October 28 2010 - Uncertainty in Measured Numbers

Today we went over uncertainty in measured numbers.

We can't have perfect measurement. Measurements have uncertainty due to the limits of instruments.

For example,

Bob weighs himself on his bathroom scale. The smallest divisions on the scale are 1-pound marks, so the least count of the instrument is 1 pound.


Bob reads his weight as closest to the 142-pound mark. He knows his weight must be larger than 141.5 pounds (or else it would be closer to the 141-pound mark), but smaller than 142.5 pounds (or else it would be closer to the 143-pound mark). So Bob's weight must be

weight = 142 +/- 0.5 pounds
---------------------------------------------------------------------------------------------

In general, the uncertainty in a single measurement from a single instrument is half the least count of the instrument.

--------------------------------uncertainty in weight

percentage uncertainty = --------------------------- * 100%

---------------------------------value for weight

-----------------------------------0.5 pounds

-------------------------------= --------------- * 100% = 0.35%

----------------------------------142 pounds

Precision refers to how close together a group of measurements actually are to each other.

Accuracy refers to how close one comes to an accepted value.

Ordinarily the more precise measurement if the more accurate.

Happy Mole Day To You ...

Had to post this for the lulz.

October 26 2010 -- Significant Digits

Today we reviewed the significant digits (or significant figures).

A significant digit is any digit that contributes to giving precision for a number.

Non-zero digits are always significant, however, zeroes follow rules based on their position within a number :

1. Zeroes placed before other digits are not significant; 0.046 has two significant digits.
2. Zeroes placed between other digits are always significant; 4009 kg has four significant digits.
3. Zeroes placed after other digits but behind a decimal point are significant; 7.90 has three significant digits.
4. Zeroes at the end of a number are significant only if they are behind a decimal point as in (c). Otherwise, it is impossible to tell if they are significant.

We also learned how to write answers in proper form; in multiplication/division calculations, the answer must have the same number of significant digits as the number with the lowest amount of significant figures within the problem. In addition/subtraction calculations, the answer must have the same number of decimal places as the number with the lowest amount of decimal places within the problem.

October 19 2010 - Lab 3B

Today we did a cool Lab where we did "Separation of a Micture by Paper Chromatography".

-Where we use 3 color dyes (unknown, green and either red, blue, and yellow)

-We used chromatography paper as the object to place the dyes onto and see the filtration of colors happen

-Our color dyes as the solute in this experiment

-The solvent used was water

-As time passed we watched our different dyes, yellow, green and unknown creep up the chromatography paper

-As the water is absorbed and reaches higher and higer, we can see the colors filter out into the primary colors and the dye stretch out and up

-We later understood that unknown dye is a mixture of all the 3 primary colors

-The yellow just filtered out to more yellow

-The unknown dye later filtered out to red, yellow, and blue

-While the green dye filtered out to yellow and blue dye

Here is another video, very similar to the experiment we did :

October 15 2010 -- Separating Mixtures

Separating shows the different components and different properties of the mixtures. You need to think of a process that can separate between components with different properties.

http://sciencepark.etacude.com/projects/separations/separation1.php
This website lists and explains several basic techniques for separation of mixtures, such as filtration, evaporation, crystallization, distillation, and chromatography.

October 13 2010 - Naming Acids

Today we went over naming acids.

Naming Acids

Acids = compound composed of hydrogen ions and a negatively charged ion are dissolved in water (aqueous(aq)).

October 7 2010 -- Ionic and Covalent Compounds

Today we reviewed the properties of ionic and covalent compounds.



Ionic compounds are compounds that are made of 2 or more oppositely charged particles. They are held together by electrostatic forces and electrons are transferred from metal elements to non-metal elements.

In covalent compounds, the electrons are shared between two non-metal elements.

October 5 2010 - Lab

Today w did the lab "Heating and Cooling Curves of a Pure Substance"

-We investigated the heating an

d cooling process for solid and liquid dodecanoic acid. So we'll be able to determine or compare the freezing and melting point.

