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Alt 04-11-2008, 21:21   #1
Yaso
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Standart Name : Yasİn

NAME : YASİN

SURNAME : GÜVEN

CLASS : 8/D

NUMBER : 322

TEACHER : ADEM DEMİRKOL

SCHOOL : K. ORUÇREİS A. L.

TOPIC : Pysical and chemical Changes

Reaction
The reaction is the heart of the study of chemistry. All chemical reactions involve the breakage and reformation of chemical bonds of molecules to form different substances. Chemistry, then, can be defined as the science of substances--their composition, structure, and properties and the reactions that change one substance into another.
A simple chemical reaction occurs when hydrogen gas combines with oxygen gas to form the compound water. On the molecular level, two molecules of hydrogen react with one molecule of oxygen to produce two molecules of water.
Chemical Reaction versus Physical Change
Care must be taken not to confuse a chemical reaction with a physical change, such as a change in state. Water, like all types of matter, can exist in either the solid, liquid, or gaseous state depending on the conditions of temperature and pressure.
Energy
The reaction between hydrogen and oxygen releases a large quantity of energy, most of which is present as heat. Such an energy-releasing reaction is said to be exothermic. Once the water molecules are formed, however, they cannot again be converted to hydrogen or oxygen merely by cooling (withdrawing the energy from) the newly formed water. The reverse chemical reaction to convert water back to hydrogen and oxygen can be accomplished only if an equal quantity of energy (possibly in the form of electrical energy) is returned to the water molecules to break the bonds that hold the hydrogen and oxygen atoms together. Such a reaction, which requires energy, is said to be endothermic.
Property Changes
Chemical reactions are identified by the changes in the chemical properties of the substances during the course of the reaction. For example, table salt (sodium chloride), when melted, can be decomposed to its elements by a passage of an electrical current through the melt. At the negative electrode the silvery molten metal sodium is formed, and at the positive electrode the poisonous, greenish yellow gas chlorine is released.






Pysical Change




When you tear a sheet of paper into pieces , the size of paper changes . However , the pieces of paper still contain the same substances as the original sheet of paper. When a heavy log is cut into boards or a board is shaped into a shelf , it is still wood. Only its size and shape have changed . When ice melts , it changes into water, Onlt its state have changed . When an object changes in size , shape or state , it undergoes a pysical change .
In such changes the kinds of particles , that make up substances are not changed . That is , no different kinds of molecules are formed . A few examples are given for the pysical changes on this page .


The changes in physical states of matter are the most comon pysical changes. Boiling, melting, evaporation, and freezing are changes in the state of matter.When ice melts it changes into liquid. When water boils it changes from liquid to gas. However, the kinds of molecules do not change they are still water molecules.





Chemical Properties and Change


What happens when a sheet of paper or a piece of wood is burned?

EXPERIMENT

Take a sheet of paper or a match stick.Burn it and observe what is happening? Compare the residue and the original paper.
Has the substance been changed?

EXPERIMENT

Place a small amount of mercury (II) oxide in a test tube. By holding the tube with a test tube holder, heat it in a strong Bunsen burner flame. Observe if there is a color change. Hold a glowing splint in the mouth of the tube. Observe what happens. Compare the substance before heating and after heating.
Has the substance been changed?
Now, allow the contents of the tube to cool. Does the mercury oxide return to its original red color?
EXPERIMENT


Cut a piece of sodium metal with a knife. By holding it with a tong, place it in a beaker which is 2/3 full of water. Observe what is happening.
Has the sodium metal been changed?
Do you think the change gives off heat?
After the reaction stops, place a piece of red litmus into the beaker.
Has the color of litmus changed?
Is the resulting solution acidic or basic?


As shown in the experiments above, when a sheet of paper or a match stick is burned, heat, light, and smoke are given off. The resulting residue is a small pile of ashes. This shows that the original substance changed into new substances which have exactly different properties.
When red mercury (II) oxide is heated it darkens and decomposes to give oxygen and mercury , which condenses as silver droplets on the cooler part of the tube. The composition of mercury (II) oxide has changed. New substances with different properties appear. When this happens, a chemical change has occurred. Chemical changes usually involve the release pf heat, light, or electricity.
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