Solutions But first a look at the big

Solutions

But first, a 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 everything you pick will be a mixture. )

But first, 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.

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.

Mixtures are not pure substances; their composition can vary. In each example above, there’s more than one thing there!

Mixtures of hydrogen and oxygen could be 1% oxygen, 5% oxygen, 18% oxygen or 64% oxygen. But in a compound (like water), the % oxygen is always the same.

Old TAKS question: u Which of these are composed of two or more different substances that are chemically combined in a definite ratio? F Compounds G Mixtures H Elements J Solutions

Another TAKS question: What characteristic of water remains the same no matter what is dissolved in it? A The ratio of hydrogen to oxygen B The ability to refract light C The hydroxide ion concentration D The freezing temperature

Check your understanding: How many of these are mixtures? (And which ones are they? )

Another practice: Given the diagrams X, Y, and Z below: Which diagram or diagrams represent a mixture of elements A and B?

Identify each as an element, compound, or mixture.

Back to the big picture… Note: A pure substance can be represented by a chemical formula. Examples: Fe, H 2, Na. Cl, CO 2 Give four more examples in your notes.

Mixtures

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 mixture that would be heterogeneous?

A solution is a homogeneous mixture: has the same properties throughout “homo” = the same

Solutions occur in all phases u u u Solid solution: steel Gas solution: air Liquid solutions: most common in chemistry

Alloys u An alloy is a solution where two or more solids are combined. • A homogeneous mixture of metals Why use alloys instead of pure metals? • Can increase strength and durability by combining properties of different metals

Now Practice! Are these examples of elements, compounds, homogeneous mixtures, or heterogeneous mixtures? 1. Air 2. Bronze 3. Calcium 4. Raisin Bran 5. Orange juice (with pulp) 6. Salt water

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

Recap: Solutions • Solution: homogenous mixture ~ Mixture: parts are not chemically combined ~ each component of the solution keeps its identity ~ won’t settle out composed of solute and solvent gets dissolved does the dissolving

Solutions • name the solute/solvent in the following solutions… solute salt water atmosphere brass (Zn and Cu) salt other gases Zn solvent water N 2 Cu

Why does anything even dissolve? Why doesn’t it just sit there?

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 nonpolar 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 a partially positive end.

It’s the polarity of water that makes it such a good solvent: Let’s see how it works: http: //www. mhhe. com/physsci/chemistry/essentialchemistry/flash/molvie 1. swf

A little more vocabulary… Saturated solution: a solution that is holding all the solute it can hold at a given temperature (Think of a saturated sponge: it can’t take any more!) u Unsaturated solution: a solution containing less than the maximum it can hold (Like a dry sponge: it can still hold more) u

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.

• Which substance is most soluble at 40°C? Na. NO 3 • How many grams of NH 4 Cl will dissolve at 50°C? • What two substances have the same solubility at 24°C? • How many grams of Na. NO 3 will dissolve in 300. 0 g of water at ~ 240 grams (80 x 3) 10. 0°C? ~ 50 grams KNO 3 and Yb 2(SO 4)3

A future test question: u How does soap work? If your mother cooks bacon, and asks you to clean the pan, will the pan get clean if you just rinse it off?

Soap CH 2 CH 3 CH 2 OP CH 2 O- O-

Soap CH 2 CH 3 CH 2 u Soap has a long non-polar end P CH 2 O- CH 2 O- O-

Soap CH 2 CH 3 CH 2 OP CH 2 u And end O- O- a polar

O- CH 2 CH 3 CH 2 P CH 2 _ O- O-

u. A drop of bacon grease in water u Grease is non-polar u Water is polar u Soap lets you dissolve the non-polar in the polar.

Non-polar ends dissolve in grease

Polar ends dissolve in water

u Soap molecules tie the grease to the water, so when you rinse away the water, the grease goes along with it!.

Factors that affect the rate of dissolving: 1. 2. 3. Temperature (solutes dissolve faster in hotter solvents) Why? Surface area (sugar granules dissolve faster than a sugar cube of equal mass) Stirring (brings fresh, unsaturated solvent in contact with the solute)

Dissolving Gases u If you’re trying to dissolve a gas in a liquid, 2 factors affect solubility: • Temperature of the solvent (less gas will dissolve in a warmer solvent) • Pressure (increasing the pressure will increase the amount of gas you can dissolve)

Practical Applications: u This gas solubility – temperature relationship can be remembered if you think about what happens to a soft drink as it stands around for awhile at room temperature. It gets flat since more of the carbon dioxide bubbles have escaped.

Practice Question: u Thermal pollution is merely heat that has been transferred to water or air. You may think that heat is not really pollution. But how is the concentration of dissolved oxygen in water affected by thermal pollution?