Mr. Skelton’s 8 th Grade MCAS Science Review Part 2.

Mr. Skelton’s 8th Grade MCAS Science Review

Part 2

Valence electronsValence electrons are like the outer ring of skydiversouter ring of skydivers in this formation… because it’s much easier for them to pull away from or join the outer levelouter level of the formation than it is to pull away from or come into the middlemiddle of the formation.

Chemical Bonds are formed Chemical Bonds are formed when atoms when atoms gaingain, , loselose or or shareshare

valence electrons.valence electrons.

are composed of 2 or more different 2 or more different elementselements in a fixed ratiofixed ratio (proportion)

a compound’s properties differdiffer from those of the individual elements

+

Example: table salt (NaCl) is composed of 1 atom of Sodium (Na),Sodium (Na), a highly reactive alkali metal, and 1 atom of Chlorine (Cl),Chlorine (Cl), a gas that is poisonous to humans.

Sodium (Na) Sodium (Na) explosive explosive

metalmetal

Chlorine (Cl) Chlorine (Cl) poisonous poisonous

gasgas NaCl (salt)NaCl (salt)

CompoundsCompounds

• a compound’s properties differdiffer from those of the individual elements

+

Example 2: water (H2O) is composed of 1 atom1 atom of Oxygen (O)Oxygen (O), a flammable gas, and 2 atoms2 atoms of Hydrogen (H)Hydrogen (H), another flammable gas.

Hydrogen (H) Hydrogen (H) flammable flammable

gasgas

Oxygen (O)Oxygen (O) flammable gasflammable gas

HH22O (water) O (water) very stable very stable

liquidliquid

CompoundsCompounds

The composition of molecules is shown by a MOLECULAR FORMULAMOLECULAR FORMULA

HH22OO - water CC88HH1010NN44OO22 - caffeine

AA MOLECULEMOLECULE is formed when is formed when 2 or more atoms 2 or more atoms bond togetherbond together (combine). (combine).

AA MOLECULEMOLECULE is the is the smallest unit of a compoundsmallest unit of a compound that has the chemical properties of the compound.that has the chemical properties of the compound.

Click on the link to see an animation of some molecules.

3-D Molecules

These are all ways to show a carbon dioxidecarbon dioxide molecule (COCO22).

HH22OO

subscriptsubscript– shows the # of atoms of # of atoms of the element before itthe element before it. So, this molecule has 2 H2 H (hydrogen) atoms.COCO22

If there is no subscriptno subscript after an element, that means there is 11 of that type of atom. So, this molecule has 1 carbon atom1 carbon atom and 2 2 oxygen atomsoxygen atoms.

Molecular FormulasMolecular Formulas

Sometimes, you’ll see a molecular formula that looks like this: MgMg22 (PO (PO44))22

When you see this, you multiply everything inside the multiply everything inside the parenthesis by the subscript after the parenthesisparenthesis by the subscript after the parenthesis. So, this molecule has 2 Magnesium atoms2 Magnesium atoms, 2 Phosphorus 2 Phosphorus atoms (1 x 2)atoms (1 x 2), and 8 Oxygen atoms (4 x 2)8 Oxygen atoms (4 x 2).

Sometimes, you’ll see a molecular formula with a coefficientcoefficient, that looks like this: 6CO6CO22

This means: 6 molecules of CO6 molecules of CO22

Molecular FormulasMolecular Formulas

atomsatoms

smallest particles of smallest particles of elementselements

elementselements

simplest substancessimplest substances

cannotcannot be broken down be broken down any furtherany further

Examples: Fe, Au, O, H, CExamples: Fe, Au, O, H, C

make up

moleculesmolecules2+ atoms combine to make2+ atoms combine to make

smallest particles of smallest particles of compoundscompounds

compoundscompounds

cancan be broken down into be broken down into elementselements

Examples: water (HExamples: water (H22O)O)salt (NaCl)salt (NaCl)

carbon dioxide (COcarbon dioxide (CO22))

make up

Atoms & MoleculesAtoms & Molecules

2+ elements combine to make2+ elements combine to make

ELEMENTS THAT EXIST AS ELEMENTS THAT EXIST AS DIATOMICDIATOMIC MOLECULES MOLECULES

(molecules with 2 atoms of the same (molecules with 2 atoms of the same element)element)

ELEMENTS THAT EXIST AS ELEMENTS THAT EXIST AS DIATOMICDIATOMIC MOLECULES MOLECULES


Remember:

BrINClHOFBrINClHOF

These elements only exist as

PAIRS.

