partially negative ends of water, with positive charges attracting negative charges (just like the + and - ends of magnets). When there are many water molecules relative to solute molecules, as in an aqueous solution, these interactions lead to the formation of a three-dimensional sphere of water molecules, or hydration shell, around the solute. Hydration shells allow particles to be dispersed (spread out) evenly in water.
Water molecules forming hydration shells around Na+ and Cl- ions. The partially positive ends of the water molecules are attracted to the negative Cl- ion, while the partially negative ends of the water molecules are attracted to the positive Na+ ion.
How does the formation of a hydration shell cause a solute to dissolve? As an example, let's consider what happens to an ionic compound, such as table salt (NaCl), when it's added to water.
If you stir table salt into water, the crystal lattice of NaCl will begin to dissociate into Na and Cl ions. (Dissociation is just a name for the process in which a compound or molecule breaks apart to form ions.) Water molecules form hydration shells around the ions: positively charged Na ions are surrounded by partial negative charges from the oxygen ends of the water molecules, while negatively charged Cl ions are surrounded by partial positive charges from the hydrogen ends. As the process continues, all of the ions in the table salt crystals are surrounded by hydration shells and dispersed in solution.
Nonpolar molecules, like fats and oils, don't interact with water or form hydration shells. These molecules don't have regions of partial positive or partial negative charge, so they aren't electrostatically attracted to water molecules. Thus, rather than dissolving, nonpolar substances (such as oils) stay separate and form layers or droplets when added to water.
Want to join the conversation?
- why cant oil spread evenly there water?(21 votes)
- Well... There are 2 reasons for this...
1) Oil has a density less than water, so it just sits there on the top, not spreading out into the more dense water below.
2) Oil is very nonpolar, meaning that a polar molecule like water doesn't get attracted to it. Oil is a covalent bond, and doesn't dissolve into ionic bonds like water. Because of this, the oil just keeps to itself, repelling all the water molecules causing it to be so independent and for it to not spread evenly in the water.(15 votes)
- Im still can't wrap my head around hydrogen shells how are those formed?(19 votes)
- So since water is more positive on one end and more negative on the other end compounds like NaCl start to dissolve because one of them is attracted to the partial positive side of the water and the other is attracted to the partial negative side. Then this process continues until the water kind of coats each of the atoms.(43 votes)
- In the second last paragraph, it is said that NaCl dissociates in water that is understood but why does only after NaCl dissolves in water is it able to conduct electricity is it because of free moving electrons or free moving ions through out the liquid?(16 votes)
- If ionic bond is stronger than hydrogen bond, why does "Dissociation" happen?(8 votes)
- The accumulation of several partially polar H2O molecules overpowers the singular charge of the ionic bond. So when you have one ionic bond vs one hydrogen bond, ionic wins, but one ionic bond vs several hydrogen bonds, that leans towards the hydrogen bonds.(9 votes)
- so does our saliva dissolve stuff because of its water content, enzymes (such as amylase) or both?(8 votes)
- Water is the solvent and enzymes help to degrade larger molecules into smaller ones. I would not call degradation of starch into maltose dissolving.(5 votes)
- If non polar substances are not soluble in water, how does oxygen get dissolved in water,which supports marine life?(5 votes)
- Oxygen as gas can dissolve in water but there is no reaction between oxygen and water.
Oxygen and water both retain their molecular form. So that is the way how aquatic animals perspire, by using dissolved oxygen. :D(8 votes)
- Why do things dissolve? For example why wouldn't the NaCl just stick together? They are attracted by an ionic bond, so is the "pull" from the hydrogen and oxygen stronger than what they have to offer one another?
does this only happy with ionic bonds?
If I understand correctly.. electronegativity determines a molecules polarity. So does hexane not dissolve because it has a low or no electronegative number?
And another question about breaking ionic bonds. What is salt was suddenly in the presence of some molecule or element of that has the same charge as the Na or Cl. Could it forgo its ionic bond with one another and bond with an equally satisfying atom? Why or why not?(5 votes)
- To answer the first question, a molecule must have a charge to dissolve in water, because for a substance to dissolve, it has to form bonds with the solvent, and it cant do that unless it is polar
2. It happens with polar molecules (like methanol), as well as ionising molecules (like any acid)
3. Hexane does not have an charge because the electronegativity of the carbon and hydrogen is negligible.
4. When like 3 H2O molecules bond to the Na or Cl, they overpower the ionic bond between the Na and Cl, which allows them to dissociate.
Hope that helps, if it doesnt, please ask away :)(6 votes)
- Is water being a good solvent a chemical or physical property? Also, is adhesion a physical property, as it is due to waters polarity?(5 votes)
- I think it's a chemical property. Also, I believe adhesion is a physical property. Did this answer your question?(4 votes)
- What happens when there is more NaCl than H2O? The same thing but only to some of the NaCl instead of all of it?(3 votes)
- Exactly right! When there is too much of a solute for a solvent to dissolve, the solvent stops dissolving the solute. At this point, even if you add more solute, it won't dissolve. We call this state supersaturation.(5 votes)