-We heated and cooled the dodecanoic acid with a hot plate, beaker with water and tube clamps

-When the acid was freezing you can see the white crystals forming and it texture

-When the acid heats up, it is very noticeable, like melting ice but the acid is yellowish white

-We used a thermometer to measure the temperature s of the freezing and melting points of the acid

-We determined that the freezing point was 37'C and melting was 47'C

October 1 2010 -- Matter Is Made of Atoms

Atoms
• Atom – smallest possible piece of something


Elements
• Elements – pure substances that cannot be broken down
• Contains only one kind of atom
• Can exist as a solid, liquid, or gas
• Molecules – particles made of more than one atom
• Have different melting points and boiling points because they vary in mass and size
o The larger the particle, the higher the boiling point

Compounds
• Made by combining elements in definite properties
• Can also exist as solids, liquids, and gases
• Ions – particles that have an electrical charge

October 1 2010 -- Matter in the Macroscopic World (text)

What You Know About Matter
• Water normally exists as a liquid that pours freely to fill any solid container, and will solidify if cooled, or evaporate if heated
• Properties such as color and taste of characteristics of matter

Purifying Matter
• Mixture – two (or more) kinds of matter that have separate identities
o Easily separated into component parts
o Said to be impure
o Mixtures like salt water or sugar water that look uniform throughout and do not scatter light are called solutions
o Separating solutions like salt water or sugar water by boiling them dry on the stove is called distillation

Characteristics of Pure Substances
• Pure substances have a constant boiling point
• Freezing point – the temperature at which a liquid solidifies
• Melting point – the temperature at which a solid liquefies

Chemical and Physical Changes
• Density – a property of matter that describes its mass per unit volume
• Chemical change – changes that produce a new kind of matter with different properties
• Decomposition – when one kind of matter breaks apart to create two or more kinds of matter
• Physical change – changes that are easily reversed to get the original material back again; do not appear to produce new kinds of matter

Compounds and Elements
• Electrolysis – involves passing an electric current through a substance to make it decompose into new kinds of matter
• Compounds – pure substances that can be decomposed into new kinds of matter
• Elements – elemental building blocks of all kinds of matter; cannot be decomposed
o 109 known elements

October 1 2010 -- New Laws and the Heating/Cooling Curve of a Pure Substance (notes)

On Friday, we started a new topic about matter and the heating/cooling curve of a pure substance.

Law of Definite Composition
Compounds will have a definite composition. For example, water (H2O) will be water anywhere (it always have 2 hydrogens and 1 oxygen).

Law of Multiple Proportions – when two or more compounds with different properties of the same elements can be made. For example, carbon dioxide (CO2) x 2 = dicarbon tetraoxide/oxalate (C2O4).


A : At this point, the pure substance is in the solid state, because the pure substance is below the melting point. The particles are packed together closely in an orderly manner. Particles can only vibrate at a fixed position and the forces between the particles are very strong.

A-B : When the pure substance is heated, the heat energy is converted to kinetic energy. The molecules vibrate faster in their fixed positions and the temperature increases when the kinetic energy increases.

B : The pure substance is still solid, but the melting stage has begun – the solid begins to change into a liquid. The temperature remains the same.

B-C : The pure substance exists in both solid and liquid states. The heat that is supplied to it is used to overcome the forces of attraction that holds the particles together because the temperature remains constant. This constant temperature is called the melting point. The latent heat of fusion is the heat energy absorbed to overcome the intermolecular forces.

C : The pure substance has completely melted, and the solid has turned to liquid.

C-D : The pure substance is in liquid state, and as the liquid is heated and the temperature is increased, the particles move faster because the kinetic energy is increasing.

D : The pure substance is still in liquid state, and the molecules have gained enough kinetic energy to overcome the forces of attraction between the particles. Some of the molecules begin to change into gas.

D-E : The pure substance is in both liquid and gaseous states. The temperature is unchanged, and the heat energy absorbed is used to overcome the molecular forces between the particles of the liquid rather than increase the temperature. This constant temperature is called the boiling point.

E : All of the pure substance liquid has turned into gas.

E-F : The gas particles continue to absorb more energy and move faster. The temperature increases as heating continues.