chemistry: an introduction “i don’t trust atoms. they make up everything.”

CHEMISTRY: AN INTRODUCTION“I don’t trust atoms. They make up everything.”

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UNIT 1: MATTER, MEASUREMENT, AND UNIT CONVERSIONS≈

SI, METRIC, AND ENGLISH UNITS

Learning Goal: You will be able to identify and classify units into the correct category.

Success Criteria: You will know you have met the learning goal when you can correctly identify units 90% of the time.


In our daily lives, we tend to use English system units. The English system is used primarily in the United States, and includes feet, inches, miles, pounds, ounces, pints, gallons, etc.

The metric system is used in most countries around the world, and is used by scientists almost everywhere (including in the U.S.). These include meters, kilometers, centimeters, grams, kilograms, milligrams, liters, etc.


Here’s a chart that shows some of the comparable units for the two systems.

These units indicate:Length

Length

Weight and Mass

Weight and Mass

Volume

Distance


There are dozens of prefixes used in the metric system. The ones you should know for this class are:

Nano Billionth n .000000001

Micro Millionth μ .000001

Milli Thousandth

m .001

Centi Hundredth

c .01

Deci Tenth d .1

Deca/Deka

Ten da 10

Hecto Hundred h 100

Kilo Thousand

k 1,000

Mega Million M 1,000,000

Giga Billion G 1,000,000,000

Mem

ori

ze

Q: Rank from smallest to biggest:

1 liter5 milliliters3 dekaliters5000 milliliters


There is a subset of metric units called SI units. SI units are the units that are used in most scientific equations. It is important to know them because when you come across an equation that asks for a certain quantity, such as mass, you need to know what unit you need to use.

Length = meterMass = kilogramTime = second

Temperature = Kelvin

Q: What units for mass and the speed of light should you use in E=mc2?

Memorize

TAKING MEASUREMENTS

Learning Goal: You will be able to take accurate measurements using a variety of instruments and you will be able to report your results with the correct degree of certainty.

Success Criteria: You will know you have met the learning goal when you are able to take accurate measurements and report the results with the correct degree of certainty 90% of the time.

HOW SHOULD YOU READ A RULER?

HOW SHOULD YOU READ A GRADUATED CYLINDER?

You should always report the level of the liquid from the bottom of the meniscus. The meniscus is the curved shaped surface that a liquid acquires in a small cylinder.

IDENTIFYING SIG FIGS

Learning Goal: You will be able correctly identify the digits in a number that add to the precision of the number, aka the significant figures.

Success Criteria: You will know you have met the learning goal when you are able to correctly identify the correct significant figures 90% of the time.

WHAT ARE SIGNIFICANT FIGURES?

Vague (estimated) numbers vs precise numbers:

1) 4002) 2913) .00074) .0039145) 10.00Note that “precise” does not mean “small”. 291 is a more precise number than .0007. A more precise number has more sig figs.

A little too vague


You live 200 miles from Portland, Oregon.There are 160934.4 centimeters in one mile.200 x 160934.4 = 32,186,880.Do you actually live 32,186,880 centimeters from Portland?

I added 10 gallons of gas to my car.There are 3.7854 liters in one gallon.10 x 3.7854 = 37.854.Did you add 37.854 liters of gas to your car?


Rules for determining significant figures:1) Does the number have a decimal?2) If it has a decimal, unbound zeroes on the left are not significant, if it doesn’t have a decimal, unbound zeroes on the right are not significant.What are unbound zeroes? They are the zeroes that come at the very beginning (.00035) or very end (4200) of a number. They are not zeroes in the middle of a number (10.06). A number such as .0030 has unbound zeroes on both the left and the right.255 3006 10.032 88 6.0331

250 3600 7080 50 90,000,100

.00455 .062 .0002500 .032060 10.0

40.030 1050.0 20.001000 3420400 1.30500

MULTIPLYING AND DIVIDING WITH SIG FIGS

Learning Goal: You will be able to round products and quotients to the correct number of significant figures.

