classifying matter - scientific method - the case for the modern atomic theory

matter

anything that has mass and takes up space

nonmatter

everything else, usually energy

light

radio waves

sound

heat

solid phase

definite shape and definite volume

particles vibrate as a function of temperature

particles do not move relative to each other (fixed in place)

particles can be elements or compounds

can be crystalline or amorphous

incompressible

liquid phase

definite volume - incompressible

shape is defined by container

particles move as a function of temperature

particles move relative to each other but stay close together - closely packed but can move

particles can be elements or compounds

gas phase

volume expands or contracts to fit container - compressible

shape is defined by container

particles move as a function of temperature

particles move relative to each other but stay far apart

particles can be elements or compounds

what would happen if we compressed water vapor until it had the same volume as its liquid counterpart

it would condense into water

how many types of atoms

92 natural and several man-made types of atoms

elements

are pure substances made up of only one type of atom

compounds

are pure substances made up of two or more types of atoms in a definite proportion

homogeneous mixture

a mixture which is uniform in composition throughout its entire volume

• sweet tea, saltwater, store bought milk

heterogeneous mixture

a mixture which varies in composition throughout its volume

ex. italian dressing, sand and water, raw milk

scientific method

a predictive, explanatory, self checking, process based on observation, model creation, experimentation, verification, and controversy

steps of scientific method

1.We observe something in nature (collection of data)

2.We create a model (hypothesis, theory, or law) based on those observations

3.We test and refine our model by performing a series of controlled observations known as experiments to see if our model can predict or explain the observations from these experiments. (is our model valid)

4.If our model is not valid, we

  (a) improve it – Change it so that it models the observations

  (b) qualify it – Predictive only under certain conditions  (Ideal Gas, Newtonian Physics)

  (c) scrap it – Start over! (Spontaneous generation, Flat Earth, etc)

5.We share our model with others so they can test our model also. (Publish research)

6.Others attempt to “break” our model.  (This is the controversy part)  If it breaks we/they improve it, qualify it or replace it. It doesn’t break we have further validate our model (it levels up … a little). 

7.We continue to test the model

Be Careful Here:  we never prove the model to be true in the absolute sense it is just the best model to fit observed data at the time.

scientific law

summary of many past observations that can be used to predict future events (what)

hypothesis/theory

interpretation or explanation of the observed phenomena (why and how)

experiment

a trial of set of trials that are used to test the validity of a hypothesis, a theory or a law

control group

baseline

no treatments, nothing changed

experimental group

one variable (the experimental variable, treatment) is changed all else is in control group

response

this is the observed change in the experimental group attributed to the change in the experimental variable

as a rule in experiments

we change one experimental variable at a time. otherwise, it is difficult or impossible to determine which variable caused the response

models

sometimes a model is correct under specific conditions but not correct under all conditions

the model is only as good as its predictive value

newtonian physics

good at macro scale and velocities far from the speed of light

realistic physics

good at velocities near the speed of light

quantum physics

good at the small, atomic scale and smaller

keep in mind

-Science is not dogma, you don’t “believe in” or “have faith in” science – Always require evidence!

-Science is not authority - “Scientific Leaders” are often proven wrong – this is how science has progressed from the beginning. 

-Always consider an experiment’s source, motivations, funding, and experimental design – especially if youare the researcher. 

-Remember dissention is a good thing.

-X could/may/might do Y, experts say.

Greece around 500 BC - Leucippus and democritus

proposed that all matter is composed of atoms

small hard particles

different types shapes and sizes

always moving randomly through empty space

they lacked the technology to explore these hypotheses

laviosier

discovered law of conservation of mass, and disproved phlogiston theory

law of definite proportions

Joseph proust

all samples of a given compound, regardless of their source, or how they were prepared have the same proportion of their constituent elements

2 samples of carbon monoxide are decomposed to carbon and oxygen

proved that elements combine to form compounds in specific (definite) proportions, by mass

this was before atomic theory was proposed

law of multiple proportions

John Dalton

when two or more elements combine to make two or more different compounds, the mass ratios of the elements for each compound are integer multiples of each other

consider a sample of carbon monoxide and carbon dioxide both are decomposed to carbon and oxygen

carbon dioxide contains 2x as much oxygen, relative to carbon as carbon monoxide. if carbon monoxide is CO carbon dioxide must be CO2

Atomic Theory

john dalton

1.Elements are composed of tiny indestructible particles called atoms.

2.All atoms of a given element have the same mass and other properties that distinguish them from atoms of other elements. 

3.Atoms combine in simple whole number ratios to form compounds.

4.Atoms of one element cannot change into atoms of another element.  In a chemical reaction, atoms only change the way that they are bound together with other atoms.

discovery of the electron

J. J. Thomson discovered that if you put a very high voltage across two separated electrodes in a vacuum, that a beam of electrons would form.  These electrons came out of the electrodes.  He discovered that this beam of electron could be deflected by an electromagnetic field.  By this method he was able to determine the mass to charge ratio of the electron.

degree of deflection

determined the mass to charge ratio to be -1.76 × 10^8 Coulomb/g. but not the charge or the mass specifically

degree of deflection

• directly proportional to electron charge

• inversely proportional electron mass

charge of electron

Robert Millikan’s Oil drop experiment:

1.Sprayed fine drops of oil into an ionization chamber sandwiched between two charged plates. 

2.Ionizer resulted in integer numbers of electrons on each oil drop

3.Field strength between plates were increased until the drops floated in mid-air Electrostatic Force = Gravitational force.

4.Charge on drop calculated based on field strength and weight of droplets.

5.Charges varied but were always an integer multiple of -1.60 x 10^-19 C.

6.Charge of electron found to be -1.60 x 10^-19 C.

millikan determined the charge of the electron and then used thompsons charge to mass ratio as a conversion factor to determine the mass of an electron. we build on those who come before us

discovery of nucleus

Gold Foil Experiment:

1.Shot alpha particles (He nuclei) through gold foil.

2.Based on Thompson’s “Plum Pudding” model, the much heavier alpha particles would not be deflected very much if any.

3.Alpha particles were deflected, suggesting presents of very small very heavy regions in the atom.

rutherford proposed nuclear theory - discovery of nucleus

Rutherford Proposed Nuclear Theory:

1.Most of the mass and all of the positive charge in an atom is contained in a small core called the nucleus.

2.Most of the volume of the atom is empty space dispersed with tiny negatively charged electrons.

3.Since the atom is electrically neutral there must be as many electrons outside the nucleus as protons in the nucleus.

4.Later proposed the neutron to make up the missing observed mass in the atom.

accuracy

a measure of how close the average of the measures values are to the true value - function of systematic error

precision

a measure of how close the individual values are to each other - function of random error

systematic error

can be attributed to a readily identifiable (and fixable) cause and is repeatable under identical conditions.  Results in the average deviating from the true value.  Can have high or low scatter depending on the degree of random error in the measurement.

Independent of Random Error

  Cause can be identified and fixed

repeatable/predictable

  results in poor accuracy

  Can be due to:

  -  Faulty Equipment

  - Scale Error

  -  Improper Tare

  - Offset Error

  -  Operator Error

  -  Calibration Issue

  -  Poor protocol

  Fix it by Fixing the Problem!

random error

unpredictable/unrepeatable

  cannot be (easily*) removed

  results in higher scatter/less precision

  average is close to true value

  decreases as number of repeat measurements increase

Examples –

  temperature fluctuations

   electrical noise in circuits

  air currents

  ambient barometric pressure effects