A mixture is an intimate combination of two or more substances, both of which retain their chemical identities. Mixtures can be classified as either heterogeneous or homogeneous, depending on their appearance.

In heterogeneous mixtures, the mixing is not uniform and the mixtures have regions of different composition.

Mixing is uniform in homogenous mixtures, and they have the same composition throughout.

Homogeneous mixtures are further classified as either solutions or colloids, according to the size of their particles.

Solutions, the most important class of homogeneous mixtures, contain particles the size of a typical ion or small molecule—roughly 0.1–2 nm in diameter. Colloids, such as milk and fog, are also homogeneous in appearance but contain larger particles than solutions—in the range 2–500 nm diameter.

Solute: A substance that is dissolved in a solvent.

Solvent: The substance in which another substance (the solute) is dissolved.

Solvation :The clustering of solvent molecules around a dissolved solute molecule or ion.

Miscible means mutually soluble in all proportions.

Saturated solution A solution that contains the maximum amount of dissolved solute at equilibrium.

Solubility is the maximum amount of a substance that will dissolve in a given amount of solvent at a specified temperature.

Solids that are more soluble at high temperature than at low temperature can sometimes form supersaturated solutions, which contain even more solute than a saturated solution.

According to Henry’s law, the solubility (or concentration) of a gas in a liquid is directly proportional to the partial pressure of the gas over the liquid. If the partial pressure of the gas doubles, solubility doubles; if the gas pressure is halved, solubility is halved.

Mass/mass percent concentration: Concentration expressed as the number of grams of solute per 100 g of solution.

Volume/volume percent concentration: Concentration expressed as the number of milliliters of solute dissolved in 100 mL of solution.

mass/volume percent concentration: Concentration expressed as the number of grams of solute per 100 mL of solution.

Parts per million (ppm): Number of parts per one million 10^6 parts.

Molarity (M) is the number of moles of solute dissolved per litre of solution.

• Molarity= Moles of solute/Volume of solution.

The key fact to remember about dilution is that the amount of solute remains constant, only the volume changes.

Dilution factor :The ratio of the initial and final solution volumes.

Electrolyte is a substance that produces ions and therefore conducts electricity when dissolved in water.

• Distilled water contains virtually no ions and is nonconducting, ordinary tap water contains low concentrations of dissolved ions and is weakly conducting, and a concentrated solution of NaCl is strongly conducting.

Strong electrolyte: A substance that ionizes completely when dissolved in water.

Weak electrolyte: A substance that is only partly ionized in water.

Nonelectrolyte: A substance that does not produce ions when dissolved in water.

Colligative properties: properties that depend on the concentration of a dissolved solute but not on its chemical identity.

• Vapor pressure is lower for a solution than for a pure solvent.

• Boiling point is higher for a solution than for a pure solvent.

• Freezing point is lower for a solution than for a pure solvent.

• Osmosis occurs when a solution is separated from a pure solvent by a semipermeable membrane.

The vapor pressure of a liquid depends on the equilibrium between molecules entering and leaving the liquid surface. Only those molecules at the surface of the liquid that are sufficiently energetic will evaporate.

• If, however, some of the liquid (solvent) molecules at the surface are replaced by other (solute) particles that do not evaporate, then the rate of evaporation of solvent molecules decreases and the vapor pressure of a solution is lower than that of the pure solvent.

Solute molecules are dispersed between solvent molecules throughout the solution, thereby making it more difficult for solvent molecules to come together and organize into ordered crystals. Thus the freezing point is low in solutions.

Semipermeable membranes allow water and other small molecules to pass through, but they block the passage of large solute molecules or ions.

Osmosis is the passage of solvent through a semipermeable membrane separating two solutions of different concentration.

• The phenomenon of osmosis: A solution inside the bulb is separated from pure solvent in the outer container by a semipermeable membrane.

• Solvent molecules in the outer container have a higher concentration than molecules in the bulb and therefore pass through the membrane more frequently. The liquid in the tube therefore rises until an equilibrium is reached.

• At equilibrium, the osmotic pressure exerted by the column of liquid in the tube is sufficient to prevent further net passage of solvent.

Osmotic pressure: The amount of external pressure that must be applied to a solution to prevent the net movement of solvent molecules across a semipermeable membrane.

Osmolarity :The sum of the molarities of all dissolved particles in 1.0 litre of solution.

• Isotonic solutions have the same osmolarity.

• Hypotonic solutions have an osmolarity less than the surrounding blood plasma or cells.

• Hypertonic solutions have an osmolarity greater than the surrounding blood plasma or cells.

If red blood cells are placed in pure water or in any hypotonic solution , water passes through the membrane into the cell, causing the cell to swell up and burst, a process called hemolysis and if red blood cells are placed in a solution having an osmolarity greater than the cell contents (a hypertonic solution), water passes out of the cells into the surrounding solution, causing the cells to shrivel, a process called crenation.

The most important medical use of dialysis is in artificial kidney machines, where hemodialysis is used to cleanse the blood of patients whose kidneys malfunction.

Blood is diverted from the body and pumped through a long cellophane dialysis tube suspended in an isotonic solution formulated to contain many of the same components as blood plasma.