Why do certain things dissolve?

Question

Why does sugar dissolve in water and flour does not?

Answer 1

Size and polarity.

When something dissolves, its molecules become evenly distributed amoungst what it is dissolved in. The smaller a molecule is, the easier it is for it to slip between the spaces of water molecules. Sugar molecules are small enough to be able to do this. Flour consists of starch and protiens. Both compounds are extremely large molecules and have difficulty fitting between molecules of water.

Water is a polar liquid. Polarity is defined as an uneven distribution of electron density in a molecule. Water molecules act like tiny magnets with a positive pair of hydrogens and a negative oxygen. Therefore, anything else which is polar also dissolves in water. Sugar contains many hydroxyl groups, and these are somewhat polar. oil however is not polar. Its molecules are evenly surrounded by hydrogen atoms and this is why oil and water do not mix. Starch consists of many sugar molecules which have been joined together in a long chain. It would be soluable if its molecules were not so big. Protiens are similar, but are composed of amino acids. Most amino acids are also soluable, but again, proteins are so large they can not completely dissolve. More often, water molecules get trapped between the protein molecules. This is not a true solution, but a mixture known as a colloid. Gelatine is a good example of a protein colloid.

Answer 2

Substances dissolve if molecular attractions between the substance and the solvent (in this case, water) are greater than attractions between the molecules or ions of the substance itself. The rule of thumb is "like dissolves like," which refers to polarity; polar compounds are more likely to dissolve in a polar solvent like water.

Answer 3

Science or magic! :0