Lesson · B.1.2

The chemistry of water

Why water's polarity, hydrogen bonding, and solvent behaviour turn up across the HESI A2 Biology subtest, from cell membranes to pH.

ObjectiveB.1.2Read5 minExam weightFoundations of Biology ≈ 24% of Biology

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What you will be able to do

  • Explain why water is polar and how that polarity produces hydrogen bonding.
  • Name water's key properties, cohesion, adhesion, high specific heat, and solvency, and tie each to a real consequence.
  • State what pH measures and place common body fluids on the pH scale.

What the lesson concludes

  • Water is polar because oxygen pulls shared electrons more strongly than hydrogen does, leaving a partial negative charge on oxygen and a partial positive charge on each hydrogen.
  • Hydrogen bonds between water molecules explain cohesion, adhesion, high specific heat, and the fact that ice floats.
  • Water's polarity makes it an excellent solvent for ionic and polar substances, but a poor one for nonpolar substances such as fats.
  • pH is a log scale from 0 to 14, with 7 neutral, below 7 acidic, and above 7 basic, and small changes represent large shifts in hydrogen ion concentration.
  • Blood is normally kept in a narrow range around pH 7.35 to 7.45, and even small drifts outside that range are treated as a medical emergency.