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Instruction 2-2

Physical Changes | Chemical Changes | States of Matter

Chemical Changes
http://www.chem4kids.com/files/matter_chemphys.html
CCSTD Grade 5 Science 5.1.a, f., h.

Do you ever make toast for breakfast? Believe it or not, when you make toast, you are making a chemical change happen! When you breathe, a chemical change happens. Our lives are filled with chemical changes.

Remember that substances are made of molecules, which are groups of atoms. During a chemical change, the molecules break apart. The atoms form new combinations. This means the substance is actually changed and a new substance is formed.

When you make toast, the toaster adds heat energy to the bread. The bread turns brown and the texture changes. The toast is not the same substance as the breaCaked. It cannot be changed back to bread.

When milk gets sour, this is because a chemical change has happened. Bacteria have caused this change. The milk is no longer food.

When we bake a cake, we use many ingredients. We may use flour, eggs, butter, sugar, yeast, and salt. When the cake is baked, the heat used combines the ingredients to make a new substance: cake. The ingredients cannot be separated from each other or brought back to their original form. This is because a chemical  change has taken place.

Here are some clues that may show you a chemical change has taken place:

  • the color changes
  • the texture (the way something feels) changes, and can't be changed back
  • gas bubbles are formed
  • a new substance is formed
  • energy is taken in or given out in a reaction—burning and electricity are examples
  • something changes into something else, and you can't change it back.

Many times it is hard to see what happened during a chemical change. When gasoline burns, it mixes with oxygen gas, which is found in air. This burning produces water and carbon dioxide gas. The water may be steam, so you may not see it. This is why most of the time all you would see would be the flame of the fire and the gasoline disappearing (see http://virtual.yosemite.cc.ca.us/lmaki/Chem150-99/chapters/chapter1/lessons/phys_chem/phy_c_1.htm  for a fun activity).

Remember that if you just change the shape of something, this is a physical change. If you melt candle wax, the wax melts and then hardens as it cools into a different shape. However, the wax is still wax. The substance hasn't changed. However, if you burn a candle, a chemical change takes place. Some of the candle is changed into gases, and heat energy (see: http://www.bbc.co.uk/scotland/education/bite
size/standard/physics/energy/heat_energy_rev1.shtml
)  is produced. You cannot change the melted candle back into the original candle. The molecules of the candle and candlewick broke apart when the candle was burned, and new molecules formed (also see http://www.dmturner.org/Teacher/Library/5thText/ChemPart6.htm#chemchange).  

Some chemical changes happen very slowly. You may notice that your bike is rusting. This is because of a chemical reaction of the metal parts of the bike with water. A new substance is formed, which is rust. You can tell this change has happened because of a color change. The silver metal turns orange.

 



 

Some chemical changes happen very quickly, like when fireworks explode. Fireworks change color and release energy in the form of light energy.
Sometimes it is tricky to know whether a physical change or a chemical change has taken place. Salt is made of the elements sodium (Na) and chloride (Cl). If you dissolve salt (NaCl) into water (H2O), the sodium and chloride atoms of the salt are pulled apart by the water. The salt disappears. This means a chemical change has taken place. However, if the water evaporates, the sodium and chloride atoms will rejoin. The salt will form again and will be in the bottom of the container after the water evaporates. So this chemical change can be reversed.

Remember:

  • during a physical change, no new substance is formed
  • during a chemical change, at least one new substance is formed

No matter what kind of change takes place, the amount of matter stays the same. Matter is never created or destroyed. The particles of one substance can be rearranged to form another substance. However, the number of particles stays the same.

 

Experiments

http://faculty.dbcc.cc.fl.us/swansoj/Changes_-_Physical_or_Chemical.htm

http://wow.osu.edu/experiments/chemistry/cpchanges.html

Reading List
 
  Magic School Bus Gets Baked in a Cake: A Book About Kitchen Chemistry

The Usborne Illustrated Dictionary of Science

 

for Students, Parents and Teachers

Now let's do Practice Exercise 2-2 (top). Choose printer friendly or online exercises. Printer friendly version requires the Adobe Acrobat Reader 5. Click HERE to obtain a free copy.

 

  

Next Page:  States of Matter (top)