International A Level Biology / Chemistry for biologists
Water: the dipole and why it dissolves things
Edexcel International A Level Biology (YBI11), chapter 1: Chemistry for biologists, specification point 1.1. Free lesson.
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Water is a dipolar molecule, so it attracts ions and other polar substances and can carry them in solution. This explains why substances dissolve in blood plasma and plant xylem, while non-polar lipids do not dissolve in water.
Why water is a dipole
Inside each water molecule, oxygen and hydrogen share electrons in covalent bonds. Oxygen pulls the shared electrons towards itself, so the sharing is uneven. This gives oxygen a slight negative charge, written δ−, and each hydrogen a slight positive charge, written δ+. A molecule with two oppositely charged ends is a dipole. The charges are partial, not full charges. The δ+ hydrogen of one water molecule attracts the δ− oxygen of another. This weak attraction between different molecules is a hydrogen bond. It is not the bond holding oxygen and hydrogen together inside one…
How water dissolves ionic substances
Sodium chloride is a crystal made from positively charged sodium ions and negatively charged chloride ions. Water's partial charges attract these ions and pull them away from the crystal. Each ion becomes surrounded by water molecules, with the molecules facing the right way. The δ− oxygen ends face the positive Na⁺ ions. The δ+ hydrogen ends face the negative Cl⁻ ions. Opposite charges attract, so the ions separate and become surrounded by water.
Which substances dissolve in water
The key is whether a substance has charged regions. Ionic substances have full charges, while polar substances have partial charges. Water is attracted to both, so they dissolve. Water can also form hydrogen bonds with polar molecules. Na⁺, Cl⁻, glucose and amino acids dissolve in water. Glucose is not ionic. Its polar OH groups form hydrogen bonds with water. Lipids are non-polar: they have no charged regions for water to be attracted to, so they do not dissolve. Substances that dissolve in water are hydrophilic. Substances that do not dissolve are hydrophobic.
How water transports substances
A substance must be dissolved to be carried in a liquid. Water is the transport medium in both blood plasma and plant xylem, but the substances carried and the forces moving the water differ. Blood plasma is the liquid part of blood. It travels through blood vessels around the body and carries dissolved glucose, amino acids, ions, urea and hormones. Pressure from the heart moves it. Xylem vessels carry water and mineral ions, such as nitrate, from roots to leaves. Evaporation from the leaves pulls the water upwards. Hydrogen bonds make water molecules stick together, helping to keep…
Keeping hydrogen bonds separate from covalent bonds
A common error is to call the bond inside a water molecule a hydrogen bond. The O to H bond inside one molecule is covalent. A hydrogen bond is a much weaker attraction between two different molecules. A hydrogen bond runs from the δ+ hydrogen of one water molecule to the δ− oxygen of another. It is not an attraction between hydrogen and hydrogen, or between oxygen and oxygen.