DIATOMICDIATOMIC MOLECULES MOLECULES

NoteNote:

They are MOLECULESMOLECULES of an elementelement.

SOME ELEMENTS EXIST AS SOME ELEMENTS EXIST AS DIATOMIC MOLECULESDIATOMIC MOLECULES


SOME ELEMENTS EXIST AS SOME ELEMENTS EXIST AS DIATOMIC MOLECULESDIATOMIC MOLECULES


What this means is…

ELEMENTSELEMENTS can exist as either atoms or either atoms or moleculesmolecules, but but

COMPOUNDSCOMPOUNDS can only be moleculesmolecules. If it’s only got 1 element symbol1 element symbol in the formula, it’s an elementelement. Examples: OO22, N, N22, Cl, Cl22

If it’s only got more thanmore than 1 element symbol1 element symbol in the formula, it’s a compoundcompound. Examples: COCO22, ,

HH22O, NaCl, CO, NaCl, C66HH1212OO66

A simple way to tell is: If it’s on the Periodic tableon the Periodic table, and there’s only 1 element symbol1 element symbol, it’s an ELEMENT!ELEMENT!

If it’s NOTNOT on the Periodic table, and there’s more than 1 element symbolmore than 1 element symbol, it’s a

COMPOUNDCOMPOUND.Learning Learning √√

Are the following substances elementselements or compoundscompounds?

Au

N2

CuCl2

CH4

SO2

SiO2

F

O3

He

Element or Compound?Element or Compound?

Br2

NaOH

MgCl2

The Big Ideas… so far1.1. An An atomatom is the smallest piece you can have is the smallest piece you can have

of an of an elementelement. . 2.2. Everything in the universe is made up of a Everything in the universe is made up of a

combination of 92 elements.combination of 92 elements.3. Atoms combine to make 3. Atoms combine to make moleculesmolecules..4. A 4. A moleculemolecule is the smallest piece you can is the smallest piece you can

have of a have of a compoundcompound..5. Compounds are very different from the 5. Compounds are very different from the

elements they are made of. elements they are made of. Ex: NaCl Ex: NaCl (salt)(salt)

6. Atoms combine by gaining, losing or sharing 6. Atoms combine by gaining, losing or sharing valence electrons. They try to either fill their valence electrons. They try to either fill their outer level, or get 8 electrons in it.outer level, or get 8 electrons in it.

H2

A moleculemolecule is formed when two or more atoms when two or more atoms are bonded togetherare bonded together..

Let’s BondLet’s Bond

Learning check: Learning check: HowHow do atoms bond? do atoms bond?

When atoms come together by sharing sharing electronselectrons, the bond is a covalent bondcovalent bond.

Water -- A Very Important MoleculeWater -- A Very Important Molecule

Note how fills all outer electron levels; oxygenoxygen has 8 8 valence electrons, and the hydrogenshydrogens both have 22. By sharing their electrons, everybody is happy! Water is one of the most stable molecules around.

covalent bondingcovalent bonding

The electron dot diagrams on the right show molecules with covalent bonds.covalent bonds.

Electron Dot DiagramsElectron Dot Diagrams show an atom’s valence electronsvalence electrons

one atom gives electrons, one atom takes electrons

Sodium gives a single electron to chlorine to complete its outer electron level. Chlorine is happy to take it.

The result is ion formationion formation.

Ion FormationIon Formation

Ions and Ion Formation

An ionion is an atom or molecule with a positive or atom or molecule with a positive or negative chargenegative charge.

Since sodiumsodium loses one electron, it has a positive charge of +1, written as Na+.Na+.

Since chlorinechlorine gains one electron, it has a negative charge of -1, written as Cl-Cl-..

Two oppositely charged ions are attracted to each Two oppositely charged ions are attracted to each other, and bond together.other, and bond together.

This type of chemical bond is an ionic bondionic bond.

SaltsSalts are solids held together by ionic bonds.

Elements in Group 1Group 1 (Alkali metals)(Alkali metals) all have 1 valence 1 valence electronelectron they want to give away, so they bond easily with elements in Group 17Group 17 (Halogens)(Halogens), which are short 1 valence short 1 valence electronelectron, so they want to take one.

Ionic BondsIonic Bonds

Why do Group 1 and Group 17 react so easily with one another?

