Chemistry Exam Review Packet

Chemistry Exam Review Packet A. Separating Mixtures

Mixtures are not unique to chemistry; we use and consume them on a daily basis. The beverages we drink each morning, the fuel we use in our automobiles, and the ground we walk on are mixtures. Very few materials we encounter are pure. Any material made up of two or more substances that are not chemically combined is a mixture.

The isolation of pure components of a mixture requires the separation of one component from another. Chemists have developed techniques for doing this. These methods take advantage of the differences in physical properties of the components. Mixtures are all able to be separated by using physical properties. No chemical changes are involved, so the substances will retain their chemical identity throughout the separation process.

The method used to separate the components of a mixture depends on the physical properties of the components. Since substances have different physical properties, different mixtures require different methods to separate them. Some methods are more effective than other methods.

Magnetism

If one component of the mixture has magnetic properties, you could use a magnet to separate the mixture. Iron, nickel, and cobalt are all materials that are magnetic.

Hand Separation

You can separate the components of a dry mixture by manually picking out each component. This is most effective when the substances to be separated are large enough to be seen clearly. For example, the parts of a salad can easily be separated using your fork.

Filtration

Sometimes you will need to separate solid matter from a liquid. In the process of filtration, the mixture is passed through a porous material. This separates the non soluble solid from the liquid. Filters range widely in sophistication. Common ordinary filter paper (as might be used in a coffee maker) is inexpensive. Cloth and sand are also inexpensive materials that can act as a filter. However, the filter used in an oil filter for a car costs a bit more and so on. These materials allow the liquid to pass through but not the solid. You use this process when you strain spaghetti or make coffee. The spaghetti remains in the colander while the water continues through and coffee grounds are caught by a coffee filter while the water can pass through.

Sifting (Sieving)

You can separate a dry mixture containing substances of different sizes by sifting. In this process, materials of different sizes are passed through a sieve, a device containing tiny holes. Smaller grains pass through the holes leaving the larger-sized solids behind. A sieve is a common beach toy. You can scoop up sand and shake it side to side. The larger pebbles and shells are left in the sieve while the smaller sand grains pass through. Sieves can come in many different sizes.

Extraction and Evaporation

In the process of extraction a soluble solid like salt and be separated from an insoluble solid like sand. You could try to manually separate the sand and salt using a magnifying glass and tweezers, but is there an easier way? Is there some property that salt has that sand does not? Salt dissolves in water and sand does not! In extraction a solvent is used to selectively dissolve one component of the solid mixture. For example, mix the sand/salt with water. The salt dissolves, the sand does not. When this mixture is then poured through a filter, the sand is separated from the salt water.

How could you separate the salt from the water? The answer is evaporation. Changing a liquid into a gas can often separate liquid mixtures. This can be done by natural evaporation or by boiling the liquid mixture. When the saltwater is heated, the water evaporates leaving the salt behind.

Chromatography

You can separate numerous dissolved substances in a solution using chromatography. For example, paper chromatography can be used to separate an ink solution. In this case, ink from a felt-tip pen is dissolved in alcohol. A tiny dot of ink is placed at the bottom of a piece of absorbent filter paper. The very end of the filter paper is then placed into the alcohol. The alcohol soaks into the absorbent filter paper carrying the ink with it. Colored ink is a mixture of several pigments which bind to the paper to different extents. Those pigments that hold loosely move quickly up the paper compared to those that hold more firmly. This results in the separation of different pigments on the filter paper. B. The Periodic Table

Scientists all over the world use the same symbols for the elements. Chemical symbols are a shorthand way of writing the chemical names of the elements. Symbols for the elements usually consist of one or two letters. Some man-made elements have three letters in their chemical symbols. If the chemical symbol is just one letter, that letter is a capital letter. If the symbol has two or more letters, then the first letter is a capital and the following letters are lower case. This standard for writing symbols for the elements makes it easy for scientists to identify the specific elements. In addition to having a chemical symbol, each element has an atomic number. The atomic number is the number of protons in an atom’s nucleus. This number is also usually equal to the number of electrons in the atom as well. The atomic number is a unique property that identifies an element. No two elements have the same atomic number. Elements are arranged on the Periodic Table according to their atomic number. Atomic numbers increase as you read the elements from left to right.

Another important property of elements is atomic mass. Atomic mass is equal to the number of protons and number of neutrons in an atom. Since electrons are so small, their mass is not figured into the atomic mass. The atomic mass that appears on the periodic table is the average mass of an element’s atoms. Atomic mass generally increases with atomic number.

All of the elements are organized on the Periodic Table of the Elements. On the right side of the periodic table, you will notice a zigzag line (See Figure 1). This line is very important in distinguishing among elements. All of the elements that are located to the left of this zigzag line are considered to be metals. Elements that are to the right of the zigzag line are considered to be nonmetals. Metals and nonmetals have very different properties. As a result, metals and nonmetals will combine to form new substances.

