Chapter 15, Problem 4A

Interpretation Introduction

Interpretation: The reason for some molecules to be soluble in water and others to be not needs to be explained. The structural features that makes a molecule soluble in water needs to be determined.

Concept introduction: Dissolution of solid solute in the solvent to form a solution depends on both the nature of the solute and the solvent.

Answer to Problem 4A

Due to the phenomenon “like dissolves like” some molecules are soluble in water and others are not. Polarity of the molecules especially molecules with $O-H$ bond makes them soluble in water.

Explanation of Solution

 It has been experimentally observed that polar solutes tend to dissolve in the polar solvent and non-polar solutes tend dissolve in non-polar solvents. The concept is called “like dissolves like”. Since sugar ( $C_{12}{H}_{22}{O}_{11}$ ) and ethyl alcohol ( $C_2{H}_{5}OH$ ) are polar molecule the get dissolved in polar water molecule. Petroleum consists of mostly long chain non-polar hydrocarbon chain and hence they are insoluble in water.

The partial positive charge on hydrogen attracts the partial negative charge of an electronegative oxygen atom in water molecule forming what is called hydrogen bonding. The molecules with $O-H$ bond can also form hydrogen bonding with water molecule and in this way, dissolves readily into water.

Sugar ( $C_{12}{H}_{22}{O}_{11}$ ) and ethyl alcohol ( $C_2{H}_{5}OH$ ) are not only polar but also have $O-H$ bond and hence they dissolve rapidly in water.

Conclusion

Due to the phenomenon “like dissolves like” some molecules are soluble in water and others are not. Polarity of the molecules especially molecules with $O-H$ bond makes them soluble in water.