ELEMENTS, MIXTURES, AND COMPOUNDS

Remember that matter is anything that has mass and takes up space.

All materials have physical and chemical properties:

Chemical Property: Are observed when the substance changes its chemical make up

Physical Property: Can be observed without changing the chemical makeup of the substance.

What are some physical properties of iron?

What are some chemical properties of iron?

CLASSIFYING MATTER:

Quantitative: numerical or how much you have

Qualitative: What you have needs no measurement

Lets review the three phases of matter solids, liquids and gases. Here is a good page to review their structure. One might think that a good way to classify matter would be by what phase they are in. However, remember that substances can be found in more than one phase. For example we have ice, liquid water, and steam. All three are different phases of water. So if we classify by phase, where would we put water? Also if we classify by phase we would have many substance together that do not have anything in common. For example gold and sand are both solids but the types of particles that make them up are very different.

A better way might be to classify matter by the chemical make-up of the substance. A good way to look at how to classify matter is to look at the flow chart in your handout.

Homogenous matter: Has the same composition throughout, does not settle out, and particles are dissolved.

Heterogeneous matter: has varied composition, it can settle out (muddy water, oil and water) and the particles are not dissolved (concrete) Therefore heterogeneous matter is classified as a mixture. Some of these, that settle out, are also classified as suspensions

If it is homogeneous it can be a homogeneous mixture or a pure substance. A homogeneous mixture can be separated by physical means (salt water) but a pure substance cannot (water, iron)

Types of homogeneous mixtures (solutions)

First the two parts of any solution are the solute and the solvent. The solute is what gets dissolved and the solvent is what does the dissolving. In salt water the salt is the solute and water is the solvent. You can have many different types of solutions, below are a few examples:

1.)Solid in a liquid = salt water

2.) Gas in a liquid = soda pop (carbon dioxide in water) or in a lake dissolved oxygen in water.

3.) Gas in Gas = Air around us is mixture of mainly nitrogen and oxygen

4.) Liquid in liquid = alcohol in water

5.) Solid in a solid = these are also known as Alloys. For example Brass us a mixture of copper an zinc and stainless steel is a mixture of iron and chromium.

Solubility: Refers to whether you can dissolve one substance in another. For example you can dissolve salt in water but candle wax will not dissolve in water. This is usually due to differences in density or the polarities of the molecules. Liquids and solids that have similar polarities will mix and ones that are dissimilar will not mix. Water and oil will not mix because water is polar and oil is not. Polar means that the molecule has a positive and negative end due to uneven sharing of electrons in a covalent bond. Oil is non-polar means it has not net charge and this is due to the equal sharing of electrons in a covalent bond. (Think about the starch pellets and styrofoam pellets in the acetone and water) Substances as emulsifiers can be used to make two unlike substances stick together. Soap is used to do this when you clean your hands and eggs are used to mix oil and oil together to make substances like mayonnaise. (remember this demo from class?)

When a substance dissolved their is usually an energy change associated with the dissolving process. This is because bonds solute-solute bonds are broken and solute-solvent bonds are made. This means the temperature of the solution will increase (exothermic) or decrease (endothermic) Different substances will cause different changes in temperature. Here is an excellent web site that shows the temperature change as well as a diagram as to what it looks like on the particle level.

General solubility rules:

1.) When dissolving a solid in a liquid usually the amount of solid dissolved can be increased by increasing the temperature of the solution. Here is a copy of a solubility table for some various compounds. Which one is the most soluble? Least soluble?

2.) Not all solids are soluble in water and not all to the same extent.

3.) When a solvent such as water as a solute dissolved in it it lowers the freezing point and raises the boiling point. See this link for a simulation of this.

3.) For gases in liquids the temperature trend is OPPOSITE. That is as you increase the temperature of the solution LESS gas dissolves. That is why you pop goes flat when it sits in a room. As a soda warms up the dissolved gas in the soda warms up and leaves the solution instead of staying in it. This is also why warm water pipes freeze before cold water pipes. Since cold water can dissolve more air, it will freeze slower than warm water even though it has a lower initial temperature. Here is some more information on the hot water/cold water freezing problem.

Characteristics of Mixtures:

1.) They are not chemically combined. If you mix salt and water together, the salt and water does not form a new substance with the water.

2.) Each component of the mixture retains their original properties. Again, if you have a solution of salt and water, you can still taste the salt and of course you still have water in the jar.

3.) The components are not in a definite proportion. If you add sugar to water you have a mixture. You can add a little bit of sugar or a lot of sugar but you still have a mixture.

4.) They can be separated by physical means. If you have a solution of salt water, if you leave it sit long enough the water will evaporate leaving the salt behind. You do not need a chemical reaction to separate the salt and the water.

Chemical changes: New substances are formed. Here the molecules are broken apart and new molecules are formed. They cannot easily be changed back, energy changes occur. Examples: Burning a piece of wood, fireworks, food digestion, etc. Here is the video of the hydrogen balloon from class.

