Download Molecules - Net Start Class

Molecules (7.6C)
Overview
Student Expectation
The student is expected to recognize how large molecules are broken down into smaller
molecules such as carbohydrates can be broken down into sugars.
Key Concepts
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Key Concept 1: A molecule is formed when two or more atoms join together.
Molecules can consist of two or more atoms of the same element or from more
than one type of element.
Key Concept 2: Large molecules are made of chains of smaller units.
Key Concept 3: Our bodies take larger molecules and break them down through
chemical changes during digestion to create smaller molecules that can be
absorbed by the body.
Key Concept 4: Carbohydrate chains are broken into simple sugars.
Key Concept 5: Protein chains are broken into amino acids.
Key Concept 6: Lipid chains (fats) are broken into fatty acids.
Fundamental Questions
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What is a molecule?
What does our body do with large molecules we consume?
When an organism consumes a carbohydrate, what molecule(s) do carbohydrates
become during digestion?
When an organism consumes a protein, what molecule(s) do proteins become
during digestion?
When an organism consumes a lipid (fat), what molecule(s) do lipids become
during digestion?
Background Information (same as physical and chemical changes, as they are
related)
Chemical changes in matter produce new substances with properties different from the
original substances. In a chemical change, compounds may change into other compounds.
Then these compounds may be broken down into elements. Alternatively, elements may
combine to form compounds. In all of these types of changes, new substances with new
properties form. Various types of evidence are used to identify the formation of a new
substance through a chemical change.
Gas production is a form of evidence indicating a chemical change taking place. For
example, when baking soda and vinegar react one of the products formed is carbon
dioxide gas. When the two reactants combine, bubbles form indicating the production of
CO2 gas. Another way to see this gas formation is to put the two reactants in a sealed
flask with a deflated balloon around the top of the flask. When the two reactants
combine, they form bubbles and the balloon inflates.
Temperature changes are sometimes used as evidence to indicate a chemical reaction has
occurred. However, a temperature change does not always indicate a chemical change.
Thermal energy is transferable in many processes, including physical changes. For
example, when ice is placed in warm water, the temperature changes but no chemical
reaction takes place. However, when two combined substances with similar beginning
temperatures produce (exothermic) or lose (endothermic) heat, then a temperature change
indicates a chemical reaction.
The formation of a precipitate can be evidence of a chemical change. A precipitate is an
insoluble compound formed out of solution in precipitation reactions. A common
example of precipitate formation is seen when mixing two clear solutions, silver nitrate
and sodium chloride. The result is a chemical change in which two products, sodium
nitrate and silver chloride, a white solid, are produced. Silver chloride is the precipitate. It
is a white solid.
Color changes, as with temperature changes, are sometimes used as evidence of a
chemical change. Not all color changes, however, indicate chemical changes. For
example, adding food color to water might appear to change the color of the water, but no
chemical reaction takes place. Many times a chemical indicator changes color when the
pH of a solution changes. When the two clear solutions, silver nitrate and potassium
iodide are mixed, a bright yellow compound called silver iodide precipitates. This color
change is an indicator of a chemical change, as well as a sediment formation.
In the digestive system, both physical and chemical changes occur as food digests. When
food first enters the digestive system in the mouth, it changes physically through the
process of chewing. Food then physically changes into smaller pieces that are easy to
swallow. Most of the physical change (mechanical digestion) is done by chewing. Saliva
mixes with food to moisten it making it easier to swallow. Digestive enzymes are present
in saliva beginning the process of chemically changing the food (chemical digestion).
Food passes from the mouth to the stomach through the esophagus. The walls of the
esophagus produce mucus, which lubricates the food making it easier to move. Next,
food enters the stomach where the breakdown of protein in food begins. The muscles in
the stomach churn the food contributing to mechanical digestion (physical change). Food
is also mixing with digestive juice containing mucus, pepsin (an enzyme that digests
proteins) and hydrochloric acid. These help to break food down chemically. As food
leaves the stomach and passes into the small intestine, the food becomes a thick liquid
called chyme. Muscles move food through the small intestine by peristalsis. Most
chemical digestion occurs in the small intestine. Food mixes with several digestive juices
produced by the small intestine, liver and pancreas.
Once food is digested, it absorbs into the bloodstream in the small intestine. This is where
nutrients pass through all body parts. When food enters the small intestine, the pancreas
releases enzymes that help digest starch, proteins and fats. The liver produces bile, which
helps break apart large clusters of fat into tiny droplets making it easier for enzymes to
digest. The last part of the digestive system is the large intestine. Undigested food
entering the large intestine contains lots of water and minerals. After the water and
minerals are reabsorbed by the large intestine, feces remain as waste until it is eliminated
from the body.
As food is digested, larger molecules break down chemically into smaller molecules with
the help of digestive enzymes. Catabolism is a process that helps break down larger
molecules into smaller units and releases energy. In this process, large molecules such as
polysaccharides, lipids, nucleic acids and proteins are broken down into
monosaccharides, fatty acids, nucleotides and amino acids.
Carbohydrates create smaller subunits called saccharides. This includes sugars, starches
and fiber. Through hydrolysis, water breaks long chains of polysaccharides into smaller
chains or simpler carbohydrates. The body converts digestible (non-fiber) carbohydrates
into glucose, which cells use as fuel. Some carbohydrates (simple) break down quickly
into glucose while others (complex) slowly break down and enter the bloodstream
gradually. During digestion, all carbohydrates break down into glucose before entering
the bloodstream where insulin helps the glucose enter the body’s cells. Some glucose is
stored as glycogen in the liver and muscles for future use. Simple carbohydrates are
composed of 1 or 2 sugar units. Simple carbohydrates are digested quickly. Complex
carbohydrates (starches) are made up of many smaller units and take longer to break
down and be digested.
