B3

Unit B Reading NotesPage 1 of 4

B-1 Reading:

Chemistry is the study of the composition, structure and properties of matter and the changes it undergoes. A chemical is any substance that has a definite composition. This means that a sample of a given chemical will have exactly the same composition, no matter where it is found.

We say that a chemical change or chemical reaction occurs when one or more substances is converted into (a) different substance(s).

Every type of matter is made up of particles, and most matter is in one of three states: solid, liquid, or gas. The text talks about the macroscopic and microscopic appearances of the three states of matter. Macroscopic refers to what you can see with your unaided eyes, and microscopic refers to what you would see if you could see individual particles (atoms or molecules). It is easy to show you macroscopic pictures of matter, but microscopic is much more difficult, so we use the concept of microscopic views or particle diagrams (I use the second term most of the time, but you should know that they are the same thing) to help you visualize the arrangement of particles in different states of matter.

The text gives an example with water in Figure 2 on page 6, and the chart below discusses the general macroscopic and microscopic properties of solids, liquids, and gases of a given substance:

State of Matter / Macroscopic Properties / Microscopic Explanation of Macroscopic Properties
SOLID /
  • Fixed volume
  • Fixed shape
/
  • Particles are held tightly together by strong attractive forces so that they have a rigid structure.
  • Particles vibrate about a fixed point.

LIQUID /
  • Fixed volume
  • Takes the shape of container
  • Flows
/
  • Particles are very close together, but not as tightly held as when they are in the solid state.
  • Because the structure is less rigid (and less organized), the particles in this state can slide past each other.

GAS /
  • No fixed volume
  • Takes shape of container
  • Flows
/
  • Particles move independently at very high speeds and are relatively far apart compared to liquid and solid states.
  • Attraction between particles is much weaker than in the solid or liquid states.

Properties and Changes in Matter:

A property is a characteristic that helps define a substance. In general, an extensive property depends on the amount of a given substance that you have. Examples of this are volume, mass, and the amount of energy in a sample of a substance. Intensive properties, such as melting point (MP), boiling point (BP), density, and thermal and electrical conductivity, are independent of how much of the substance you have.

Physical properties are those characteristics that can be observed or measured without changing the identity of the substance (e.g., MP, BP). Any change that occurs that doesn’t change the substance into another is called a physical change (e.g., cutting, grinding, melting, evaporating, etc.). A change of state (liquid, gas, or solid) is always a physical change or process.

A chemical property relates to the a substance’s ability to combine with other substances to become a different substance. Conversely, if a substance is able to break down into constituent substances, that is also a chemical property. We say that a chemical change or chemical reaction occurs when one or more substances is converted into (a) different substance(s). The substances that react in a chemical change are called reactants, and the substances formed in a chemical change are called products. We can describe chemical reactions with word equations:

Hydrogen gas + oxygen gas  water

Substance(s) to the left of the arrow are the (reactants), and substance(s) are the product(s).

There is usually evidence of a chemical change:

  • Every change is associated with an energy change (heat, light, etc)—when energy is released by a system during a physical or chemical process, it is called an exothermic process. When energy is absorbed by a system during a physical or chemical change, it is called an endothermic process (Figure 5c, page9).
  • A gas could be produced (bubbling) (Figure 5a, page 9).
  • Formation of a solid precipitate. If two clear solutions are mixed together and become cloudy, a precipitate has formed (Figure 5b, page 9).
  • A color change occurs (Figure 5d, page 9)

Finally, the products of a chemical change will frequently have very different properties compared to the reactants. The formation of water above is a good example. Both hydrogen and oxygen are highly flammable gases at room temperature. Their product is a clear liquid that is used to put out fires!

B-2 Reading:

Matter is usually defined as anything that has mass and occupies space. (I THINK OF IT AS “STUFF”)—all materials found in nature are matter.

Mass is a measure of the quantity of matter (HOW MUCH STUFF DO YOU HAVE?)

Mass is not affected by temperature, location, or any other factor. We measure the mass of a sample of matter with a balance. Don’t confuse mass with weight, which is a measure of the downward force exerted by gravitational acceleration on an object having a given mass. Because gravitational acceleration can change depending on your position, the mass will be unchanged while the weight can vary.

The standard units for mass are kilograms (kg), but we are more likely to use the unit of grams (g) for our measurements. The two units are related by the following equivalency: 1 kg = 1000 g

Elements and Compounds

A pure substance is a kind of matter, all samples of which have the same properties

3 classifications of matter: Elements, compounds and mixtures

Elements: a substance that cannot be broken down into other substances by ordinary chemical change (Hand out periodic table). Samples of elements have only one type of atom (see the periodic table).