Success Criteria: You will know you have met the learning goal when you can round products and quotients to the correct number of significant figures 90% of the time.

MULTIPLYING AND DIVIDING WITH SIG FIGS

Multiplying and dividing numbers and reporting your answer with the correct number of sig figs is easy. Just figure out the number of sig figs in your least precise number and round your answer to that number of sig figs.

22 x 400 = 8800. Round to 9000

.045000 ÷ 371.0 x 1,297,100 = 157.3302. Round to 157.3

30.68215 ÷ 3 = 10.1207. Round to 10

ADDING AND SUBTRACTING WITH SIG FIGS

Learning Goal: You will be able to round sums and differences to the correct number of significant figures.

Success Criteria: You will know you have met the learning goal when you can round sums and differences to the correct number of significant figures 90% of the time.


Adding and subtracting numbers, then reporting your answer with the correct number of sig figs is not complicated. Just find your answer like you normally would, then round to whatever place (hundreds, tens, ones, tenths, hundredths, etc.) you can based on the numbers that went into your answer.

1250 + 12.3 = 1262.3 Round to 1260.

1250 has sig figs down to the tens place. 12.3 has sig figs down to the tenths place. The tens place is higher, so we have to round our answer to the tens place.


8800 - 728.5 + 6210

8800- 728.5+ 6210= 14281.5

Round to 14300

.000291 + .0030 - .004620

.000291+ .0030- .004620= -.001329

Round to -.0013

SCIENTIFIC NOTATION

Learning Goal: You will be able to convert numbers between standard notation and scientific notation.

Success Criteria: You will know you have met the learning goal when you can convert numbers between standard notation and scientific notation 90% of the time.

SCIENTIFIC NOTATION

Often in science, we deal with very large or very small numbers. Sometimes, writing out every place holding zero is cumbersome or impractical. For example, we will learn later in this course about a number called Avogadro’s number, which has a value of 602214129000000000000000

Writing this number out takes too much time, too much space, and can result in errors if you add up the number of zeroes wrong. Instead, we can write it as 6.02214129 x 1023. Normally, we will just use the first three significant figures, so we’ll write it as 6.02 x 1023.

SCIENTIFIC NOTATION

The way you determine the value of the exponent (the 23 in the previous example) is by how many places from the ones place you have to move the decimal. For example, to write 7600 in scientific notation, you would have to move the decimal three places, so it would be 7.6 x 103. Notice that numbers written in scientific notation only include the sig figs.

SCIENTIFIC NOTATION

In your calculator, (at least in TI calculators) you can input numbers in scientific notation using the EE button. This saves time and reduces math errors later in the course, so I encourage you to get comfortable using this function.

4.32 x 108 = 4.32E8

5.10 x 10-4 = 5.10E-4

Convert between scientific and standard notation.3.08 x 105 = 1.200 x 107 =7.28E6 = 3.900E-3 =

.00452 =

.03100 = 6,040,000 =129,000,000,000 =

ACCURACY AND PRECISION

Learning Goal: You will be able to distinguish between accuracy and precision and be able to cite examples of both.

Success Criteria: You will know you have met the learning goal when you can distinguish between accuracy and precision and correctly cite examples of them 90% of the time.


We often use the terms accurate and precise interchangeably, but in science they mean very different things. Accurate refers to how close to the true value our measurements or our calculations are. Precise refers to how close our measurements or calculations are to each other. The best measurements are accurate and precise.


How would you describe the archers who shot the arrows at the four targets on the right?

Accurate and precise

Accurate, but not precise

Not accurate, but precise

Neither accurate nor precise


Q: Suppose you were asked to measure out 53.0mL of HCl for three trials of an experiment. Your partner checks your volumes and records them in the following data table.

Trial 1 Trial 2 Trial 3 Are you accurate? Are you precise?

51.2mL 51.3mL 51.2mL

Q: What might be causing this?

MANIPULATED, RESPONDING, AND CONTROLLED VARIABLES

Learning Goal: You will be able to distinguish between manipulated, responding, and controlled variables and be able to cite examples of all three.

