Unit 1 (Chapter 5): Units and Measurement

Quantitative vs Qualitative

Has a measurement (quantity), even if it's just an approximation

The distance is 5 meters

25 Liters of smoke poured out of our failed experiment

It took me about 5 seconds to realize my beard was on fire because of poor Bunsen burner technique

Is purely descriptive (quality)

I jumped a long way

The smoke was black

It took me a while to put out the fire in my beard, and left me with charred bald patches on my chin

Quantitative vs QualitativeQualitative observations are great, and you should be making them on every lab...

BUT...

...chemistry is a quantitative science. Almost every great discovery in the field came from very carefully measuring something.

And if you measure something, you need units.

So what?

Three topics to read about:

1. The NASA Mars Orbiter

2. The Gimli Glider

3. Medication Errors

(these are links, by the way, if you're using the Powerpoint version)

What does this mean for you?

If you have any desire to pass Honors chemistry, you will:

1. Put units on every single number

2. Put units on every single number

3. Not talk about Fight Club*

4. Rules 1-2 also apply to showing work.

*OK, so maybe we won't enforce this one

What does this mean for you, part two (the Revenge)?

You need to understand the difference between symbols, and units. Let's take a simple equation:

d = m/V(density equals mass over volume)

d, m, and V are all symbols, not units.

Each could be measured in many different units; volume could be in pints, gallons, liters, milliliters, cubic inches, hogsheads...or many others

What does this mean for you, part three (Judgment Day)?

Let's say our mass is 15 grams, and our volume is 5 milliliters, then our equation becomes:

d = 15 g / 5 mL

where g and mL are the units. The units of the answer just come from the math you did. We divided grams by milliliters, so our density is in.....g/mL!

d = 3 g/mL

symbol value unit

A few easy mistakes to watch out for

m is mass as a symbol, meters as a unit, and 'milli' as a metric prefix.

Miles are usually abbreviated “mi” to avoid confusing them with meters

Gallons are usually abbreviated “gal” to avoid confusion with grams

There are a limited number of letters, so...

Examples

The good: 1.05 m 23 ft 2.3x105 mL

The bad: 1.05 23 distance 2.3x105 volume

The ugly: 1.05 kg*m2/s2

(This is an example of a complicated, but valid unit. We won't worry about what it means yet, but whatever it measures, you multiply kilograms by the square of meters, and divide by the square of seconds to get this value)

Proper Measurement Technique

You can always read to at least halfway between the smallest markings on a tool. Let's use the example of a graduated cylinder:

20 mL

25 mL

30 mL

At worst, this measurement should be:26.5 mL

If you feel confident in your ability to tell whether it's exactly halfway or not, you could call it

26.3 mL

If it were right on the line, it would be26.0 mL

It should never be26 mL

Summary Chemistry is a quantitative science; we measure.

All measurements must have units

All numbers that come from measurements have units

When you measure something, use the smallest markings on the tool, and then try to determine if it's right on the line or not.

If you don't follow these directions, you might crash a $330,000,000.00 spaceship into Mars