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Concentration and Molarity Help

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By — McGraw-Hill Professional
Updated on Aug 28, 2011

Introduction to Concentration and Molarity

In the laboratory, chemists must make use of what they have on hand. It would be a waste of time to run out and buy 20 different concentrations of hydrochloric acid, for example, when they could dilute down the concentrated hydrochloric acid on the shelf and get whatever lower concentration was needed.

Solutions

A solution is a homogeneous mixture of a solute , the element or compound that dissolves, in a solvent , the solution that disperses the solute. Solutes and solvents can be elements or compounds. Commonly, the larger sample is called the solvent and the smaller sample is called the solute. When salt is dissolved in water, the salt is the solute and water (known as the universal solvent ) is the solvent.

A solute is an element or compound dissolved to form a solution. A solvent is a liquid to which an element or compound has been added to form a solution.

To understand how atoms, elements, and molecules interact in solutions, it is important to understand concentration. You have seen how a high concentration of an element or solution can be much different from a low concentration. Consider salt water. Different concentrations can have very different effects. A physiological saline, 0.85% NaCl solution, given intravenously and used to stabilize patients’ fluid levels in hospitals, is much different from the highly concentrated salt brine (30−50% NaCl) used to preserve codfish. In fact, confusing the two could be deadly.

When two or more liquids are able to form a solution, they are called miscible . Alcohol is miscible in water. In fact, there is an entire area of study in physics called fluid dynamics that tests the miscibility of fluids and the way they flow at different concentrations.

When two liquids don’t mix, like oil and water, they are immiscible . Picture a lava lamp. There are generally large round globules of “lava,” made of a specially compounded and colored wax, floating lazily around in a specially formulated liquid (most likely water). This mixture is heated with a light bulb in the base of the lamp that causes the lava to heat and expand until it becomes less dense than the contained liquid above. When this happens, the lava begins to rise to the top of the container where it slowly cools. When the lava is cool enough to sink to the bottom, it is heated again and the process is repeated. This is truly an example of chemistry as entertainment.

A colloid is like a homogeneous solution, but it is made up of larger particles of one solution mixed and spread all through another solution; think of chocolate chip cookie dough. There are two parts to a colloidal mixture, the dispersing medium and the dispersed phase . The dispersing medium is the substance in a colloidal mixture that is in the greater amount, like the cookie dough. The dispersed phase is the substance in the colloidal mixture that is in the smaller amount, like the chocolate chips.

Colloidal mixtures are not true solutions. Remember, in solutions the solute dissolves in the solvent. In colloidal solutions, the components don’t dissolve, they just mix. When the parts of a compound mix in this way, they are said to have colloidal properties.

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