Success Criteria: You will know you have met the learning goal when you can distinguish between manipulated, responding, and controlled variables and be able to cite examples of all three of them 90% of the time.


Imagine you are conducting an experiment to determine how temperature affects how fast gas is produced in a chemical reaction. You conduct several trials where the reaction takes place at different temperatures. The variable that you directly alter in the experiment (the temperature) is called the manipulated variable or the independent variable. The variable that you observe, the one that is affected by the changes you make (how fast gas is produced) is the responding variable or the dependent variable.


Trial 1

Trial 2

Trial 3

Trial 4

Temp. °C 20 50 80 100

Seconds taken to produce 5.0 mL of gas

13.2 9.3 7.5 4.0

You raised the temperature of the reaction vessel (manipulated variable), which resulted in gas being produced faster (responding variable).


For the results of your experiment to be trustworthy, everything but the manipulated and responding variables must be kept the same. This might include the length of time the reaction takes place, the equipment used to measure the volume of gas, and the equipment used to heat the reaction vessel. These are all controlled variables. If you were to change them, then you couldn’t reliably conclude that the changes you made to the manipulated variable were indeed what caused the changes to the responding variable.


Imagine you want to test out a new diet to see if it can help you lose weight. However, instead of just going on the diet, you also start exercising three times a week and you start taking a diet pill every day. Imagine you lose 15 lbs over two months. Can you conclude that the diet caused your weight loss? No!!! You didn’t properly control the other variables (exercise and the diet pills) that could reasonably be expected to affect your weight, so any conclusion you make about the efficacy of the diet is unreliable.

INTERPRETING GRAPHS

Learning Goal: You will be able to determine information about manipulated and responding variables from graphs.

Success Criteria: You will know you have met the learning goal when you can determine information about manipulated and responding variables from graphs 90% of the time.

INTERPRETING GRAPHS

Q: What are the variables?Manipulated:Responding:Controlled:

Q: How long did it take the sample of water to reach 55 degrees Celsius?

Q: What temperature was the sample of water after being heated for 650 seconds?

CLASSIFYING MATTER

Learning Goal: You will be able to classify matter into several distinct and overlapping categories.

Success Criteria: You will know you have met the learning goal when you can classify matter into several distinct and overlapping categories 90% of the time.

CLASSIFYING MATTER

Some categories that we place matter into, such as solids, liquids, and gases, you already know well. There are several more precise categories that substances can fall into as well. For example, a substance can be a mixture of multiple substances, such as curry powder, or it can be pure, such as gold or helium. We’ll learn today about many of these categories.

CLASSIFYING MATTER

An element is the most basic type of matter. Elements are what you see on the periodic table. A sample of an element is made of only one type of atom. Examples include silver, mercury, and neon.

A compound is made up of elements chemically bound together in a fixed ratio. Examples of compounds include water (H2O), carbon dioxide (CO2) and glucose (C6H12O6).

CLASSIFYING MATTER

The elements on the periodic table can be classified in a number of ways. One of the broadest and most useful ways is to divide them into metals and nonmetals. Metals (calcium, iron, uranium, lead) are, with the exception of hydrogen, the elements on the left and in the middle of the periodic table (about 75% of all elements!). They tend to be solids at room temperature, conduct electricity, and form positive ions. Nonmetals (oxygen, neon) are on the right side of the periodic table. They are a wider variety of phases and textures at room temperature, generally don’t conduct electricity, and tend to form negative ions. There’s a third category called metalloids or semimetals (silicon, antimony) that have properties between the metals and nonmetals (see staircase).

CLASSIFYING MATTER

A mixture is a combination of compounds and/or elements not bound together in a fixed chemical ratio. Rock, milk, air, and bread are all examples of mixtures. We generally further classify mixtures into two subcategories: homogenous mixtures and heterogeneous mixtures.

