science lessons jan. 7

8/14/2019 Science Lessons Jan. 7

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CHEMISTRYC H E M I S T R Y A S T U D Y O F S U B S T A N C E S , T H E IR P R O P E R T I E S A N D T H E IR IN T E R A C T I O N S

Welcome to the Yellow section. Here w e focus on the discipline of chemistry: the study of differ-ent kinds of stuff (substances) an d what they do to each other (their interactions). In this section wewill learn som e of those fun and seemingly magical chemical reactions; bu t more importantly, we will.earn how their "magic" works. Please begin now with the first lesson.1 : Impress Your Friends!

Recall that al l matter is composed of at-o m s , an d that atoms are composed of protons,neutrons an d electrons. Protons and neutronsare packed t ight ly together in the center ornucleus of an atom, and electrons orbit thenucleus at high speed. The n u m b er of posi-tively charged protons tends to be the same asthe number of negat ively charged electrons,keeping the ch arge of an atom neutral (neitherpositive nor negative). Protons and neutronsmake up most of the mass of the atom, whileelectrons have somebut l it t lemass.

Second, recal l that an element is madeup of atoms having a certain number of pro-tons in their nuclei. Each atom of hydrogen,fo r example, has one proton in its nucleus. Allmatter is made up of some combination of el-ements. If we could group atoms together by

An atom is the smallest stable form ofmatter. Two examples of individual atomsare illustrated at the top of the diagram.An element consists of several of the samekind of atom; so the diagram shows tw oseparate pure elements made up of the twodifferent types of atoms. When atoms cometogether under their forces of attraction,they bond together to form a molecule;i f there is more than on e type of atom inth e molecule it is called a compound. Theword compound can refer to either a singlemolecule or to a collection of moleculesof th e same type. Since there is only onetype of molecule represented in the groupat the bottom of the diagram, this grouprepresents a pure compound.

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Atoms

Elements

Molecule

Compound

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CHEMISTRY

their number of protons, the result wouldbe separate pure elements. Pure gold is acollection of atoms of gold, each having79 protons. Because they are common oruseful , some elements (like copper andmercury) have names we recognize.

No element is present in nature inits absolutely pure form. Although wecan purify them from their natural forms,in nature they are found combined withother substances. So when you lookaround you in your world, you will findmany different substances. But these arenot simple elements. They are complexcombinations of elements. The study ofchemistry involves taking a closer lookat those substances and seeing what theyare made of and how they are arranged.

Atoms of different elements or ofthe same element can be joined togetherby the natural (mostly electric) forces toform molecules. For example, an atomof oxygen can join together with two at-oms of hydrogen to make a molecule ofdihydrogen oxide, which we usually callwater. Molecules made up of atoms fromtwo or more elements joined together arecalled chemical compounds. Pure water(water made up of only dihydrogen oxidemolecules and no other) is a single com-pound.

We begin our study in the next les-son by looking at the different forms ofmatter.

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Although the minerals found in the earth are madeof elements, there are few pure elements found there.These elements ar e combined with other elements inways you will grow to understand throughout the nextseveral weeks.

Exercises:1. Silver is the name of an element. How many types of atoms make up silver?2. A is made up of atoms held together by natural forces.3. Two or more elements joined together make up a .

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3/11

CHEMISTRY2: Packaging S tuff

So you want to understand stuff, eh? I mean, yo u want to understand the makeup of matter. Well,you are on your way, bright student.Once upon a time there was a stuff packager. Her job was to package stuff. She picked the rightsized box to put in just the right amount of stuff. But this w asn't just an y stuff packager: she had thespecial ability to pack stuff in space. What I mean is, she could move stuff around in space and itwould stay where she put it. That's why they paid her the big bucks. Sometimes sh e left out the space

and packed things tightly. At other times she would put in a little space and pack things more loosely.At yet other times, she would pack things so that there was m ostly empty s pace and very little stuff.

The "fog" that is often seen lying above a pool of water on a humid morning is water that hasevaporated and cooled there. It is no longer a gas because it is not completely spread out: instead, itis tiny droplets of water that have gathered. The droplets are not massive enough to overcome frictionwith the air and fall to the earth un der the pull of gravity, so they linger in the air until the energyfrom the sun becomes intense enough to evaporate them again.