The answer lies in the valence electrons. The Alkali metalsAlkali metals in Group 1Group 1 all have 1 valence electron 1 valence electron

they want to they want to get rid ofget rid of.. The HalogensHalogens in Group 17Group 17 all have 7 valence electrons, and want to 7 valence electrons, and want to gaingain one one to fill their outer

electron level. Each atom has what the other needs. Result: 2 happy atoms.

Group 1 and 17

IonsIons

• To tell the difference between an atomatom and an ionion,

look to see if there is a chargecharge in the superscript!

Examples: Na+ Ca+2 I- O-2 are all ionsions.

Na Ca I O are NOTNOT ions.

In general

metalsmetals lose electrons positive ionspositive ions

nonmetalsnonmetals gain electrons negative ionsnegative ions

Sodium mixing with chlorine

Compounds always have properties that are different from the elements they’re made of.

For instance: Na (sodium) is a metal that explodes in water, and Cl (chlorine) is a poisonous gas, but when they combine, they make NaCl (salt) which we eat every day.

Cool, huh?

ElementsElements

simplest substances can’t be broken down into other substances

92 occur naturally

everything in the universe is made of 1 or more elements

Examples: carbon, oxygen, gold

are made of

AtomsAtoms

CompoundsCompounds

combination of 2 or more combination of 2 or more elementselements

ExampleExample:: water (H2O)

OH H

ExampleExample:: carbon dioxideCO2

CO O

are made of

MoleculesMolecules

2 or more atomsatoms bonded together (connected)

All pure substancespure substances fall into 2 categories:

MMiixxttuurreess

A mixture is a combination of 2 or more substances, which have been physicallyphysically

but not chemicallynot chemically combined.

Examples:Examples:

trail mixtrail mix dirtdirt air air saladsalad

chocolate chip cookieschocolate chip cookies salt watersalt water

LemonadeLemonade sandwichsandwich

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.







How are mixtures different from How are mixtures different from compounds?compounds?

MMiixxttuurreess CompoundsCompounds

• pure substance (only one substance)

• chemically combined

• cannot be separated physically

• have a molecular formula

• more than one more than one substancesubstance

• physically combinedphysically combined

• can be separated can be separated physicallyphysically

• do not have a do not have a molecular formulamolecular formula

There are 2 types of mixtures

Homogeneous Homogeneous mixturesmixtures

HHeetteerrooggeenneeoouuss mixturesmixtures

• the substances in the mixture are spread evenly throughout it (homo = samehomo = same)

Examples: air, steel, salt water, Gatorade

• the substances in the mixture are not spread evenly throughout (heterohetero = different= different)

Examples: dirt, Lucky Charms cereal, chocolate chip cookies

Learning Check:Learning Check: Pure SubstancePure Substance or M Miixxttuurree??

WaterWater

SaltSalt

Salt waterSalt water

Carbon dioxideCarbon dioxide

Ammonia (NHAmmonia (NH33))

GoldGold

Ice Melt (CaClIce Melt (CaCl22))

Rust (FeRust (Fe22OO33))

IronIron

HeliumHelium

LeatherLeather

MilkMilk

NeonNeon

ChocolateChocolate

Learning Check:Learning Check: Pure SubstancePure Substance or M Miixxttuurree??

AnswersAnswers

WaterWater PSPS

SaltSalt PSPS

Salt waterSalt water MixMix

Carbon dioxide Carbon dioxide PSPS

Ammonia (NHAmmonia (NH33)) PSPS

GoldGold PSPS

Ice Melt (CaClIce Melt (CaCl22)) PSPS

Rust (FeRust (Fe22OO33)) PSPS

IronIron PSPS

HeliumHelium PSPS

LeatherLeather MixMix

MilkMilk MixMix

NeonNeon PSPS

ChocolateChocolate MixMix

ATOMSATOMS

and

ELEMENTS:ELEMENTS:

Which is Which?

If an atom is like a….

Scoop of ice cream

Then elements are like…

92 different flavors

An element can only have one flavor of ice cream….

or

But a compound has more than one flavor…

This would be an “atom” of an element…

And this would be a “molecule” of the same element…

And these would all be “molecules” of different compounds…

AtomsAtoms are the smallest units of an element that have the properties of that element.

ElementsElements are substances that cannot be broken down into simpler substances. They are the simplest substancessimplest substances.

Every substance in the universe is a combination of one or more elements.