Symbols of the elements:

Abbreviations for the names of elements, mostly 1 or 2 letters—first is capitalized, second one is lower case. Some of them are weird—W for tungsten, Cu for copper, Sb for antimony, etc. These elements derive their symbols from their Latin, Greek, or German roots.

Elements can exist as distinct atoms, monatomic gases (the noble gases that don’t react with much), and diatomic (two-atom) molecules such as the elements nitrogen, oxygen, fluorine, chlorine, bromine, iodine, and hydrogen (N2, O2, F2, Cl2, Br2, I2, and H2).

Some elements will have more than one form. They have different properties, but are still made up of only one type of atom. They are called allotropes.

Compounds: a substance made up of atoms of 2 or more elements chemically combined. They can be broken down (decomposed) into simpler substances by a chemical change—an example is sugar—heat to form carbon and water, electrolyze (split apart with electricity) water and it becomes oxygen and hydrogen.

Compound Characterisitics:

  • Elements making up a compound are combined in definite mass ratios (always 16g of O for every 2 g of H in water). These mass ratios are the same in all samples of a pure compound. We represent compounds with chemical formulas that show the relative number of each type of atom in a single formula unit of that compound. Water has the formula H2O, which means that there are two hydrogen atoms for every oxygen atom in a molecule of water.
  • Chemical and Physical props of a compound differ from those of the constituent elements (props of water—liquid, used to put out fires—different from those of hydrogen and oxygen—gases, flammable)
  • Compounds can be formed from simpler substances and decomposed into simpler substances by chemical change into elements, elements and simpler compounds, or just simpler compounds.

Varieties of Matter—Mixtures

Mixtures consist of 2 or more substances, each of which retains its original props (this is the main difference from compounds).

3 types of mixtures:

  • An element is mixed with 1 or more other elements (e.g., air).
  • An element is mixed with one or more compounds (sugar/salt).
  • One or more elements is mixed with 1 or more compounds.

Characteristics:

  • Element or compound has 1 set of properties, but components in a mixture retain their own properties.
  • While composition of an element or compound is fixed, composition of mixture can vary wildly.
  • Mixtures can be homogeneous or heterogeneous. Those with uniform propertiess are likely to be homogeneous—a sample of this type of mixture will have same composition throughout because the particles are mixed at the molecular level. If a sample of matter has visible “islands” of matter that are different from one another, the mixture is heterogeneous.

We can use the properties of the individual substances in a mixture to separate them out. For example, a mixture of iron metal and sulfur can be separated because only the iron pieces will be attracted to a magnet. You can also use differences in boiling point to purify a homogeneous mixture of two liquids (distillation). Differences in particle size can be used to separate particles by filtration.
PLEASE REMEMBER THAT WE CAN USE MICROSCOPIC VIEWS (PARTICLE DIAGRAMS) to represent elements, compounds, and mixtures.

B-3 Reading

EXPERIMENTAL ERROR ANALYSIS AND PERCENT YIELD

Scientists often question how good their measurements are. When scientists are not sure of the quality of data they assume that there is always some doubt, or uncertainty in the measurements. Sources of uncertainty in measurement are 1) human error in using measurement equipment, and 2) errors generated by poorly calibrated measuring equipment.

It is very important for scientists to understand how good their experimental data is. Scientists often use a concept that I will stress repeatedly in our data analysis is percent error to test the value of their results. The definition that we will use is slightly different from that used in the text (p. 45), and I want you to use this one:

% Error =

It is clear to see from this equation that an experimental value that is greater than an accepted or standard value will give a % error that is positive, while an experimental value that is smaller than a literature standard will have a % error that is negative. We say that our data in these cases exhibit positive or negativedeviation respectively from standard values.

Neither type of deviation is necessarily incorrect—it just shows how your data compares with standard values. The most important thing is for the magnitude of your % error to be as small as possible: ±15% is usually considered an acceptable error range for student experimental work.

Let’s do a sample problem:

Calculate the percent error in a length measurement of 4.25 cm if the correct value is 4.08 cm.

% error = = = 4.2% (Notice that the cm units on the top and bottom cancel out.)

Another way to analyze how good experimental technique is is to do a percent yield calculation:

% Yield =

For the copper lab that we do, the theoretical yield is the mass of copper you start with, and the actual yield is the mass of the copper that comes out of the end of the experiment. True experiments will never provide 100% yield due to losses of materials during container transfer, incomplete purification, and other causes. The value you calculate should NEVER be more than 100%, because it means that you may have introduced materials or incompletely purified your products along the way.