In addition to the zigzag line, the periodic Group table contains vertical columns of elements Figure 1 as well as horizontal rows of elements. The vertical columns are called groups. There are eighteen groups of elements (See Figure 1). Within each group, the elements have similar but not identical properties. These Period properties are determined by the valence electrons for an element’s atoms. Valence electrons are the outermost electrons of an atom. The electrons are important because they are involved in the chemical bonding of elements to make compounds. As a result, these electrons help to determine the properties of the elements in a group. All of the elements in a given group have the same number of valence electrons. For example all of the elements in Group 1 or IA have one valence electron. This characteristic gives the elements in the family similar properties. In general, all metals have 1, 2, or 3 valence electrons. Nonmetals will have 4, 5, 6, 7, or 8 valence electrons. Lastly, there are seven horizontal rows or periods (See Figure 1) on the periodic table. Unlike the elements in each family, the elements in each period do not share common properties. The properties of the elements change dramatically across any given period. However, there is still a pattern to the periods of elements. The first element in a period is always an extremely active metal. The last element in a period is always an inactive gas. For example, in period 3, sodium (Na) is an active metal while argon (Ar) is a stable, non-reactive gas.

As stated earlier, elements in the same group have similar properties. Metals usually react by losing electrons to other atoms. The reactivity of metals tends to decrease as you move from left to right across the periodic table. The metals in Groups 1 and 2, such as magnesium and sodium, are the most reactive elements and are never found by themselves in nature; they are always combined with another element. The metals in Groups 3 through 15, such as gold and silver, are less reactive. Their lack of reactivity is the reason for why gold coins and jewelry from thousands of years ago are still just as beautiful today.

Nonmetals usually react by gaining or sharing electrons with other atoms. Most nonmetals, such as chlorine, fluorine and oxygen, are highly reactive and readily form compounds. However, Group 18 elements, such as helium and neon, hardly ever react to form compounds because they do not lose or gain electrons.

C. States of Matter

The three states of matter are solids, liquids, and gases. In a solid, the molecules vibrate in place and form a rigid, regular structure. There is very little space between the molecules. It is because of these molecular behaviors, that a solid will retain its shape and volume regardless of the container. As heat is applied, the solid will melt into a liquid because the heat causes the molecules to move faster and farther apart. Because of this, the molecules of a liquid will slide past each other allowing the liquid to flow and causing it to take the shape it its container but not change in volume. If the liquid is heated, it will eventually turn into a gas. In this state, the molecules are moving very rapidly and thus are spreading far apart. This causes gases to take the shape of their container and also change in volume when put in different sized containers.

Solid Liquid Gas D. Conservation of Matter

The Law of Conservation of Matter states that no matter how many reactants and products are in a chemical reaction, all of the atoms present at the beginning of the reaction are present at the end of the reaction. The arrangement of the atoms might change, but the total number of atoms must remain the same.

Since the amount of matter in a chemical reaction does not change, the total mass of the reactants must equal the total mass of the products. This principle is called the conservation of matter. Simply stated, during a chemical reaction, matter is neither created nor destroyed. It simply changes form. The following equation shows a chemical equation that represents the conservation of matter: Fe + S  FeS

In this equation, one atom of iron reacts with one atom of sulfur to produce a new compound. Even though the atoms are rearranged, the number of atoms for each element in the reaction equals the number of atoms of each element in the product. Therefore, the mass of the reactants must also equal the mass of the product.

E. Acids and Bases Every liquid you see will probably have either acidic or basic traits. One exception might be distilled water. Distilled water is just water and is considered to be neutral-neither an acid nor a base. Acids generally taste sour, corrode metals and turn blue litmus paper red. Bases on the other hand, taste bitter and don’t react with metals but instead feel slippery to the touch. Bases turn red litmus paper blue.

Scientists use something called the pH scale to measure how acidic or basic a liquid is. The scale goes from values very close to 0 through 14. Distilled water is 7 (right in the middle). Acids are found between a number very close to 0 and 7. Bases are from 7 to 14. Most of the liquids you find every day have a pH near 7. They are either a little below or a little above that mark. When you start looking at the pH of chemicals, the numbers go to the extremes. If you ever go into a chemistry lab, you could find solutions with a pH of 1 and others with a pH of 14. There are also very strong acids with pH values below one such as battery acid. Bases with pH values near 14 include drain cleaner and sodium hydroxide (NaOH). Those chemicals are very dangerous. F. Chemical and Physical Changes

There are two kinds of changes in matter: physical and chemical. Some examples of physical changes include what happens to paper when you tear it, to a nail when you bend it, or to wool when you spin it into yarn. A physical change alters the form of a substance, but does not change it to another substance. Matter can also change by means of a chemical change. When a substance undergoes a chemical change, it is changed into a different substance with different properties. Burning wood is a good example. The wood is changed into completely different substances, such as carbon dioxide gas and solid ash.

The term chemical reaction is another name for a chemical change. In some chemical reactions, one substance breaks down into two or more other substances. In other reactions, two or more substances combine, forming one or more new substances. Or compounds may change into other compounds. The one thing all chemical reactions have in common is that new substances are produced.

Evidence for Chemical Reactions: a. Color change—a color change is often a sign that a chemical reaction has occurred. The brilliant colors we see in autumn leaves are the result of the chlorophyll in the leaves breaking down allowing colors of other substances in the leaves to become visible. b. Precipitate—is created when two solutions react to create a solid. The presence of a precipitate tells you a new substance has formed. c. Gas Production—oxygen bubbles formed during photosynthesis collect on the leaves of underwater plants. Oxygen is a product of the reaction between carbon dioxide and the water inside the cells of a plant. d. Changes in Temperature—an increase or decrease in temperature can result from the changes in energy during a chemical reaction. e. Changes in Properties—sometimes the result of a chemical reaction are products with very different properties than the reactants. For example, when leaves are burned, the result is ash which has very different properties than leaves.