Chemistry is a subdivision of science and it specializes in the composition, structure, and properties of substances and the transformations in which these substances go through. This might sound confusing to some, but really it is quite simple to understand. Chemistry can be found all over the world, even in such basic things as cooking. For instance, when you boil water, Chemistry explains why water evaporates and why salt will dissolve in it. Basically, Chemistry studies chemicals (hence the name Chemistry) and applies this knowledge to modern society. To understand how to apply the Chemistry knowledge, you must first understand the basic principles behind Chemistry.
In all sciences, there is a system that describes how to go about solving problems. This system is called the Scientific Method. Basically, there are four parts to this system, that are very logical in order. First, an observation must be made. Observations can be either in the form of quantitative observations or qualitative observations. Quantitative observations have to do with numbers, whereas qualitative observations have to do with a physical characteristic of something. For example, the observer observes a two winged red butterfly. The fact that the butterfly has two wings is a quantitative observation and the fact that the wings are red is a qualitative observation. This leads to the second part of the scientific method. After making many observations, the observer formulates a question, which is normally, why did that happen? From there, the observer looks for possible reasons why, or what is called formulates theories. A theory is the observer's interpretation of the data that he has collected. The first set of theories is called a hypothesis. A hypothesis is simply an educated guess on the part of the observer to explain the question at hand. The fourth and final step is for the observer to test his theory by experimentation. If the theory is wrong the observer goes back and formulates a new theory, and if he is right, he goes back and retests his theory.
Science is in no way perfect. People do make mistakes, this is part of the human nature, but as technology improves and more people test the previous theories, the wrong ones get sorted out from the right ones and science corrects itself.
In science and chemistry, measurements are very important. Measurements are quantitative observations. An example of a measurement is that the desk is 4 feet long. Measurements in science pose a big problem to its users. This is true because there are two different systems in use. Americans use a system called the English system. The English system uses units like inches, feet, gallons, and miles, and there is no easily remembered relationship between the units, so a world-wide system of measuring was established called the Metric System, or SI system. This system is based on multiples of ten. For example, if you have 1 meter, you have 100 centimeters, 1000 millimeters, and .001 kilometers. This is obviously easier to understand.
In the SI system, there are base units for measurement. Examples of these units are meter for length, liter for volume, and gram for mass. What makes this system even easier than the English system is that the prefixes are standard for all the base units. So if you have a kilogram of water, you have 1000 grams of water and if you have a kilometer of road, you have 1000 meters of road.
It is important to understand that in science, when you take measurements the last number is always estimated. For example, if the ruler you were using to measure a piece of paper was calibrated to centimeters (meaning the smallest marked units were centimeters) and the length came out to 20 centimeters, this answer would not be acceptable because the answer has to have an uncertain value which would be the millimeter's value. (Measurements must always be one decimal place more exact than the smallest marked units on the measuring device.) In the example above, 20.0, 20.3, or 19.8 would all be acceptable values.
In the beginning of Chemistry, there are always a few terms which must be defined because their definitions are very close to each other or are important. One of the most general words in Chemistry is matter. It is important to understand matter because everything in the universe is made up of matter. Matter can exist in three phases, solid, liquid, and gas. (Plasma)Solids are rigid structures which have definite volume and definite shape. Due to the strength of solids, they are not easily compressed. Liquids are the second state of matter that matter can exist in. Liquids have definite volume, but lack definite shape. This is why liquids will fill containers with odd shapes, but are not easily compressible. Gases are the third and final state in which matter can exist in. Gases do not have definite volume and do not have definite shape and this is why they will fill containers with odd shapes and are easily compressible.
The first set of words that might pose a problem to a beginner in Chemistry is mass and weight. In today's society, mass and weight may be used interchangeably, but in Chemistry they are very different. Mass is the amount of matter that an object has. An example of the difference between mass and weight is that a person on earth may have a mass of 220 kilograms but weigh 100 pounds and on the moon that same person will have a mass of 220 kilograms and weigh 16.7 pounds. The reason why the weight changed was because the moon's gravitational pull is much less than earth's, but take note to the fact that the mass never changes no matter what the weight does.