ElementsElements

92 elements occur naturally on Earth; 23 have been created by scientists in laboratories.

•The elements and their symbols are shown on the PERIODIC TABLEPERIODIC TABLE

• How many elements are there?

PERIODIC PERIODIC TABLETABLE

listen to the “Element Song”

Elements are arranged on the Periodic Table:

Vertically into GroupsGroups

Horizontally Into PeriodsPeriods

If you looked at one atom of every element in a groupgroup you would

see…

Every atom in a group has the same number ofsame number of electronselectrons in its outermost in its outermost level.level.

For example…

The Group 2Group 2 atoms atoms all have 2 electrons2 electrons in their outer level

Be (Beryllium)

Atom

Mg (Magnesium) Atom

• The electrons in an atom’s outermost level are called the valencevalence electronselectrons.

• The number of valence electrons in an atom affects the way the atom bonds with other atoms.

• This is why elements in a group have similar properties– because they have the same number of valence electrons.

Electron Dot DiagramsElectron Dot Diagrams show the number of valence electrons in an atom.

Observe the electron dot diagrams of the Period 2 elements below.

What do you notice?

Electron Dot Electron Dot DiagramsDiagrams

Each group has distinct propertiesEach group has distinct properties• The Periodic Table is divided into groups

(vertical columns) based on the properties of each group’s elements.

Groups whose elements behave very similarly are called a familyfamily..

Soft, silvery-colored solids

Have 1 valence electron

Very reactive!!! especially with Halogens (Group 17)

video showing alkali metals mixing with water. Stand back…

Alkali Metals (Group 1)Alkali Metals (Group 1)

sodium reacting with water more Na + H2O

Lithium Sodium

Potassium

Rubidium Cesium

Alkali Metals

Francium

Alkaline Earth Metals Alkaline Earth Metals (Group 2) (Group 2)

Silvery-White Solids

Have 2 valence electrons

Fairly reactive

Many are found in rocks in the earth’s crust

CalciumMagnesium

Strontium

Beryllium

Barium Radium

Alkaline Earth Metals

Transition Metals Transition Metals Groups 3 to 12Groups 3 to 12

Malleable Malleable (easily hammered into sheets)

Most are good good conductors of conductors of electricityelectricity

DuctileDuctile (easily extruded into wire)• SolidsSolids (except for Mercury)

• High melting High melting pointspoints

• Shiny lusterShiny luster

Uses of Uses of Transition Transition

MetalsMetals

MetalloidsMetalloids

• share properties with both metals and non-metals

• Si (Silicon) and Ge (Germanium) are very important semi-conductors.



• Semi-conductors are used in computer technology.

What are semiconductors used in?

NonmetalsNonmetals

Solids are brittle (break easily, don’t bend)

Do not conduct electricity



Some are gases, some are solids, one is a liquid.

Non-Metals: Solids

Have 7 valence electrons

Reactive, especially with Alkali Alkali Metals (Group 1)Metals (Group 1)

Halogens (Group 17)Halogens (Group 17)

Exist as diatomic moleculesdiatomic molecules (F2, Cl2, Br2, I2), so that both atoms have a full outer electron layer.

Cl Cl

Remember:

BRINCLHOFBRINCLHOF

HalogensHalogens

• Fluorine is put into water supplies to kill harmful bacteria and to help keep teeth healthy.

• Chlorine is used in swimming pools to kill bacteria in the water.

• Bromine is used in pesticides. Silver bromide is used in photography.

• Iodine is used as an antiseptic on cuts, and as an indicator for starch.

Uses of the HalogensUses of the Halogens

Outer electron levels are full

Non-reactive (stable)

Gases at room temperature

Low melting & boiling points

Exist as single atoms rather than diatomic molecules (He, Ne, Ar, Kr, Xe, Rn)(He, Ne, Ar, Kr, Xe, Rn)

Noble Gases Noble Gases (Group 18)(Group 18)

HeliumHelium is lighter then air and is used in balloons and airships.

Jellyfish lamps made with noble gases artist- Eric Ehlenberger

Colors Noble Gases produce in lamp tubes:

• Ne (Neon):Ne (Neon): orange-redorange-red

• Hg (Mercury):Hg (Mercury): light bluelight blue

• Ar (Argon):Ar (Argon): purplepurple

• He (Helium):He (Helium): pale peachpale peach

• Kr (Krypton):Kr (Krypton): silvery-whitesilvery-white

• Xe (Xenon):Xe (Xenon): blue-greenblue-green

Actinide SeriesActinide SeriesLanthanide SeriesLanthanide Series

Both are silver, silvery-white, or gray metals. The metals have a high luster, but tarnish readily in air. The metals have high electrical conductivityhigh electrical conductivity. Many fluoresce under UV lightfluoresce under UV light.

