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| Chemistry is the study of matter and its reactions. All matter consists of small particles. Anything that we observe can be explained using this model. As we proceed through the year, we will develop this model. |
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Making observations is central to any science. Observations are made free of bias or preconceptions. Once these observations are made, the experimenter can develop a model that explains the observations. Observations fall into two broad categories. Qualitative observations involve descriptions with words. For example, the sky is blue or water is clear. Quantitative observations involve measurements and always have numbers with the appropriate units attached. For example, There are 500 ml of soda in a bottle or the mass of an apple is 200 g. All measurements must have units. Saying your height is 2 is ambiguous because you could be 2 m tall, 2 yards tall or 2 feet tall! Units that cannot be broken down into simpler units are referred to as fundamental units. Derived units contain a combination of fundamental units. The metric system is the preferred way of expressing units. All units are different by factors of 10. |
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Common Prefixes Used in the Metric System
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Law of Conservation of Mass
In any change, the amount of matter after the change should be the same as the amount before the change. If a system is closed, the mass stays the same because the number and type of particles involved have not changed. If a system is open, particles from the system can escape to or enter from the surroundings. This can result in a decrease or increase in the mass of the system. Is this a violation of the law of conservation of mass? |
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Factor Label / Dimensional Analysis When performing calculations involving measurements, units have to accompany all numbers that correspond to measurements. For example a conversion involves taking a measurement in one unit and expressing it in another unit. Converting the measurement from one unit changes the number and unit but doesn't change the actual quantity you are measuring. For example, if you are 6 ft tall, your height could also be expressed as 72 inches. While the number and unit have changed, you are still the same height. To do conversions using factor label: 1. First identify what you are starting with and what you will end up with. 6ft --> ? in 2. Identify the conversion equality you'll need to use. 1 ft = 12 in 3.
Identify the two possible conversion factors by first dividing both
sides of the equation by the quantity on the left. Repeat for the
quantity on the right.
1 ft = 12 in 1 ft = 12 in 1 ft 1 ft 12 in 1 ft Notice that each equation is equal to one because 1 ft / 1 ft = 1 and 12 in / 12 in = 1. 4. Choose the ratio from above that will cancel out the given units (ft) and result in an answer with the desired units (in). 6 ft | 1 ft = 0.5 ft2 OR 6 ft | 12 in = 72 in Obviously the solution on the right results in the desired units. Notice that while 12 / 1 =/= 1, 12 in / 1 ft = 1 This means that converting the measurement from feet to inches changes the number and units, but not the actual measurement itself. If we wanted to convert 32 inches to feet, we can use the same process. 32 in | 1 ft = 2. 67 ft OR 32 in | 12 in = 384 in2 |
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Uncertainty in Measurement
All measurement requires estimation. No matter how small the divisions are on a ruler, graduated cylinder or thermometer, we can estimate out to one decimal place beyond the smallest division. On the ruler below, the arrow is in between the 8.9 and the 9.0 position. Since the arrow is about half way between these two positions, we can estimate the length to be 8.95 cm |
| On the graduated cylinder below the volume at the arrow lies between the 150 ml mark and the 160 ml mark. Since the arrow is about half way between these two positions, we can estimate the volume to be to be 155 ml. |
| All measurements must be expressed to the correct estimation. Since the last place is an estimation, we cannot express digits to the right of the estimation. |
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Significant Figures
Measurement: Significant figures reflect the amount of certainty you have about a measurement. The more precise a measuring tool is, the more places beyond the decimal place there is. In class you learned how to determine the number of significant figures there are in a measurement. The rules are as follows:
Calculations: There are numerous calculations that are done with measurements. Because each measurement has significant figures, there needs to be a rule that helps us determine the number of significant figures we have in the answer. There are two rules:
Accuracy Vs. Precision
Accuracy refers to how close your measurement or calculation comes to the accepted value. For example water has a boiling point of 100 C and has a density of 1 g / ml. If you measured the boiling point of a sample of water to be 99.9 C you would be fairly accurate. If you calculated the density of water to be 8.9 g / ml, that would of course be highly inaccurate. Precision refers to the smallest division on your measuring tool. A ruler divided into 0.1 cm increments will give more precise measurements that a ruler divided into 1 cm increments. |
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