Water, Solutions, and Mixtures

Water's properties come from its polarity. Because water is polar (a partially negative oxygen and partially positive hydrogens), water molecules cling to one another and to other charged substances through hydrogen bonds. Four properties matter most:

• Solvency (the "universal solvent"). Water's polar ends surround and separate charged or polar solutes, dissolving them. Blood, urine, cytosol, and digestive juices are all water-based solutions. Nonpolar substances (fats) resist dissolving and must be carried by special transporters.
• High heat capacity. Water absorbs and releases a lot of heat with little temperature change, which buffers the body against sudden temperature swings and helps maintain homeostasis.
• High heat of vaporization. Evaporating water (sweat) carries away large amounts of heat, cooling the body efficiently.
• Cohesion and lubrication. Water molecules stick together (cohesion), producing surface tension and making water an excellent lubricant in serous and synovial fluids.

Water is also a reactant. In hydrolysis, water is added to split large molecules during digestion; in dehydration synthesis, water is removed to build them (covered in a later section). So water is not just the stage — it is sometimes an actor.

Mixtures: solutions, colloids, suspensions. A mixture blends substances physically without chemical bonding, so its components keep their properties and can be separated. Mixtures differ by particle size:

• Solutions have the smallest solutes (ions, small molecules like glucose), evenly scattered and never settling. Saline and blood plasma's dissolved salts are solutions.
• Colloids (emulsions) have larger particles that remain dispersed and can scatter light. Cytosol is a colloid that can shift between fluid and gel states.
• Suspensions have the largest particles, which are visibly distributed but settle out when left standing. Whole blood is a suspension — spin it down and the red cells sink beneath the plasma.

Concentration. How much solute is present is described several ways: percent (grams per 100 mL), molarity (moles per liter), and, in clinical settings, units like milliequivalents per liter (mEq/L) for electrolytes and milligrams per deciliter (mg/dL) for glucose. Concentration matters because physiology depends on the right amount, not just presence — and because water moves toward higher solute concentration (osmosis), a theme in the cell unit.

How it works

Classifying a mixture:

1. Do particles settle? Yes → suspension (blood cells in plasma).
2. Do they stay dispersed but scatter light / can gel? → colloid (cytosol).
3. Are solutes tiny, evenly spread, never settling? → solution (saline).

Comparisons

Common confusions

• Solution vs suspension. Solutions never settle; suspensions do. Blood is a suspension because its cells settle when not moving.
• Solvent vs solute. Solvent dissolves (water); solute is dissolved (salt, glucose).
• Mixture vs compound. A mixture is physical (separable, components keep properties); a compound is chemically bonded.
• "Universal solvent" is not literal. Water dissolves many polar/charged substances but not nonpolar fats.

Memory aids

• "Solute is the smaller amount, Solvent is the greater" (both start with "sol," so anchor on amount).
• Suspension → things Suspended eventually Sink.
• Water cools by evaporation — "sweat = built-in AC."

Quick review

• Water's polarity makes it the body's solvent and gives it high heat capacity, cooling by evaporation, and cohesion/lubrication.
• Solutions (tiny solutes, never settle), colloids (dispersed, can gel), and suspensions (large particles, settle — like blood) differ by particle size.
• Concentration (percent, molarity, mEq/L, mg/dL) describes how much solute is present and drives water movement.