LanthanidesLanthanides are relatively soft, have high melting and boiling points, burn in air, and are very reactive. Most are widely used in laserslasers.

ActinidesActinides are very dense radioactive metalsvery dense radioactive metals, which combine directly with most nonmetals.

Lanthanides & ActinidesLanthanides & Actinides

We are all carbon-based

life forms; carboncarbon is found in every living thing on Earth.

78% of the Earth’s

atmosphere is NitrogenNitrogen..

OxygenOxygen makes up 21% of Earth’s atmosphere, 47% of the Earth’s crust, and is found in 100% of all living things on Earth.

Really Important ElementsReally Important Elements

46.6% Oxygen (O)27.7% Silicon (Si) 8.1% Aluminum (Al) 5.0% Iron (Fe) 3.6% Calcium (Ca) 2.8% Sodium (Na) 2.6% Potassium (K) 2.1% Magnesium (Mg)

The 8 most common elements in Earth’s crust (by mass):

1. Oxygen (65%) 2. Carbon (18%) 3. Hydrogen (10%) 4. Nitrogen (3%) 5. Calcium (1.5%) 6. Phosphorus (1.0%) 7. Potassium (0.35%) 8. Sulfur (0.25%) 9. Sodium (0.15%) 10. Magnesium (0.05%)

Hydrogen Nitrogen

Top 10 elements in the human body

BismuthBismuth MercuryMercury

the only metal that is liquid at STP

Some Cool ElementsSome Cool Elements

Gold Silver

Platinum

Elements used in jewelry

Dmitri MendeleevDmitri Mendeleev (1834 - 1907) organized the 63 (1834 - 1907) organized the 63 elements that were known at that time (1869) elements that were known at that time (1869) into the Periodic Table. He predicted the into the Periodic Table. He predicted the existence of 3 elements that had not yet been existence of 3 elements that had not yet been discovered, based on the gaps in his periodic discovered, based on the gaps in his periodic table. The 3 elements were discovered later table. The 3 elements were discovered later (scandium, gallium, germanium).(scandium, gallium, germanium).

Mendeleev’s Periodic TableMendeleev’s Periodic Table

Early versions Early versions of Mendeleev’s of Mendeleev’s

tabletable

Glenn Glenn SeaborgSeaborg(1912-(1912-1999)1999)

• Discovered 8 Discovered 8 new elementsnew elements

• Only living Only living person for person for whom an whom an element was element was named (named (Sg, Sg, seaborgiumseaborgium, , element 106)element 106)

RutherfordRutherford

BohrBohr

CurieCurie

EinsteinEinstein

MendeleevMendeleevOther scientists who have had elements named after them…

Balancing Chemical Balancing Chemical EquationsEquations

Chemical Bonds & Chemical Bonds & ReactionsReactions

Let’s BondLet’s Bond

H2

A moleculemolecule is formed when two or more atoms are bonded together.

Chemical Bonds are formed Chemical Bonds are formed when atoms when atoms gaingain, , loselose or or shareshare

valence electrons.valence electrons.

Valence electrons are like the outer level of skydivers in this formation… it’s much easier for them to pull away than it is for the skydivers in the middle. It’s also easier for a new skydiver (electron) to join the outer levelouter level of the formation than it is to move into the middle middle of the formation.

When atoms come together by sharing sharing electronselectrons, the bond is a covalent bondcovalent bond.

Water - A Most Important MoleculeWater - A Most Important Molecule

Note how covalent bondingcovalent bonding fills all outer electron levels; oxygen has 8 valence electrons, and the hydrogens both have 2. By sharing their electrons, everybody is happy! Water is one of the most stable molecules around.

The electron dot diagrams on the right show molecules with covalent bonds.covalent bonds.

Ion FormationIon Formation: one atom gives electrons, one atom takes electrons

Sodium gives a single electron to chlorine to complete its outer electron shell. Chlorine is happy to take it.

An ionion is an atom or molecule with a positive or negative atom or molecule with a positive or negative chargecharge. Since sodiumsodium loses one electron, it has a positive charge of +1, written as Na+.Na+. Since chlorinechlorine gains one electron, it has a negative charge of -1, written as Cl-Cl-..The type of chemical bond formed in this way is called an ionic bond.ionic bond.

