Types of Changes

Physical change: the shape, phase, size,

etc. of the material can change

the material itself is the same before and after the change

The change can be “undone”

Chemical change: The substances present at

the beginning of the change are not present at the end

new substances are formed.

The change cannot be “undone.”

http://www.learner.org/courses/essential/physicalsci/session4/closer1.html

Review: Examples of Physical Changes water evaporates butter melts a can is crushed glass is broken modeling clay is

shaped

ripping paper boiling water sawing wood breaking a stick dissolving salt in

water

Review: Physical Properties

describe the object or substance can be observed or measured

without changing the identity of the substance

Review: Examples of Physical Properties color odor size shape state (solid, liquid, gas, plasma) texture (smooth, rough, soft, bumpy) density (measured in g/ml and g/cm³) melting point/boiling point/freezing point

Review: Chemical Properties

are determined by the arrangement of atoms in the molecules making up the object

describe a object or substance based on its ability to change into a new substance with different properties

cannot be observed with your senses; must be tested

aren’t as easy to observe as physical properties

Key Idea #7

Chemical changes occur when elements and/or compounds react or decompose to produce new substances.

http://www.ric.edu/faculty/ptiskus/chemical/

Chemical Properties

If the chemical properties of an object or substancehave changed, then a chemical reaction has

occurred.

Examples of Chemical Properties: pH: a measure of how acidic something is Flammability: a substances ability to burn Reactivity: The rate at which a substance reacts

Clues that indicate a chemical change has taken place include:

color change temperature change bubbling and fizzing

note: boiling water is NOT a chemical change heat, light, or smoke is given off a solid (precipitate) is formed the change is difficult or impossible to

reverse

Compare and contrast the chemical properties of the new substance with the original after a chemical change.

Examples Properties before

Propertiesafter

Evidence of a chemical change

Bike

Hair

Alka Seltzer and water

Fireworks

Copy the following chart into your notes:

http://www.suwaneedental.com/travel/Vancouver/old%20rusty%20bike.jpg

http://www.flickr.com/photos/hairport_lisbon/2318561502/

http://cache.lifehacker.com/assets/resources/2008/06/alka-seltzer-diy.png

http://z.about.com/d/chemistry/1/0/_/_/fireworks.jpg

In a chemical change one or more substances combine or break

apart to form new substances. atoms rearrange themselves during a

chemical reaction to form different molecules and compounds. For example, when hydrogen reacts with oxygen

the product is water vapor.

Baking Soda and Vinegar

Acid-base reaction. Vinegar (acetic acid): C2H4O2

Baking soda (sodium bicarbonate): NaHCO3

C2H4O2 + NaHCO3 ---> C2H3O2Na + H2CO3 The products are sodium acetate and carbonic

acid Carbonic acid quickly decomposes into carbon

dioxide and water: H2CO3 ---> H2O + CO2 The CO2 is what you see foaming and bubbling in

this reaction.

http://www.newton.dep.anl.gov/askasci/chem99/chem99096.htm

Burning Sugar Add heat

C12 H22O11 = Carbon + Water (H2O)

Add more heat Carbon + Water (H2O) = Carbon + Water Vapor

(H2O)

Add more heat Carbon + Hydrogen gas + Oxygen gas

Activation Energy is the energy needed to start a chemical

reaction.

Example: A match won't light spontaneously. When you strike a match you are providing

the activation energy needed to cause a reaction between the chemicals in the match head and oxygen in the air.

All chemical reactions include a change in energy.

Exothermic reactions release energy to their surroundings; usually in the form of heat.

Endothermic reactions need to absorb heat from their surroundings to proceed.

Conservation of Mass Mass is conserved during chemical

changes. The mass of the reactants is

always the same as the mass of the products after the chemical reaction.

In every chemical change new substances are formed the starting substances are called the

reactants the new substances are called the products

Natural gas is primarily methane.  Methane (CH4) is a molecule in which 4 hydrogen atoms

are bonded to one carbon atom.  Lighting a gas stove provides the activation energy that

causes the methane to react with oxygen in the atmosphere and release heat.

During the reaction, the atoms recombine to form carbon dioxide and water vapor.

During a chemical change

the number and kind of atoms in the reactants are the same as the number and kind of atoms in the products.

After the chemical change the mass of the reactants equals the mass of the products.

Before the reaction there were 9 atoms 1 carbon, 4 hydrogen, 4 oxygen

After the reaction the same 9 atoms were there, just in a different combination.

                                  

Iron reacts with oxygen in the air to form rust.

After the chemical change the new substances have different physical and

chemical properties than the original elements and/or compounds.

the number and kind of atoms in the reactants are the same.

Pennies get dull over time because the copper in the pennies slowly reacts with the oxygen in the air to form copper oxide.

Chemical reactions cause chemical changes. involve a change in substances and a

change in energy. cause atoms to rearrange themselves to

form different molecules and compounds.

NOTE: Matter and energy is NOT created or destroyed in a chemical reaction---only changed.

The only sure evidence for a chemical reaction is

the production of new materials.

http://www.saskschools.ca/curr_content/science9/chemistry/images/les8images/cakeaft.gif

Chemical Reaction Examples

A sparkler contains magnesium. When lit, the magnesium reacts

with oxygen in the air and produces light and heat. 

The chemicals inside a rocket, when lit, react with the oxygen in the air and produce heat, light and sound.

Alka Seltzer and WaterIngredients: Water = H2O Alka Seltzer = Sodium Bicarbonate 3(NaHCO3) + Citric Acid

C6H807

Reactants: Alka Seltzer + Water = [3(NaHCO3) + C6H807] + H2O Products: Water Vapor + Carbon Dioxide (in air) + Sodium Citrate + Water

(in beaker) = 3(H2O)+ 3(CO2) (in air) + C6H5Na3O7+ H2O (in beaker)

Open vs Closed Systems

Open system: matter interacts with its surroundings; can be transferred in and out

Closed system: no matter is transferred in or out of the system

When a substance chemically changes, it becomes a different substance with different properties but the amount of matter remains the same (Conservation of Mass).

Examples of Chemical Changes Burning sugar Baking bread Rusty nail Fireworks exploding Raw egg becomes cooked egg Spoiled milk Digesting food in your stomach Cake mix becomes cake Paper becomes ash

Why is a melting ice cube NOT an example of a chemical change?

Give at least three pieces of evidence in your answer.

Alka Seltzer Lab

Why is baking a cookie an example of a chemical change?

Give at least three pieces of evidence in your answer.

Chemical Properties and Chemical Change

www.elmhurst.edu/~chm/vchembook/105Achemprop.html

http://www.saskschools.ca/curr_content/science9/chemistry/lesson11.html

www.saskschools.ca/.../chemistry/lesson8.html