GED Science Practice Test: Types Of Mixtures And Solubility

The properties of matter can determine how two substances combine together when mixed.  A mixture consists of two or more substances that are combined together.  Mixtures can be separated by physical means, which means that they have not chemically reacted together (see the unit on chemical reations).  Because the substances in a mixture have not chemically reacted together, the individual substances retain their properties.

There are two main categories of mixtures:

  • Homogeneous Mixtures – in which the two or more substances that form the mixture are evenly distributed throughout the mixture. There are two main examples of homogeneous mixtures:
    • Solutions – One substance (called the “solute”) is dissolved in another substance (called the “solvent”), e.g. salt water is salt dissolved in water – in such a way that the salt no-longer exists as solid particles within the water.
    • Alloys – A combination of two or more metals. Common examples of alloys include bronze, brass and steels.
  • Heterogeneous Mixtures – in which the two or more substances that form the mixture are not evenly distributed throughout the mixture. The combination of oil and water found in some salad dressings is an example of a heterogeneous mixture. Another example is particles of sand in water.

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In a solution with two substances, the first  substance dissolves, and is no longer visible as a separate substance (e.g., salt dissolves in water and is no longer visible). Sand and water, however, does not form a solution because sand does not dissolve in water.  The property that determines whether or not a substance will dissolve is called solubility. The solubility of a substance depends on the physical and chemical properties of the substances as well as on temperature, pressure and the pH of the solution.  For example, due to differing chemical structures, oil will is not soluble in water. Another example is being able to dissolve more sugar in warmer water. The sugar is said to be more soluble in hot water.

In chemistry, a solution involves two substances: a solute and a solvent. A solute is the substance you are trying to dissolve. The solvent is the substance that does the dissolving. The concentration of a solute in a solution is a measure of how much solute is dissolved in the solvent. The maximum amount of solute that can be dissolved into a specific solvent is called the saturation point. If solute is added beyond the saturation point, it will not be dissolved and will begin to precipitate out of the solution, meaning that it will remain undissolved.  In order to understand the idea of a saturation point, observe the following graph showing the solubility of salt and sugar at different temperatures:

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The saturation point of sugar when water is at a temperature of 20°C can be found by finding this point on the curve for sugar.  At this temperature, the maximum amount of sugar that can be dissolved is 200 grams in 100 mL of water.  200 g/mL is the saturation point for sugar in water of this temperature.  If 201 grams of sugar are put into water at this temperature, not all of the sugar will dissolve, and some will settle out to the bottom.  If the water is heated up, the saturation point of sugar increases.  In other words, more sugar can be dissolved into warmer water than in cooler water.  You might be familiar with rock candy.  Rock candy is made by first dissolving large amounts of sugar in hot water, and then allowing that solution to cool.  As the solution cools, it cannot hold all of the sugar that was originally dissolved in it.  A solution which has more solute dissolved than it can naturally hold is called a supersaturated solution.  The extra sugar precipitates out and forms the rock candy crystals you are familiar with.

Another interesting property revealed by the solubility graph above is that the solubility of salt does not increase much with increasing temperature of the water.  Thus, it would be difficult to create a supersaturated solution of salt, because heating water would not allow much more salt to be dissolved.

Many solutions we have used as examples are based on water as a solvent. These solutions are known as aqueous solutions (aqueous means ‘referring to water’ in Latin). However, there are other solvents, such as alcohol, oil, etc.  Additionally, we have mostly been talking about solid solutes and liquid solvents.  However, gases and liquids can also be dissolved in liquids.  For example, oxygen is found dissolved in water.  Gases can be dissolved in other gases.

 

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