The result is ion formationion formation.

Ions and Ion Formation

Ionic BondsIonic BondsTwo oppositely charged ions are attracted to each other, and bond together.

This type of chemical bond is an ionic bondionic bond.

SaltsSalts are solids held together by ionic bonds.

Elements in Group 1Group 1 (Alkali metals)(Alkali metals) all have 1 valence 1 valence electronelectron, so they bond easily with elements in Group 17Group 17 (Halogens)(Halogens),, which are short 1 valence short 1 valence electronelectron.

Why do Group 1 and Group 17 react so easily with one another?

The answer lies in the valence electrons. The Alkali metalsAlkali metals in Group 1 all have 1 valence electron they want to 1 valence electron they want to get rid ofget rid of.. The HalogensHalogens in Group 17 are all short 1 valence electron, and want all short 1 valence electron, and want to to gaingain one one to fill their outer electron level. Each group has what the other needs. Result: 2 happy atoms.

Sodium mixing with chlorine

http://www.youtube.c

om/watch?v=2mzDwgyk6

QM&NR=1

The Parts of a Chemical ReactionThe Parts of a Chemical Reaction

ReactantsReactants: ingredients in a chemical reaction; ingredients in a chemical reaction; substances that you substances that you startstart with with

ProductsProducts: new substances formed in a chemical new substances formed in a chemical reaction (what you reaction (what you end upend up with) with)

ExampleExample: : PhotosynthesisPhotosynthesis

carbon dioxide + water sugar + oxygen

reactantsreactants productsproducts

sunlight

The arrow stands for “becomes”

There are 4 types of chemical reactions:There are 4 types of chemical reactions:

1. SYNTHESIS : A + B ---> AB1. SYNTHESIS : A + B ---> ABTwo elements combine to form a compound.Two elements combine to form a compound.

2. DECOMPOSITION : AB ---> A + B2. DECOMPOSITION : AB ---> A + BA compound breaks down to form simpler A compound breaks down to form simpler

substances.substances.

3. SINGLE REPLACEMENTSINGLE REPLACEMENT A + BC ---> AC + BA + BC ---> AC + BOne element replaces another in a compound.One element replaces another in a compound.

4. DOUBLE REPLACEMENT4. DOUBLE REPLACEMENT AB + CD ---> AC + BDAB + CD ---> AC + BDTwo elements replace each other, forming two new Two elements replace each other, forming two new

compounds.compounds.

CHEMICAL REACTIONSCHEMICAL REACTIONS

Let’s look at the 4 types of chemical reactions Let’s look at the 4 types of chemical reactions in another way…in another way…

1.1. Synthesis reaction Synthesis reaction (putting together a compound)(putting together a compound)

+

2. Decomposition Reaction 2. Decomposition Reaction (breaking down a compound)(breaking down a compound)

+

3. Single Replacement Reaction 3. Single Replacement Reaction (1 atom switches places)(1 atom switches places)

+ +

4. Double Replacement Reaction 4. Double Replacement Reaction (2 atoms switch places)(2 atoms switch places)

+ +

4 types of chemical reactions

Or, we could use relationships to visualize the 4 Or, we could use relationships to visualize the 4 types of reactions….types of reactions….

1. Synthesis 1. Synthesis (Get together)(Get together)

++

2. Decomposition 2. Decomposition (Break-up)(Break-up)

++

3. Single Replacement 3. Single Replacement (change 1 partner)(change 1 partner) ++ ++

4. Double 4. Double Replacement Replacement (change 2 (change 2 partners)partners)

++ ++

relationships

The The Law of Law of Conservation Conservation of Massof Mass states: ““During a During a physical or physical or chemical chemical change, change, matter is matter is neither neither created nor created nor destroyeddestroyed.”.”

This means that during a during a chemical chemical reactionreaction, the atoms on the atoms on the leftleft side of the arrow must balance or equal the equal the atoms on the atoms on the rightright side of the arrow.

4 H atoms4 H atoms

2 O atoms2 O atoms

4 H atoms4 H atoms

2 O atoms2 O atoms

WRONGWRONG

WRONGWRONG

This one is This one is CORRECT!CORRECT!

