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The Scientific MethodThe Atomic TheoryClassification of Matter

Dr. Yau

(loosely based on Chap. 1.4, 1.5, 1.6

in Jespersen, Brady & Hyslop, 6th edition)

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What is meant by the “Scientific Method?”

The scientific method is a way to solve a problem or answer a question in science.

One simplistic way to view this method is to think of it as being in 4 steps:

1. Observe

2. Hypothesize

3. Test

4. Form Theory

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The Scientific Method1. Observe (looking for a pattern in the

observations) Note: Observations could be from someone else’s experiments.

2. Form a hypothesis (which is an educated guess to explain the pattern)

3. Test the hypothesis a) Design a test that would provide

evidence that the hypothesis is correct. It MUST be reproducible.

b) Run the test and analyze the results.4. If proven correct, hypothesis becomes

theory. If not, it must be revised, re-tested or

discarded.

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Scientific TheoryA scientific theory is not “absolute.”

With improvement in instrumentation and new discoveries, additional experimental data can render a theory to become questionable.

We can never be absolutely sure that a theory is correct.

Scientists MUST keep an open mind.

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The Scientific Method is Cyclical

Fig 1.2 p. 4 Observations suggest explanations, which suggest new experiments, which suggest new explanations, and so on.

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What is a Scientific Law?It is a generalized summary of a large quantity of data

and observations for which there is no exception.

It differs from a hypothesis or a theory in that it does not attempt to explain the phenomenon.

e.g. The scientist, Boyle, noticed that when a sample of gas is allowed to expand, its pressure decreases proportionally if the temperature is kept constant. He states that the pressure (P) of a gas is inversely proportional to its volume (V) at constant temperature (T):

There is no attempt to explain why.

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Theory vs. Scientific LawBoyle’s Law (previous slide) alone did not bring us to

the theory, but together with other laws such as Charles’ Law, Gay-Lussac’s Law and Avogadro’s Law, scientists came up with the Kinetic Molecular Theory to explain the behavior of gases.

A sample of gases is made of small particles that are very far apart and are in constant random motion. It is the collision of these particles on the walls of the container that creates pressure.

When the V is small, there would be more collisions with the walls, and thus P is larger.

When V is increased, the particles have more room to move and there are less collisions with the walls and P decreases.

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Learning Check:Whenever we put a burning match to a piece of paper it will burn. Classify this statement. It is…

1. a hypothesis2. a theory3. a test4. an observation5. a scientific law6. a conclusion

Answer: #4 Why is it not a hypothesis or theory?

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Dalton’s Atomic Theory

Dalton’s Atomic Theory is based on three scientific laws:

• Law of Conservation of Mass

• Law of Definite Proportions

• Law of Multiple Proportions

(These comprise the “observations” in the Scientific Method, leading to the “hypothesis” and ultimately, the “theory.”

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Dalton’s Atomic Theory

1. Matter is made of tiny indivisible particles called atoms. (from Greek word “atomos” meaning indivisible).

2. Atoms of an element are identical in mass & other properties. Atoms of different elements have different properties.

3. Compounds are made of atoms of different elements combined in a fixed ratio of small whole numbers.

4. In reactions, atoms are not destroyed or created but merely rearrange to form new cmpds. Why are some words in red????

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What Is An Element?

• Elements - substances that cannot be decomposed into simpler substances.

• shown on the periodic table as symbols: “K” for potassium and “Na” for sodium

There are around 118 known elements.

around 92 are natural occurring

around 26 are man-made

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Models Helps Us Visualize Matter

Elements are made of identical atoms, either singly or in groups.

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Allotropes of Carbona) diamond

b) graphite

c) lonsdaleite (diamond formed

when C meteorites hit the earth)

d) buckyball

e) C540 (nested bucky)

f) C70

g) amorphous C

h) single-walled carbon nanotube or bucky tube

To keep it simple, we just write C for carbon as if it were monatomic.

e

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Chemical Symbols for ElementsChemical Symbol

– One or two letter symbol for each element name– First letter capitalized, second letter lower case

Ex. C = carbon S = sulfur

Ca = calcium Ar = argon

Br = bromine H = hydrogen

Cl = chlorine O = oxygen

• Used to represent elements in chemical formulasEx. Water = H2O

Carbon dioxide = CO2

– Most based on English name– Some based on Latin or German names

Some are given a single letter (always capitalized): H, B, C, N etc.

Some are given two letters (first letter is always capitalized, and the second letter is always in lower case): He, Li, Be etc.

If you write two letters with both upper case you would be implying a compound (made of two elements).e.g. HF is the compound, hydrogen fluoride.and Hf is the element, hafnium.