Physical changes: Alter the appearance of the substance but does not alter the chemical make-up of the material. Here the molecules are just moved closer together or farther apart. They can easily reversible. Energy changes still occur. Examples, tearing paper, dissolving salt, melting, freezing, and evaporation.

How can you separate mixture? When separating mixtures you like to use differences in their physical properties to separate them. This allows all components of the mixture to be saved. You could separate a mixture by using differences in their chemical properties. However then one of the components of the mixture will not be in it's original form. In this video, we placed a dollar bill in a mixture of alcohol and water and then separated out the alcohol by a chemical change. Can you see why distillation is usually used to separate this type of mixture? Why or why not? Below are some typical ways to separate most common mixtures.

a.) Distillation or evaporation: An example of this would be separating salt water. If we let the salt water sit out for a few days the water will evaporate and the salt will be left behind. The physical property used here is the difference in their boiling points. By the way you can also separate a mixture of some solid alloys (like brass) by the difference in their melting points. Also two liquids can be separated but then a distillation tube must be used. The following link shows a distillation setup that could be used to separate a mixture of alcohol and water. Since alcohol has a lower boiling point than water when heated the alcohol will boil off first and then condense in the beaker at the end of the apparatus.

B.) Filtration: Is separating sand from water. Water will go through the filter but sand will not. The physical property used here are differences in size of the particles. Picture A at the following link is a good picture of how something is filtered.

C.) Hand-picking: Separating peanuts and cashews from mixed nuts is an example of this. The physical property used here are differences in shape, size, color, etc.

D.) Chromatography: Is using a medium such as paper to help separate the components of a mixture. Here is a sample of the activity that we did in class The physical property used here is the differences in the size of the particles and the affinity for the strip of paper causes the different inks to separate. Here is a picture of a black vis-à-vis pen from class. Here is a picture of two chromatography strips. Do you think they are from the same pen? Why or why not? Finally at this site if you scroll down a little you will find a video of paper chromatography. It takes a awhile to load so make sure you are on a fast Internet connection. It also contains more information on paper chromatography for those of you who find this process interesting.

E.) Decanting: Decanting works well for substances that settle out. For example here is a picture of some muddy water that has settled. If you are careful you can gently pour off the water from the top and leave the dirt below. You may have used a similar technique in your kitchen to separate fat out of a mixture. The physical property used here are the differences in the densities or solubilities of the substances.

F.) Other differences in physical properties can also be used. For example iron can be separated out of a mixture of sand and salt because of its magnetic properties.

Two types of pure substances

If it is a pure substance it can either be an element or a compound. Elements are made up of only one kind of atom and will be found on the periodic table. Compounds are made up of two or more different atoms that are chemically combined. Examples are water, table salt, and gasoline. What is the world made up of mainly compounds, elements, or mixtures?

Elements:

1.) Is the simplest pure substance.

2.) An atom is the smallest part of an element that has the properties of that element

3.) Atoms of the same element are alike and atoms of different elements are different. Gold atoms are alike but different than oxygen atoms.

4.) Atoms are represented by symbols. The first letter is always capitalized and the second is always lower case. These letter notations sometimes came from Latin terms for the element.

5.) Element flash cards

Characteristics of Compounds

1.) The two elements in compound are chemically bound together.

2.) The cannot be separated by physical means.

3.) The compound has new and different properties than the elements that make it up. For example we all know what salt looks like. Go to the periodic table link and check out what sodium and chlorine look like before they are combined. Here is a site that shows the reaction between chlorine and sodium to make table salt. You can either look at the still pictures or the video. The video is quite large and takes some time to download

4.) Elements in a compound are in a definite proportion by mass. For example for water you must have exactly two hydrogen atoms bonded to one oxygen atom.

5.) A molecule is the smallest part of a compound

6.) Compounds are represented by chemical formulae that show how many atoms are in the molecule and what kinds of atoms are in the molecule. For example water is H2O meaning there are two hydrogen atoms for every one oxygen atom.

Here is an excellent link that has a video that shows most of these characteristics in a reaction between sulfur and iron.

Know that you know the main differences between elements, compounds and mixtures, here are some links that have some sample questions that will test your knowledge on them.

1.) Particle questions on elements, compounds, and mixtures.

2.) Sample fill in the blank quiz on elements, compounds, and mixtures.

3.) How far can you get in this fake money game on elements, mixtures, and compounds.

Chemical reactions: To have a chemical reaction you must have a chemical change.

Endothermic: Energy is Absorbed

Exothermic: Energy is given off

Take this quiz to see if you can tell the difference between a physical and chemical change

Indicators of a chemical reaction. The following indicate that a chemical reaction MIGHT be occurring.

1.) disappearance of a substance

2.) formation of a substance (precipitates)

3.) evolution of a gas

4.) light

5.) temperature changes