A chemical formula is a combination of element symbols and subscript numbers used to
show the composition of a substance, either as a compound or in pure element form.
Chemical formulas make it easier to describe substances by providing information about
the number and kinds of atoms creating a molecule of that substance. These formulas
identify each component element by its chemical symbol and indicate the atoms’ number
of each element found in each molecule of that substance. If a molecule contains more
than one atom of a particular element, this quantity is indicated by using a subscript after
the chemical symbol.
For example, a molecule of carbon dioxide consists of one carbon atom and two oxygen
atoms. Its chemical formula is CO2. If there is only one atom of an element in a molecule
of a compound, then there is no subscript number written. Another example, glucose, is
represented by the chemical formula C6H12O6. For each molecule of glucose, there are six
carbons atoms, twelve hydrogen atoms, and six oxygen atoms. Some elements occur
naturally in pairs of atoms rather than as single atoms. These are called diatomic
molecules. Hydrogen gas (H2), nitrogen gas (N2), and oxygen gas (O2) are common
diatomic molecules.
Chemical formulas are used in chemical equations to describe chemical reactions.
Chemical equations represent chemical reactions symbolically with the reactants written
on the left hand side of an arrow (represented by an arrow symbol and usually read as
“yields”) and the products written on the right hand side. If there is more than one
reactant or product, then they are separated by a plus sign.
The Law of Conservation of Mass states the quantity of each element does not change in
a chemical reaction. Thus, each side of a chemical equation must represent the same
quantity of each element in the reaction. Chemical equations are balanced by adjusting
the number of each chemical formula by placing a number, the coefficient, in front of the
formula. Most of the time equations are balanced using the smallest whole number
coefficient for a reactant or product.
For example, hydrogen gas (H2), can react with oxygen gas (O2), to form water (H2O).
H2 + O2 → H2O
This equation, however, is not balanced. Since the quantity of each element cannot
change according to the Law of Conservation of Mass, coefficients must be used to
“balance” the equation. In order for the amount of oxygen to be the same on each side a
“2” is placed in front of H2O. This balances the number of oxygen atoms on each side,
but also changes the number of hydrogen atoms on the right hand side. Another “2” is
then placed in front of the reactant H2 to balance both sides of the equation giving the
same quantity of hydrogen and the same quantity of oxygen on both sides. The new
balanced equation is written as: 2H2 + O2 → 2H2O
Two molecules of hydrogen and one molecule of oxygen yield two molecules of water.
Subscripts never change when balancing equations. For the example above, adding a
subscript “2” to H2O changes the compound completely (in this case from water H20 to
hydrogen peroxide H2O2). Only coefficients are used when balancing chemical equations.
A chemical reaction occurs when the chemical identity of the reactants is different than
that of the products. A new substance or substances forms with properties different from
the original reactants. Often times, chemical reactions result in changes easily observed.
Evidence of chemical reactions includes temperature changes, formation of a precipitate,
color changes and gas production.
During chemical reactions involving the gain or loss of heat, temperature changes
indicate the formation of new substances. In endothermic reactions, heat is absorbed
when transforming from reactants to products (gain in enthalpy). When it takes more
energy to break the bonds of the reactants than is released when new chemical bonds
form, the reaction is endothermic (feels colder). A reaction is exothermic (loss in
enthalpy) when more energy is released in forming new bonds than it takes to break the
original bonds of the reactants (feels warmer).
In precipitation reactions, a precipitate is a solid formed out of solution. A precipitate
forms because the solid produced is insoluble in aqueous solutions. This is a new
substance formed with new properties and is evidence of a chemical change. When gas is
produced during a reaction, it is also evidence of a new substance formed. For example,
when baking soda (solid) is mixed with vinegar (liquid), carbon dioxide (gas) is one of
the products formed. The new products formed from this reaction, including CO2 gas,
have properties different from the original reactants.
A color change represents more than just a physical change. This is the same as diluting a
substance or adding food color to a substance, is used to identify a chemical change. The
color of a substance may change when its chemical composition changes. An example of
a color change as evidence of a chemical reaction is seen when crystal violet dye is added
to a solution containing hydroxide ion. The intensity of the color decreases over time
until it disappears. The product of the reaction is colorless in water solution.
The density of a substance is a physical property defined as mass per unit volume.
Density is calculated by dividing the mass of an object by its volume. For solids, the units
are usually expressed as g/cm3. For liquids and gases, the units are usually expressed as
g/ml. Each element and compound has a unique density associated with it. Often times in
chemistry, the density of a substance is compared to the density of water (1.0 g/ml).
Objects that sink in water have density values greater than 1 (g/ml or g/cm3) and objects
that float on water have density values less than 1 (g/ml or g/cm3).
Calculating the density of an unknown substance is sometimes used to identify the
substance by comparing it to known density values of elements and compounds. Density
is just one of many physical and chemical properties used to identify an unknown
substance. To identify an unknown substance using density, the mass and volume of an
unknown are measured. For an irregular shaped solid, the volume is found by placing the
object in a known amount of water and determining the volume of the displaced water to
find the volume of the object. The mass of the object is then divided by the volume to
determine the density. That density value is then compared to a list of known density
values to help identify the object. Density alone may or may not be enough to determine
the identity of an unknown. Other physical and chemical properties may be needed as
well.