Pure Substances and Mixtures

Look at the big picture… u Pick out any object in the room, and it can be one of two things: a pure substance or a mixture (Almost anything you pick will be a mixture.)

A look at the big picture…

Elements are pure substances… Elements are substances that are composed of only one kind of atom. Elements cannot be broken down by chemical change.

An element cannot be broken down by a chemical change (chemical reaction): Compounds can be broken down into elements: CaCl 2  Ca + Cl 2 But an element can’t be decomposed: Fe  ?

Practice question: u Which of these could not be decomposed in a chemical reaction? A. Sodium B. Carbon dioxide C. Carbon D. Sodium chloride

Compounds are also pure substances… A compound is a pure substance made of different elements, chemically bonded together. ALWAYS in the same proportions. A compound has a chemical formula.

Compounds are pure substances u A compound has definite and unchanging composition and properties. u Water, for example, is always H 2 O. Always 2 hydrogen atoms for every 1 oxygen atom.

Practice question: An unknown liquid is heated. At 85 o, the liquid starts boiling, and about one-third of the liquid turns to gas. The remaining liquid doesn’t boil until the temperature reaches 135 o. Is the unknown liquid an element, a compound, or a mixture? Why?

Mixtures are not pure substances; their composition can vary. When two substances are physically mixed together, they keep their own properties. When iron is in a mixture, it keeps its magnetic properties. If iron is in a compound, it loses its own properties.

Practice question: u Which of these are composed of two or more different substances that are chemically combined in a definite ratio? (Hint: chemically combined means chemically bonded.) F Compounds G Mixtures H Elements J Solutions

Identify each as an element, compound, or mixture.

Back to the big picture… Let’s talk about mixtures…

Mixtures u the parts of a mixture are physically combined, but not chemically bonded together. u the parts of a mixture can be separated by physical means, like filtering or boiling.

Heterogeneous Mixture u Mixture that does not have the same properties throughout the mixture u Individual substances remain distinct u Can you think of another example of a heterogeneous mixture?

Granite is a heterogeneous mixture.

A solution is a homogeneous mixture: has the same properties throughout

Memory trick: “hetero” means different. “homo” means the same.

Recap… u If all portions of a mixture are in the same state, have no visible boundaries, and are uniform throughout, then the material is homogeneous. Examples of homogeneous mixtures are the air we breathe and the tap water we drink. Homogeneous mixtures are also called solutions. Thus air is a solution of nitrogen, oxygen, water vapor, carbon dioxide, and several other gases; tap water is a solution of small amounts of several substances in water. The specific compositions of both of these solutions are not fixed, however, but depend on both source and location; for example, the composition of tap water in Boise, Idaho, is not the same as the composition of tap water in Buffalo, New York.

Which picture below shows a heterogeneous mixture?

AAn element BA compound CA homogeneous mixture DA heterogeneous mixture A mystery powder in the lab could not be separated by any physical changes. However, when this powder is put over a flame, a chemical change takes place, with a gas of carbon dioxide being produced along with a different looking powder. Based on this evidence, which type of matter is this mystery powder most likely to be?

Solutions

Solution Vocabulary u The solvent does the dissolving: whatever you have more of. u The solute is dissolved; it’s the thing you have less of. u Memory trick: “solvent” has more letters than “solute,” so it’s the thing you have more of.

Why do some things dissolve in water? Why don’t they just sit there and get wet?

What if you were soluble? (click on the link below) The Soluble Song The Soluble Song

Why does anything even dissolve? Why doesn’t it just sit there? Solubility guideline: “Like dissolves like.” Polar solvents (like water) dissolve polar things. Non-polar solvents (like oil) dissolve non- polar things.

Polar? Non-polar? Something is polar if it has two opposite ends. The earth is polar because it has a north pole and a south pole. A person is bi-polar if they have two opposite moods.

Polar? Non-polar? u In chemistry, a molecule is polar if it has one end with a + charge and one end with a – charge.

Polar? Non-polar? What determines whether a molecule is polar? Remember electronegativity? Electronegativity is the ability of an atom to attract shared electrons towards itself.

u When atoms share electrons in a covalent bond, they have a tug of war over the electrons. If the two atoms have different electronegativities, they won’t share the electrons evenly. One atom will pull harder on the electrons than the other atom.

Oxygen has a higher electronegativity than hydrogen, so oxygen wins the electron tug-of-war. The electrons spend more time on the oxygen end of the molecule.

Be sure to know that water is a polar molecule because the oxygen atom does not share the electrons equally with the hydrogen atoms. Therefore, a water molecule has a partially negative end and a partially positive end.

It’s the polarity of water that makes it such a good solvent: Let’s see how it works:

How could we make that salt crystal dissolve faster? Factors that affect the speed of dissolving: temperature (the water molecules will move faster at higher temperatures – like playing that animation at faster speed) stirring or agitation (brings fresh water molecules into contact with the salt) surface area (grinding up the solute into smaller pieces increases the area of contact between the salt and water)

Solubility u Solubility: the amount of a solute that will dissolve in a given solvent u What factors affect solubility?

Temperature Effects u Higher temperature usually increases the solubility of a solid in a liquid. But not always! u We must read the effect of temperature from a graph of experimental data. u The graphs are called solubility curves.

Solubility Curves Use the graph in your notes to answer the questions follow.

Saturated Solutions You read from the solubility curve that 36.0 g NaCl will dissolve in 100 mL of water grams is the most you can dissolve. If you dissolve 36 grams, you will have a saturated solution.

Saturated Solutions Supersaturated solutions can be made by heating the solvent, dissolving a lot of solute, and letting it cool off.

Making a supersaturated solution: Click here Click here

Does water conduct electricity? Actually, pure water does not conduct electricity!

Solutions that contain ions can conduct electricity: u When electricity is applied to a solution, ions in solution migrate toward the oppositely charged rod or plate to complete an electrical circuit, but neutral molecules in solution do not. Thus solutions that contain ions conduct electricity, while solutions that contain only neutral molecules do not.

Electrolytes and nonelectrolytes

Last definitions: u An electrolyte is any compound that can form ions when dissolved in water. Electrolytes may be strong or weak. u Strong electrolyte: A compound that dissociates completely into ions, thus producing an aqueous solution that conducts electricity very well. u Weak electrolyte: A compound that produces relatively few ions when dissolved in water, thus producing an aqueous solution that conducts electricity poorly.

The Summary Video u click here click here