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Tyndall Effect • Solutions are not colloids or suspensions. • Colloids have solute particles that are aggregates or large molecules between 1 nm and 1000 nm. • The particles (called the dispersed phase) cannot be separated from the solvent (called the dispersing medium) by filtration or settling. A colloid will scatter light (Tyndall effect).
Solutions are homogenous mixtures. This means solutions contain two or more substances, and are mixed enough to “look all the same”. Solutions do NOT have to be liquids, bronze is a solid solution and air is a gaseous solution.
All solutions have two parts. The substance doing the dissolving is the solvent and the substances being dissolved are the solutes. The solutes must have particles between 0.01-1nm. The solutes can be atoms, molecules, and ions. The solutes do not settle, scatter light, nor can be filtered. • If you don’t know which is which, the substance you have the most of overall is the solvent.
The most common solvent is water. It is so common and dissolves so many things that it is often called “the universal solvent” (the solute is hydrated.) This is not entirely true, as only polar solutes will dissolve in water (we say they are soluble or miscible). Nonpolar substances will not dissolve in water (we say they are insoluble or immiscible.) This is due differences in the types of intermolecular forces – H-bonding and dipole for polar, but only London forces for nonpolar. • If alcohol is the solvent, then the solution is often called a tincture.
In general it is said “like dissolves like” meaning polar solvents only dissolve polar solutes and nonpolar solvents only dissolve nonpolar solutes. Polar substances include salts, acids, bases, and nonsymmetrical molecules like water and NH3. Nonpolar substances include most carbohyrates, fats, oils, and other symmetrical molecules. The more similar two substances are the more miscible they will be. (Similar intermolecular forces!)
Even when things are soluble in water, or any other solvent, different solutes will dissolve to different amount. The actual amount (in grams) that will dissolve in 100 grams of a solvent is called the solubility. When discussing the solubility of solutes, solutions are classified into 3 types: saturated, unsaturated, and supersaturated.
A saturated solution has dissolved as much solute as it can. If you add sugar to iced tea and sugar is just sitting at the bottom of the glass your iced tea is saturated for sugar. (Even though it seems there is no change, there is an equilibrium occurring between dissolution and crystallization.)
A supersaturated solution has been “tricked” into dissolving more solute than it should be able to. If any more solute is added to a supersaturated solution, all the extra solute will undissolve (precipitate). This is how sugar crystal candy (rock candy) is made.
The amount (in grams) of how much will dissolve at a particular temperature is the dependent variable (it depends on the temperature). This is shown on the left side of the graph. Notice this is for all the solutes in 100 grams of water.
If the temperature and amount of dissolved solute is above the line, then the solution is supersaturated. If the saturated solution of 70 grams of NH3 is very carefully heated, it will still have 70 grams of NH3 at 20 °C. As this amount is above the line, this would be a supersaturated solution.
Gases are affected by pressure. With high pressure a lot of gas can be squeezed into a little bit of space. High pressures will help gases dissolve in a liquid, and low pressure will help remove the gas from a liquid. This only works for dissolving a gas in a liquid, and is why a carbonated beverage goes “flat” after it is opened (called Effervescence).
C) When a solid dissolves in a liquid, it takes time for the liquid molecules to grab pieces of the solid and float away. If the liquid is stirred, it speeds up the floating away processes, so more liquid can grab more solid in the same amount of time. Thus stirring or shaking will help a solid dissolve faster in a liquid.
When a gas dissolves in a liquid, it is basically trapped by the liquid molecules. If the liquid is stirred, it removes the water molecules that are trapping the gas, and the gas will form bubbles and eventually leave the liquid. This is why shaking a carbonated beverage makes so many bubbles.
Just knowing that a solution is unsaturated does not indicate much about the solution. Is there only a little bit of solute, or is there a lot and it is almost saturated? The only way to tell is to know the solutions concentration.
In general if there is a lot of solute in the solvent, the solution is called concentrated, and if there is only a little bit of solute in the solvent it is called dilute. But chemists need a more exact way to know how concentrated or dilute the solution is.