Interestingly, when sh e packed differently, the stuff acted differently. W h e n sh e packed tightly,the stuff got stiff. Stiff stuff ca n sometimes be useful. When she packed more loosely, she found that


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CHEMISTRYth e stuff fil led th e container from th e bot-tom up, but the stuff never got stiff. Shecould put her hand into the s tu f f and i tw o u ld m o v e out of the way for her. Wh enshe took her hand out of the container ,some of the stuff would be on her handand she would have to wipe it off . Final-ly , when she packed with most ly emptyspace, the stuff m o v e d f reely about theb o x . S o m e t i m e s s tu f f b u m p e d t o g e t h e rwith other stuff . She had to shut the lidquickly or the stuff would bum p r igh t outof the box to spread out within the pack-aging room. But if she got the box closedon the stuff, it wou ld jus t spread i tse lf ou twithin the box. If you listened carefully,you could hear that stuff bum ping a roundin there.

What magic d id th i s l ady possess?None. You see, th is lady was extremelys m a l l . T h e s t u f f s h e p a c k e d w a s p a r -t i c l e s a t o m s a n d m o l e c u l e s . P a r t i c l e sthat pack together tightly and have a lovefo r each other make stiff solids. They arer ig id, and the movement of their particlesis minimal . Par t ic les of l iquids have lessl o v e f o r e a c h o t h e r a n d m o v e a r o u n dmore freely, but not as freely as particleso f gases . Whi le l iqu ids f i l l a con ta inerfrom th e bottom up, gas m olecules spreadout to fil l any con taine r all the way to thel imits .C o m p a r e th e t y p e s o f m a t t e r t h a t

we've seen. The f loor is solid, water isliquid, and air is gaseous. That may notseem l ike much of an observation, but ifyou think about it, almost every kind ofmatter is either a solid, a l iquid, a gas or some combination of these. There are also a few substanceslike gelatin and ice cream that we call semisolids, but for the most part everything fits nicely into oneof these three categories called the phases of m atter.Matter sometimes undergoes a change from one of these phases to another. The way you causematter to change phases is to put in or take away energy. For example, tightly packed solid molecules,when heated up, s tar t moving more rapid ly; th is movement expands the substance and creates moredistance between the molecules. If enough heat is applied, the forces binding the par t ic les together

A glacier is a large mass of ice that grows forwardor shrinks back depending on the temperature of thesurrounding air.


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CHEMISTRYare broken and the particles become more loosely arranged. At that point they become liquid. If youcontinue to add heat, they ca n break apart even further to become gas. To return these particles to theiroriginal state, all that is necessary is to cool them dow n.

When you put water in the freezer, heat leaves it and its temperature drops. When the tempera-ture drops, the liquid changes to a solidice. W hen it warms back up to room temperature, i t changesagain from solid to liquid. This is not so thrillin g beca use we 've seen it since we w ere little kids. W hatis nifty is that other liquids also change to solids when they get cold (although some liquids have toget really cold before they turn to solids). Also, gases change to liquids. This illustrates that gases arejust liquids hanging loose, an d that liquids ar e solids hangin g loose. How loose th e particles of a sub-stance are depends on the strength of the forces attracting those particles and on the amount of heatenergy around them.

What happens when you boil water? Steam c o m e s off th e water. We say i t evaporates, or rumsto vapor. And if you boil th e water long enough, there wil l be no water left in the pan. That's becauseit al l turns to steam. But what is steam? It's a gas. You might say it's water gas. When steam cools,what happens? It turns back into water.

-40 C 0 C 2 0 C 100'C-40 F 3 2 F 212 F

This diagram shows th e relationship between th e Fahrenheit and Celsius scales. Notice that there isnearly a two-degree change in Fahrenheit fo r each degree change in Celsius. Tlie nvo scales cross atapproximately -4 0 .

Water turns to ice when its temperature falls below 0C. It turns to gas if the temperature risesabove 100C. But what about other liquids? D o they change from solid to liquid or from liquid to gasat the same tem peratures? N o. Each solid has its own unique melting point at which it turns to liquid,and every liquid has its own unique boiling point at which it turns to gas. These are j u s t nvo facts ofnatureobservations about the way heat affects matter. At a later time, w e will learn how importantthe melting and boiling points of different compounds are. In fact, we will learn that life as we know itwould be impossible without th e melt ing and boi l ing points of water being just as they are.When a pan of water is placed on a burner , th e temperature of the wate r starts going up. The

heat passes from molecule to molecu le . O ne mo lecule heats up and passes some of that heat off to itsneighbors. Occasionally a molecule near th e surface of the water will become so energet ic with all ofth e heat it has received that it will yell "yowee" an d break free into th e air. Its neighbors wil l heat up


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same w ay unti la r e a l l b o u n c -g a r o u n d t r y i n g tot out of there. Eacha hotter moleculeth e pan, it takesS o w h i l e th e hea theatsth e water, th e leav-g of the overhea ted

cools itThe temperaturew a te r t ha t re -stayse s a me u n t i l ev e r yw a te r has

boiled out. Thatcallede b o i l i n g p o i n t , i s100C. How100C? Simple.scientist made it.