Balancing Chemical EquationsBalancing Chemical Equations

Motion

Speed, Velocity, and Acceleration

Frames of Reference The object or point from which

movement is determined Movement is relative to an object

that appears stationary Earth is the most common frame

of reference

Motion Motion is a change in position

relative to a frame of reference Speed is the distance traveled

in a given amount of time Speed=distance time

Speed

The units for speed: meters/second (m/sec) kilometers/hour (km/hr) Speed that does not change

is constant speed

Average speed

Total distance divided by the total time

Formula: Total distance total time

Velocity Speed in a given direction Velocities in the same direction

combine by adding Velocities in different directions

combine by subtracting

Interesting fact

Rockets are launched in the same direction of the earth’s rotation to get an extra boost of 1800 km/hr to its speed

Acceleration The change in velocity Acceleration is measured in

m/sec/sec or m/sec2

Formula is: (final velocity - original

velocity)/time

3 types of Acceleration 3 types of Acceleration (change in velocity)(change in velocity)

1. 1. IncreaseIncrease in speed (speed in speed (speed up)up)

2. 2. DecreaseDecrease in speed (slow in speed (slow down)down)

3. Change in 3. Change in directiondirection

Deceleration vs. Acceleration A decrease in velocity is

deceleration or negative acceleration

A distance-time graph for acceleration is always a curve

Centripetal Acceleration

Acceleration directed toward the center of circular path

Momentum Momentum is equal to the mass of an object times its velocity

momentum= mass x velocity

MomentumMomentum

MomentumMomentum = mass x velocityThink of momentum as moving moving

inertia, inertia, or how hard it is to stop an how hard it is to stop an object’s motionobject’s motion. . An object with a large mass or high velocity will be much harder to stop than an object with a small mass or a low velocity.Which has more Which has more

momentum? Why?momentum? Why?

“mass in motion”

Conserving Momentum The law of conservation of momentum states that the total momentum is NOT lost, only transferred

Friction: Friction: The Force The Force

ThiefThief

Friction wastes energy, usually in the form of heatheat or soundsound.

FrictionFriction is when two surfaces rub two surfaces rub against each other.against each other.

Newton’s First Newton’s First Law of MotionLaw of Motion

An object at rest will remain at rest, and an object that is moving will continue moving in a straight line at a constant speed, unless the object is acted upon by another force.This tendency of objects to keep doing what they are doing is called inertiainertia.

Newton’s Second Newton’s Second Law of MotionLaw of Motion

Force = Mass x accelerationF =

maSituation 1: If the mass of an object stays the same, then a greater force will cause a greater acceleration.Example: When Babe Ruth hit a baseball, it went a lot faster than it would if a Little League player hit it.

Example: When you throw a baseball, it travels a lot faster than a bowling ball would.

Situation 2: If the force on an object stays the same, then a smaller mass will have a greater acceleration than a larger mass.

Newton’s Third Newton’s Third Law of MotionLaw of Motion

For every action, there is an equal and opposite reaction.

Momentous SituationsMomentous Situations1. Which has more momentum, a crawling baby or a running tiger? Why?

2. What is the momentum of a 20-kg dog running at a speed of 3 m/s?

3. Which has more momentum, a 250 kg dolphin swimming at 6 m/s, or a 450 kg manatee swimming at 2 m/s ?

4. A golf ball (mass 45 g.) travels at 16 m/s, while a baseball (mass 140 g.) travels at 7 m/s. Which has greater momentum?

The total momentum of any group of objects remains the same unless outside forces

(like friction) act on the object.

Examples: Newton’s cradle, billiard balls

Law of Law of Conservation of Conservation of

MomentumMomentum

Potential & Kinetic Potential & Kinetic EnergyEnergy

Potential energyPotential energy is energy energy stored stored within an object due to within an object due to its position.its position.

ExampleExample: a yo-yo with the string completely wound up.



Kinetic energyKinetic energy is moving energy energy (energy that an object has because of its motion).

ExampleExample: a yo-yo that is moving.

Examples of Potential & Examples of Potential & Kinetic EnergyKinetic Energy

GravityGravity

GravityGravity is an attractive force that allall objects in the universe possess.

Everyone knows that the Earth exerts gravity on us, but did you know that you exert gravity on the Earth?The difference is that the Earth has a lot more massmass than you do, so its gravitational pull is much stronger than yours.

Mr. Skelton’s 8 th Grade MCAS Science Review Part 2.

Documents

Transcript of Mr. Skelton’s 8 th Grade MCAS Science Review Part 2.