Chemical Symbols for Elements

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e.g. Co is the element, cobalt.

and CO is the compound, carbon monoxide.

It is essential you learn not to capitalize both letters if you had meant to refer to an element.

In giving the name of an element, do not capitalize the name unless it is at the beginning of a sentence.

The name of an element is NOT a proper noun.

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Examples of elements: C, Fe, Al, O2, N2, P4

Examples of compounds: CO2, C12H22O11, NaCl

What do the subscripts mean?

First we must examine the concept of "atoms."

An atom is the smallest particle of an element. (Note: It can be broken down further but it would become a different element.)

HOMEWORK: Learn the names and symbols of elements. Quiz next period includes this type of question.

Example: Give the name or symbol of the following:

S = ________Ti = ___________ = potassium___ = sodium

Note: It is not sulphur, but sulfur.titanium, not Titanium

not P! What is P?

not S!

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Note that there are symbols in the periodic table with more than 2 letters. Which ones??

These are elements not yet named.Elements are usually named by the scientists who

discovered/made them.Sometimes it is difficult to prove who discovered

them first.Meanwhile they are given 3 letters:Uuu = unununium (Latin for 111) now named roentgenium (symbol = Rg)Uub = ununbium (Latin for 112) now named copernicium (symbol = Cn)Uut = ununtrium (Latin for 113) not formally

named.

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Some chemical names originated from Latin or German

English Name Chemical Symbol Latin Name

Sodium Na Natrium

Potassium K Kalium

Iron Fe Ferrum

Copper Cu Cuprum

Silver Ag Argentum

Gold Au Aurum

Mercury Hg Hydrargyrum

Antimony Sb Stibium

Tin Sn Stannium

Lead Pb Plumbum

Tungsten W Wolfram (German)

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Elements

Getting bored? Have some fun?Click here:

http://www.privatehand.com/flash/elements.html

diamond = carbon gold sulfur

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ElementsYou should have already learned (and still remember!) the

names and chemical symbols for many of these elements.

This semester you will be learning a few more.

If you took CHEM 107 on the Catonsville Campus you would be familiar with the Nomenclature Tutorial for Prep Chem.

This semester you will be using the one designated for Gen Chem. There will be frequent quizzes on sections of this Nomenclature Tutorial. Check the Assignment page for what the quizzes will cover. They will always be cumulative and may contain material other than nomenclature.

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What are Compounds?Compounds are substances made of more

than one element combinedMixtures can also be substances made of

more than one element. e.g. Brass = Cu + Zn e.g. Bronze = Cu + SnWhy are these not “compounds?”There is more than one answer to this. Try

to think up as many as you can.

in a fixed ratio.

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What Is A Compound?

• Compounds - formed from two or more atoms of different elements combined in a fixed proportion

• Have different characteristics than the elements that compose them

• Can be broken down into elements by some chemical changes

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Fixed Ratios in Compounds

Water is composed of H and O in a fixed ratio of 2 to 1,

(1:8 by mass).

Hydrogen peroxide is composed of H and O in a ratio of 1:1, (1:16 by mass).

Hydrogen peroxide

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Fixed Ratios in Compounds

Methane

Methane is composed of C and H in a fixed ratio of 1 to 4.

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Fixed Ratios in Compounds: Oleic Acid from Cow’s Milk

C17H34O2

What does this tell you about Dalton’s Atomic Theory?

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Matter is either a pure substance or a mixtureMixtures may be separated into pure substances

using physical means such as distillation, filtration, chromatography (You did all this in CH108!)

Classification of Matter

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Separation by physical means:They are based on the difference in physical properties of the components.e.g. Filtration of sand from water(Difference in solubility)e.g. Distillation of water from salt water

(Difference in boiling point)e.g. Distillation of crude oil into useful fractions. (Difference in boiling point)

At the oil refineries, crude oil is heated and various fractions are collected at different temperatures based on their boiling points.

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Gases (methane, ethane, propane, butane)C1 to C4 hydrocarbonsBP below 20C

Gasoline (naphthas) (C5 to C12)BP 20-200C

Kerosene (C12 to C15)BP 175-275C

Fuel Oil (Diesel) (C15 to C18)BP 250-400C

Residue (asphalt) (more than C20)

Lubricating Oil (C18 to C20)BP above 350C

Crude Oil is vaporized

Distillation Tower at the Oil Refinery

gas

liquid

solid

Heat Source Distillation is separation by physical means.

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Classification of Matter

Mixtures can be classified as homogeneous mixtures or heterogeneous mixtures.