B e c a u s e of t h i swater can be usedkeep a constanttells you to s immer

th e soup wi l lat i ts boi l ing(which wi l l b ethe boiling pointas long as

provides enoughto keep the soupWhen all ofe water is evaporat-there are no

There ar e some scenes where al l three phases of matter ar e present atthe same time. Once again, th e gaseous water cannot be seen, bu t wherethere ar e droplets of water in the air, they have often formed from th econdensation of gaseous water.


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CHEMISTRYmore particles of water to keep taking heat away from the pan. The temperature will rise quickly (andthe stuff left in the pan will burn).

Once steam is made by heating water, the steam will go out into the cool air and its tempera-ture will drop to below the boiling point again. Steam will cool and the tiny water droplets willcombine to make liquid water again. This conversion of steam to water is called condensation.Condensation is the process by which rain forms in clouds, dew forms on morning grass, and fogforms in cool m oist air.

A hoarfrost occurs when fo g f rom the air crystallizes directly on the leaves of trees in The cold morningair.

Exercises:1. W hy doesn't fog form in the middle of the afternoon?2. On which day is snow more likelya day when the temperature is -20C. 0C. or 20C?3. If you wanted to melt a stick of butter w ithout burning it, you could melt it at 100C. What

is a practical way of doing this?


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CHEMISTRY

3: How Much Stuff Is in Stuff?If we're going to understand matter, we have to understand how to measure it. Let's sa y that Istep on my bathroom scale and find that I weigh 60 kilograms. Then I step off the scale. The scalereading returns to zero because I am no lon ger standing on it, but how m uch do I now weigh ? Un-

less something strange is going on, I still weigh 60 kilograms. Why? Because the weight of a per-son do esn't chang e just b ecause he steps from here to there. Weight is the effect gravity has on acertain am ount of stuff.

If th e nucleus of an atom of an element were one centimeter in diameter, 100grams of that substance would fill the Grand Canyon.

Now let's take our bathroom scale out into outer space and stand on it. Why does it still readzero? Becau se there is no gravity. Does that mean we now weigh zero? Yes, it does. But I'm still asomething; that is, I haven't turned into a nothing. W hy then is my weight zero? It's because thereis no gravity, and weight depends on gravity. There is something about a person that doesn't de-pend on gravity that stays the same when he leaves the Earth. That's the amount of stuff in the per-son. The amount of stuff is the person's mass . Mass is a measure of an amount of stuff. On Earth,the mass of a body is equal to its weight. But in outer space, mass is still the same even thoughweight becom es meaningless.Of course, atoms are too small to weigh on your bathroom scale, but they still have m ass. Andon Earth their wreight is the same as their mass. So we may speak of atomic weight or atomic mass,but we're still talking about the same thinghow much stuff makes up the atom.


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CHEMISTRYDif fe ren t elements have di fferent masses (o r weights) for the same number of atoms. For ex-ample, the nucleus of a single hydrogen atom has but one proton and no neutrons, while a heliumatom has two protons and two neutrons in its nucleus. The helium nucleus is about four t imes asmassive as the hydrogen nucleus. Because we can't weigh them on the bathroom scale, it makessense to have a smaller unit of measure for describing how much stuff is in an atom. We couldweigh them in pounds or kilograms, but their masses would make really small numbers.Instead, we express the masses of atoms in atomic mass units (AMU). One atomic massunit is the mass of a proton (all protons have the same mass). While the mass of hydrogen is 1

puny AMU, the mass of helium is 4 AMU. These are the two elements with the least mass. Lead,a more massive atom, has an atomic mass of 207 AMU. Through the use of a special machinecalled a mass spectrometer, scientists have measured the masses of the atoms of all of the ele-men ts. You could think of the mass spe ctrom ete r as a bathroo m scale for atoms.Let's try for a moment to unde rs tand jus t how small an atom is . Scient is ts often measureweights (or masses) of substances in gr am s. (O n e gram of a substance is approximately equal toth e weight of two regular-sized paper clips.) One gram of hydrogen has approximately 602,200.000,000,000,000,000,000 atoms of hydrogen in rfSo an atom must be pretty small, huh? Thathuge number, called Avogadro's number, is nam e d af te r a physicis t and chemist from days goneby. His name was Lorenzo Romano Amadeo Carlo Avogadro di Quaregua e d i Cerreto, and helived in Italy from 1776 to 1856, that is , from th e t ime of the U .S. De c lara t ion of Independence to