Another set of words that must be defined is precision and accuracy. Once again, these words are very easily interchanged in today's society, but in Chemistry they have very different meanings. Precision refers to how close the values are to each other. Accuracy is how close the values are to the true value.
To understand precision and accuracy more, lets assume that three student wants to find out what the boiling temperature of water is. We all know that water boils at 100 degrees Celsius, so this is our true value. Below are the results of the three trials of the three students.
Student 1 Student 2 Student 3 Trial 1 102.5 102.6 100.4 Trial 2 95.6 102.5 100.0 Trial 3 99.4 102.4 100.2
Student 1's results were obviously all over the place, so he was neither accurate nor precise. Student 2's results were all very close together which means that his results were precise, but all his results were not close to 100 so, his results were not accurate. On the other hand, Student 3's results were both close together and close to 100, so his results were both precise and accurate.
In Chemistry, there are two main types of error. One is called systematic error. This type of error occurs when the values that are found are consistently high or consistently low. An example of why systematic error might occur is that a thermometer might be miscallibrated. (Student 2, the example above had systematic error.) The other type of error is random error. This type of error occurs when the chances of being high or being low are equal. For example, if you throw five darts at a dartboard, you have equal chance of being high or low. (Student 1, in the example above had random error.)
One of the most tedious parts of Chemistry is Significant Figures, or more commonly called Sig Fig's. Have you ever heard someone say that a chain is only as strong as its weakest link? This is what happens with Sig Fig's. An answer can only be as exact as its least exact number , so if two and two are multiplied together the answer is obviously four, but what if you measured one value at 4.1 (remember that .1 is estimated) and multiplied it by another value that you measured at 5.526? The answer can't be 22.6566, because how can you have your answer more exact that the least exact value that you have. Because 4.1 is the least exact value, the final value can only be 23 because of the rules that the scientific community came up with to deal with this problem called the Rules of Sig Fig's.
TRY IT FOR YOURSELF: Practice Problems
Temperature, in Chemistry, plays many important roles, especially in solving equations. There are three main units in which temperature can be written in. They are Kelvin, Celsius, and Fahrenheit. Fahrenheit is one of those American units which are not often used in the scientific community. Celsius is the most used unit of temperature in Chemistry, but many formulas require temperature in them, and when computing formulas, Kelvin is more often used. Because each unit has its own reason for being used, it is important to understand how Kelvin is related to Celsius. The formulas for converting Kelvin temperature to Celsius temperature and vice versa, are as follows:
Temp(Kelvin) = Temp (Celsius) + 273
Temp(Celsius) = Temp (Kelvin) - 273
Another very important value in Chemistry is density. Density is a ratio of mass to volume. For every substance at a certain temp, and pressure there is only one unique density value. This value is the same no matter how much or how little of the substance is used. Density can be used to determine what certain substances are. For example, if there is a substance which has a density of 1.00 g/mL, a chart of densities can be used to show that the unknown substance is water.
In Chemistry and the real world, substances are not always pure. Pure substances are substances that have constant composition. Mixtures are substances or a group of substances which are variable in composition and can be separated by physical means. An example of a mixture might be sand and gravel. There are two types of mixtures. One type is a homogeneous mixture. In this type of mixture, the substance is the same through out. The second type of mixture is heterogeneous. A heterogeneous mixture is a mixture containing regions of different properties. An example of heterogeneous mixture is when you take a bucket of water and dump dirt in it. Some of the dirt stays suspended in the water while most of the dirt falls to the bottom, creating regions of different properties.
We defined pure substances (in the previous paragraph) as substances that have constant composition, and because of this, pure substances can be broken down into small groups called compounds and elements. A compound is a substance with constant composition which can be broken down into its elements by chemical processes. Chemical processes are processes where the atoms in a substance are reorganized to form a new substance. After substances are broken down into its basic elements, elements can no longer be broken up. [Please note that physical processes can be reversed, such as filtration, but chemical processes can not, such as combustion.]
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Unit 1 - Section 2