Pure Substances and Mixtures

Pure Substances Elements cannot be broken down chemically into anything else – they are pure substances. Compounds can be broken down into their component elements, but are still considered pure substances. Pure substances are homogeneous throughout At a given temperature and pressure, pure substances have definite densities

Mixtures A mixture is a combination of two or more pure substances Mixtures can be homogeneous or heterogenous Homogeneous mixtures are called solutions. Solutions can be composed of solids, liquids, or gases, or some combination of the three.

Solvents and Solutes The major component of a solution is the solvent. The minor component(s) of a solution are solutes. Examples: Air: Solvent-N 2 Solutes O 2, Ar, H 2 O, CO 2 Seawater Solvent- water; Solutes: sodium chloride, oxygen, carbon dioxide, minor salts of Ca, Mg, K, Br, F, etc

Aqueous Solutions Since 70% of our planet is covered with water, we usually talk about aqueous solutions more than any other type of solution. Because it is asymmetrical, with highly polar bonds, water has the ability to dissolve many (but not all) ionic compounds. Ionic bonds are disrupted by collisions with huge numbers of water molecules

Aqueous Solutions Polar covalent compounds also dissolve well in water. Polar covalent compounds contain electronegative elements (O, N, F, Cl) that create unequal charge distributions within molecules.

Separating Components of Mixtures The components of mixtures have differing physical and chemical properties. Separating mixtures into pure components involves taking advantage of the physical differences between pure substances.

For example, How would you separate the salt from sea water? Water and sodium chloride have different types of bonding, and respond differently to heating. Melting points, boiling points, solubility, magnetism – these are examples of physical properties that can be used to separate mixtures. Separating two liquids: distillation Separating two solids: smelting

Lab 12 Separating Components of a Mixture Purpose To classify the components of a mixture and determine its % composition by mass. Background You will be given a mixture of three substances. Your teacher will give you some information about the composition of the mixture. Given a variety of equipment, you will determine the mass of each component and calculate its percentage.

Procedure Make a list of each piece of equipment you use. Write a procedure and check with the teacher to see if it is valid. Draw a labeled diagram of any apparatus you use. Data Make a data table that includes the mass of any component or piece of lab equipment, if necessary. Calculations Show all calculations for the your percent composition of your mixture

Questions Define the following terms: a) elementb) compound c) homogeneous mixtured) heterogenous mixture e) solution Classify each component of your mixture as an element or a compound. Give a reason for each classification. If all the students in the class received a sample of the same mixture, would each person (or team) obtain the same result for the percentage composition? Why? If the purpose of the experiment was to determine the % composition of a compound, rather than a mixture, should each person (or team) obtain the same result for % composition? Why?

If the purpose of the experiment was to determine the % composition of a solution, should each person (or team) obtain the same result? Why? Does your experiment use physical or chemical changes to separate components of the mixture? Explain. There are a number of sources of error that are likely in this lab. These include deficiencies in your procedure, or problems inherent in separating mixtures. Cite two examples of experimental error in your % composition determination.