CLASSIFYING MATTER

A homogeneous mixture is a mixture where the substances that make up the mixture are evenly mixed. If you look at a homogeneous mixture, you won’t see the various substances it contains. Apple juice is a homogenous mixture, as is paint. A solution is a type of homogenous mixture where something is dissolved. You can usually see through a solution, such as salt water or rubbing alcohol. In salt water, the salt (the thing being dissolved) is the solute and the water (the thing doing the dissolving) is the solvent.

CLASSIFYING MATTER

A heterogeneous mixture is one where the substances that make up the mixture are not evenly mixed, or where you can see the component parts of the mixture. Rock is a heterogeneous mixture because the minerals in the rock are visibly distinct. Trail mix and pulpy orange juice are also heterogeneous mixtures. Its worth noting that the line between homogenous and heterogeneous mixtures is a blurry one, and not always obvious.

CLASSIFYING MATTER

A colloid is a type of mixture composed of two distinct states of matter. Colloids can be either homogeneous or heterogeneous, depending on how they are mixed. Smoke is a colloid because it contains solid and gaseous parts. Cheese is a colloid because it contains solids and liquids. A suspension is a type of colloid that will settle if left undisturbed. Dirty water is a suspension because if you let it sit for a long time, the dirt will sink to the bottom.

CHEMICAL AND PHYSICAL PROPERTIES AND CHANGES

Learning Goal: You will be able to distinguish between chemical and physical properties and between chemical and physical changes.

Success Criteria: You will know you have met the learning goal when you can distinguish between chemical and physical properties and between chemical and physical changes 90% of the time.


All substances have properties. Properties are characteristics such as flammability, hardness, color, electrical conductivity, melting point, and solubility that describe how a substance appears, behaves, and interacts. We can categorize the properties of substances into chemical and physical properties.


Chemical properties are those that can only be observed when the substance undergoes a chemical change, i.e. becomes a new substance. You can’t tell if a liquid is flammable until you light it on fire, which in doing so alters the identity of the liquid. Thus, flammability is a chemical property.


Physical properties are those that can be observed without changing the identity of the substance. The freezing point of water (32°F) is an example; after water freezes, it is still H2O, just in solid form. Thus, it is a physical property and not a chemical property.


Chemical changes are changes that alter the chemical identity of a substance. For example, the burning of natural gas (mostly methane, aka CH4) occurs according to the equation:

CH4 + O2 CO2 + H2O

After burning, there is no CH4, so it is a chemical change.


Physical changes are changes that do not alter the chemical identity of a substance. Melting and boiling are two examples of physical changes.

CONVERSION FACTORS

Learning Goal: You will be able to represent equivalent quantities as conversion factors.

Success Criteria: You will know you have met the learning goal when you can represent equivalent quantities as conversion factors 90% of the time.

CONVERSION FACTORS

If there is one skill you’ll learn in this class that if you master it, it will make your life easier, it is learning to use conversion factors and unit cancelling. In this lesson we’ll discuss conversion factors, and in the next, we’ll discuss unit cancelling.

CONVERSION FACTORS

A conversion factor is a fraction equal to one, with one unit (such as feet or inches) in the numerator and a different unit in the denominator (see right). As you know, multiplying any number by 1 results in the same number. We’ll use this fact to convert from the units we have to the units we want. Writing the relationships between units as conversion factors makes going from a quantity in one unit (feet) to another (inches) much easier.

We know the relationship between feet and inches:

1 foot = 12 inches

This relationship can be written as a conversion factor in two ways:

Since 12 inches = 1 foot, 12 inches divided by 1 foot equals one.

CONVERSION FACTORS

Here are some other examples of conversion factors:

Q: There are three feet in one yard. Write both conversion factors for this relationship.

Q: There are approximately 8 kilometers for every 5 miles. Write both conversion factors for this relationship.

UNIT CANCELLING AND CONVERSIONS

Learning Goal: You will be able to use unit cancelling and conversion factors to convert between quantities with different units.

Success Criteria: You will know you have met the learning goal when you can use unit cancelling and conversion factors to convert between quantities with different units 90% of the time.