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Homogeneous Mixtures

Homogeneous mixtures have the same properties throughout the sample.

Solutions are homogeneous mixtures. e.g. salt water, coffee (without the grind) e.g. Rubbing alcohol is a solution of isopropyl

alcohol and water. It can be purchased as a 90% or 70% solution.

Alloys are homogeneous mixtures of metals. e.g. Brass is a homog. mix of Cu and Zn e.g. Bronze is a homog. mix of Cu and Sn.We can have different % of Zn in brass, giving it a

whiter or more orange color. But, within one sample, the percentage is the same.

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Homogeneous Mixtures• Same properties throughout sample• Solution

– Thoroughly stirred homogeneous mixture Ex. – Liquid solution

• Sugar in water– Gas solution

• Air – Contains nitrogen, oxygen, carbon

dioxide & other gases

– Solid solution• US 5¢ coin – Metal Alloy

– Contains copper & nickel metals

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Heterogeneous MixturesHeterogeneous mixtures…• do not have a uniform composition within a

sample• components often not of the same

physical state• components are not miscible (do not mix) e.g. oil and water e.g. mixed nuts e.g. sand and sugar

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Heterogeneous Mixtures• 2 or more regions of different properties• Solution with multiple phases• Separate layersEx.

– Salad dressing• Oil & vinegar

– Ice & water• Same composition• 2 different physical states

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Mixtures• mixtures consist of varying

amounts of two or more elements or compounds

• Homogeneous mixtures or “solutions”- have the same properties throughout the sample

– Brass, tap water• Heterogeneous mixtures-

consist of two or more phases

– Salad dressing, Coca-Cola ™

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Learning Check: Pure or Mixture?

• Brass is pure.

• Natural peanut butter made only by crushing peanuts is pure.

• Because blood cells can be distinguished from plasma under a microscope, blood is a heterogeneous mixture.

• True• False

• True• False

• True• False

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Elements and CompoundsPure substances are classified as elements or

compounds.Compounds can be separated into elements only by

chemical means and not physical means, such as electrolysis, thermal decomposition.

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Separation by Chemical MeansCompounds can be separated into elements by

chemical menas:Decomposition by electrolysis:

– Chemical reaction where 1 substance is broken down into 2 or more simpler substances by passing an electric current thru the sample.

Ex.

Molten sodium chloride

Sodium metal

+ chlorine gas

Electric current

2 NaCl (s) 2 Na (s) + Cl2 (g)

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Separation by Chemical Means

ElectricCurrent

2 H2O (s) 2 H2 (s) + O2 (g)

Do not confuse this with the what happens when you boil water!

Write the equation for what happens when you boil water:

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Learning Check: Classification

Hot

Cocoa

Aluminum Foil

Ice

(H2O)

White Flour

Table Salt (NaCl)

Pure Substance

Element

Compound

Molecule

Heterogeneous Mixture

Homogeneous Mixture

X

X

X

X

X

X

X

X

Why is NaCl not a molecule?

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Metals, Nonmetals, And Metalloids

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Why it matters whether an element is METAL or NONMETAL

KNOW THIS WELL!

Nonmetal + Nonmetal = Molecular cmpd

(covalent bonds)

Metal + Nonmetal = Ionic cmpd

(ionic bonds)

Metal + Metal = Alloy (homog.mixt.)

So why table salt not made of molecules?

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Learning Check

How is table sugar classified? Table sugar is sucrose from sugar cane with the formula

C12H22O11

A. element

B. molecule

C. compound

D. ionic compound

E. homogeneous mixture.

F. heterogeneous mixture.

G. pure substanceAns. B, C, G

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Learning Check

How is S8 classified?

A. elementB. moleculeC. compoundD. ionic compoundE. homogeneous mixtureF. heterogeneous mixtureG. pure substance

Ans. A, B, G

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Learning Check

How is chalk classified?

Assume it is calcium carbonate.

A. element

B. molecule

C. compound

D. ionic compound

E. homogeneous mixture.

F. heterogeneous mixture.

G. pure substance

Ans. C, D, G

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Learning CheckWhich of the following is/are molecular

compounds?

A. P2O5 (in match heads)

B. SO2 (air pollutant from factories)

C. Fool’s gold (pyrite: FeS2) Fool’s Gold

D. H2S (gas produced from acid on fool’s gold, smells like rotten eggs)

E. I2

Ans. A, B, D, E

We can now extend the Classification of Matter further.

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MATTER

Pure Substance Mixture

Element Compound Homogen. Heterogen.

Mixture Mixture

atom molecule

molecular ionic

compound compound