near the t ime of the Civil War.Your laboratory activity kit has a chart called a pe rio di c ta ble o f the ele me n ts that shouldbe posted in a location where you can look at it often. I t pro vide s the ato mi c symbol ( two-le t te rsymbol) given to each element, along with it s a tomic number ( the n u m b e r of p ro tons in eachatom) and its atomic mass. The atomic mass is both th e m as s of one atom of tha t e l e m e n t inAMU and the number of grams of that element that contains 602,200.000.000.000.000.000.000 (o r6.022 x 1023) atoms.For example, since helium (4 AM U) is four times heavier than hydrogen 1 1 A M U ) . Avoga-dro 's number of hel ium atoms would weigh four grams instead of one . Similarly, a s ingle carbonatom weighs 12 A M U . It is 12 times heavier than hydrogen an d three times heavie r than he l ium.That same big number of carbon atoms w ould weigh 12 grams.Find the symbol Li on the periodic table. This is the symbol for l ithium. Lithium can quicklybe seen to have 3 protons (its atomic number is 3), and have an atomic mass of 6.941. A s ingleatom of lithium we ighs 6.941 AMU . Also, 6.941 grams of l ithium contain 6.022 x 10" atoms.The moral of the story is this: th e masses of ind iv idua l a toms ar e measured in A M U . an d vis -ible amounts of matter are often measured in grams. But you should realize that , whe n you mea-sure vis ible amounts of matter, you are m e as u r i ng out enormous numbers of atoms!

Exercises:1. Look on your periodic table to find out how much one atom of barium weighs in AM U?2. How m uch does one a tom of b romine we igh in AMU?3. How many grams of bromine contain 602,200,000,000,000,000,000,000 atoms?


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CHEMISTRY

4: Properties of MatterDensityNow you know that all matter hasmass, and you know that all matter ismade up of atoms. You won't be sur-prised to find out that all matter takes

up space. It would be really neat if wecould invent spaceless matter. All of ourstorage problems would be solved. Youcould fit an endless number of socks inyour drawer. But the world just isn't thatway. Everything takes up space.The question is, do equal amounts(equal masses) of two different elementstake up the same amount of space? Forexample, does one gram of copper takeup the same amoun t of space as one

gram of lead? Th e answer is "no." Everyelement has its own unique density. Themore dense a substance is, the more massis crammed into a given amount of space.A clay brick that is used for build-ing houses might weigh about six or

seven p o u n d s . In our laboratory, wehave a brick made of lead. Although itis the same size as a clay brick an d takesup the same amount of space, it weighsabout ten times more than the clay brick.It weighs over 25 pounds. You see, thedensity of lead is much greater than thedensity of clay. Because density is themeasure of the amount of matter (mass)taking up a certain amount of space (vol-ume), you can get the density by divid-ing the mass by the volume:

density = mass / volume

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These tw o bricks are approximately th e same size, bu tth e one in the bottom photo weighs many t imes more thanthe one in the top photo. Why? The matter making up theatoms o/ the lead brick (bottom) is packed more tightlythan the matter making up the atoms of th e clay brick.

If, to a lead brick, you add a second lead brick, the mass will double. Two bricks take up twicethe volume of one brick, so the volume will also double. What happens to density if both the massand the volume are multiplied by two? It stays the same. So the density of lead is the same, no matterhow much lead is present. Every element has its own unique density. That is, each element takes up acertain amount of space for a certain amo unt of mass. Elem ents that are made up of a lot of matter in a:J1 space are dense.


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CHEMISTRY

Let's take a mo men t to review th e m ain points that have been taught in these first four chemistrylessons:1. All matter has mass and fills space.2. Different elem ents have different densities because their ind ividual particles are more or lessscrunched into the same amo unt of space.3. The love that those particles have fo r each other, the closeness with which they are packed,and the amount of available energy will decide whether they are solids, liquids or gases.

The feather weighs less than th e paper clip even though it is larger. It's because th e feather has lots ofempty space inside and is made of less dense elem ents. The matter making up the paper clip is denser.

Exercises:1. Wh ich weighs m ore, a pound of feathers or a pound of bricks?2. Which takes up more space?3. Which is more dense?

science lessons jan. 7

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