There are 12 inches in one foot. How many inches are in 4.0 feet.

You can probably do this particular problem in your head. However, we will definitely move on to more complex calculations, so let’s see how to solve this problem using conversion factors and unit cancelling (CF&UC).

1. Start with the given quantity (4.0 feet).2. Multiply by a conversion factor that includes both your given (feet) and

wanted (inches) units.3. Use the version of the conversion factor that places the same units

diagonally across from each other. This is what allows for the “cancelling” in unit cancelling.


There are 2.2 lbs in 1.0 kg. There are 1000 grams per kg. How many grams are there in 38.2 lbs?

How many seconds are there in 24.39 weeks?


Convert 68 miles per hour into meters per second.

Convert 10.4 g/cm3 into lbs/in3.

1 mile 1.609 km

5280 feet 1 mile

1 km 1000 m

60 sec 1 min

1 hr 60 min

2.2 lbs 1 kg

1 kg 1000 g

16.387 cm3 1 in3

1 liter 1000 cm3

Table of Equivalencies

DENSITY

Learning Goal: You will be able to relate the density, mass, and volume of an object.

Success Criteria: You will know you have met the learning goal when you can relate the density, mass, and volume of an object 90% of the time.

DENSITY

A pillow obviously weighs less than a bag of rocks, even though the two might be about the same size. The difference comes down to density. Density is the amount of mass that fits into a given space. Rocks have more mass than a similar volume of pillow stuffing, so we say that rocks have a higher density.

DENSITY

As you may already know, a less dense object will float on a more dense object. That is why an inflated ball will float on water but a rock will sink. The same goes for two liquids or two gasses; the less dense liquid (ex: rubbing alcohol) will float on the more dense liquid (ex: milk).

Q: Why does a helium balloon float up into the air?

DENSITY

Density is given in units of mass per unit of volume, such as grams per milliliter, or pounds per gallon. In chemistry, we will commonly see density expressed in grams per cubic centimeter (g/cm3) or kilograms per cubic meter (kg/m3).

Q: Can you write 7.5 g/cm3 as a conversion factor?

DENSITY

Q: What is the density in g/cm3 of a piece of metal that has a volume of 13.4cm3 and a mass of 86.14g?

Q: What volume of a liquid with a density of .86g/cm3 will have a mass of 2.07g?

TEMPERATURE CONVERSIONS

Learning Goal: You will be able to convert between the Fahrenheit, Celsius, and Kelvin temperature scales.

Success Criteria: You will know you have met the learning goal when you can convert between the Fahrenheit, Celsius, and Kelvin temperature scales 90% of the time.


In our daily lives, when we talk about how hot or cold it is outside, we use the Fahrenheit scale. When we say “It’s about 70 degrees outside” everyone assumes we mean 70 degrees Fahrenheit (°F). In science, and in most other countries, they use the Celsius scale (°C). 70 degrees Fahrenheit is roughly 21 degrees Celsius.


Converting between these two temperature scales is slightly trickier than converting between, say, kg and pounds, because °F and °C have different zero points. Zero degrees Celsius is equal to 32 degrees Fahrenheit. In fact, the Celsius scale uses the freezing point of water as one reference point (0 degrees). The boiling point of water (100 degrees) is another reference point.


The relationship to convert between the two systems is:

°C = (°F – 32)/1.8 or °F = (°C x 1.8) + 32

Q: What is 55 degrees Fahrenheit in degrees Celsius?


Scientists have found it useful to create a third system based on the lowest possible temperature, aka: absolute zero. Absolute zero is the temperature where the atoms or molecules in the substance have zero heat energy (no motion), and is equal to -273.15 °C. This is called the Kelvin scale, and is related to the Cesius scale by the following equation:

K = °C + 273.15


Q: What is -7.5 °C in Kelvins?

Q: What is 297.2 °F in Kelvins?

chemistry: an introduction “i don’t trust atoms. they make up everything.”

Documents

si units

comparable units

english system units

subset of metric units

english units heres

metric system

unit conversions slide

measurements learning