Polarity

• Water molecules are all polar. This means one part of the molecule has a partial negative charge and the other has a partial positive charge. The negative part is attracted to positively charged ions or groups on other molecules and vice versa. Consequently, a good way to figure out what water can dissolve is the old rule "like dissolves like." In other words, water molecules are polar, so they're generally good at dissolving other polar compounds like table sugar but bad at dissolving non-polar compounds like wax.
  
  

Ionic Compounds

• Ionic compounds like ammonium sulfate are crystals made of charged ions; these charged ions are attracted to each other; the force of attraction holds them together. When water comes along, however, attraction between negative and positive parts of the water molecule and the ions can take the place of the forces holding the crystal together: the water can pry ions loose from the crystal, causing the crystal to dissolve. Many, though not all, ionic compounds like salt or potassium chloride dissolve well in water.
  
  

Gases

• Most gases found in the air are non-polar and do not typically dissolve well in water, but even the small amounts that do dissolve are essential for life. If it were not for dissolved oxygen and carbon dioxide, the world's oceans, lakes and streams would be devoid of life. The solubility of a gas depends on its partial pressure, that is, the pressure that specific gas would have if it alone occupied the volume taken up by the mixture of gases. Gases dissolve better at colder temperatures, so if all else remains equal, there will be more dissolved oxygen in cold water than in warm. Gases composed of polar molecules like hydrogen cyanide or ammonia dissolve very well in water.
  
  

Surfactants

• Surfactants are molecules that reduce water's surface tension. Soap is a common surfactant formed by combining a base with a fatty acid. In soaps and other surfactants, one end of the molecule is very polar, while the other parts of the molecule are very non-polar; this kind of arrangement is called an amphipathic molecule. Surfactants help dissolve fats and grease. When the surfactant is mixed with water, it forms clusters called micelles around fat molecules, in which the polar groups point outward toward the water molecules, while the non-polar groups point inward toward the fat molecule. That's why you use soaps and other surfactants to wash grease from your hands or from dishes.