What are the differences between a physical change and a chemical change? There are different ways you can change things. For example, you can tear up a piece of paper into small pieces. What remains is still paper. You have changed the way the paper looks, but you have not made any new substances. You have made a physical change. If, instead, you burned the paper, what would be left? What is left is no longer paper. In this case the substance has been changed. This is a chemical change. Physical Change A physical change does not change the way the atoms are linked up. The substance may look different, but no new substances have been formed. The chemical properties are not changed. In a physical change, no energy is taken in or given off unless there is a change of state. Chemical Change In a chemical change, the atoms change the way they link up. New substances are formed. The new substances have different chemical properties from the old substance. When atoms change the way they link up, we say a chemical reaction has taken place. Energy is always part of a chemical reaction. In a chemical reaction, energy is either taken in or given off. STUDYING SOME CHEMICAL CHANGES I.
A chemical equation tells the story of a chemical reaction. A chemical equation tells which substances we start out with. A chemical equation also tells us which new substances are formed. In a chemical reaction, there is never a change in the number of elements. No elements are lost. No new elements are added. They just link up in different ways. Here is an example of a chemical equation. This equation tells the story of electrolysis. Compound
Element
Element
Water Hydrogen Oxygen Æ
2H2O 2H2
+ O2 Liquid Gas gas The elements we start out with are on the left side of the arrow. The elements we end up with are on the right side of the arrow. Try to answer these questions by using the chemical equation above. 1. Name the elements we started with:______________________, ___________________ 2. Name the elements we ended with: ______________________, ___________________ 3. Did the elements change the way they link up?________________ 4. Were new products formed?__________________ 5. Did the properties change?___________________ 6. Were any new elements added?_______________ 7. Were any elements lost?______________________ EXAMPLES OF PHYSICAL CHANGE In a physical change, the atoms do not change the way they are linked up. No new products are formed. Look closely at each of the following examples of physical change. Then answer the questions about each change. I.
Tearing Paper 1. Does the paper look different after being torn?_________ 2. Is the paper still paper?________ 3. Are the atoms taking in energy? ___________ 4. Are the atoms giving off energy? ___________ 5. In a physical change, the atoms _________________ (do, do not) change the way they link up. 6. The chemical properties of the paper ________________ (are, are not) changed. 7. A chemical equation _________________ (can, cannot) be written to show a physical change. 8. A chemical equation shows only a ______________________ (chemical, physical) change. II.
Melting ice 1. Does the ice look different after it is melted? __________ 2. The formula for water is H2O. The formula for ice is also H2O. a. When ice changes to water, the link‐up of the atoms _____________ (does, does not) change. b. When water changes to ice, the link‐up of the atoms ______________ (does, does not) change. 3. A chemical equation ________________ (can, cannot) be written to tell the story of the melting of ice. 4. The melting of ice is an example of a _________________(physical, chemical) change. 5. When ice changes to a liquid, the ice ___________________ (does, does not) take in energy. 6. Usually, energy ____________________ (is, is not) part of a physical change. 7. Energy is part of a physical change only when there is ______________________ (a change of state, electrolysis). III.
Chopping Wood 1. Does the wood look different after being chopped? __________ 2. Is the wood still wood? _______________ 3. Are the atoms changing the way they are linked up? ______________ 4. Are any elements being added? __________________ 5. Are any elements being lost? __________________ 6. Are any new products being formed? ______________ 7. Is the wood taking in energy? ___________________ 8. Is the wood giving off energy? __________________ 9. The chopping of wood is an example of a_________________ (physical, chemical) change. 10. A chemical equation______________________ (can, cannot) be written to show how wood changed when it is chopped. REVIEW: TRUE OR FALSE 1. ______ a chemical reaction causes a chemical change. 2. ______a chemical change makes new products. 3. ______elements can be lost or gained in a chemical reaction. 4. ______energy can only be taken in during a chemical reaction 5. ______The substances that take part in a chemical reaction have new properties 6. ______A physical change makes new products 7. ______The boiling of water is an example of a chemical change. 8. ______An equation tells the story of a physical change. 9. ______the mixing of salt and pepper is a physical change. 10. ______toasting a marshmallow is a physical change. 11. ______cutting a marshmallow is a physical change. 12. ______the rusting of iron is a chemical change. SUMMARY: Physical Change Chemical Change
LITERACY CONNECTION: INSTRUCTIONS There are physical and chemical changes concealed in the story below. Find as many as you can (hi‐lite them, underline them,circle them, ...) and identify each as either a (P) physical or (C) chemical change. Jessica and Chris wanted to surprise their parents with a special anniversary dinner. While Jessica cut up the vegetables and cheese for a salad and boiled an egg for slicing, Chris placed ice cubes into some bubbling orange pop, which he had just poured, from a freshly opened bottle .They had decided to make and cook some hamburgers and to cut up potatoes and make their own French fries! Chris mixed the ingredients for the burgers while Jessica cut up the potatoes and placed them in a pan on broil. Earlier that day, Jessica had mixed some Jell‐O powder in hot water and mixed in fresh strawberries. She put it in the refrigerator so it could cool and set. Once she had prepared the Jell‐O, she decided she would make a chocolate cake for the special dinner. She put the cake mix and other ingredients into a bowl, stirred it up and placed it in a pan and put it in the oven. When the hamburgers were cooked right through and the French fries were golden brown, they surprised their parents in the dining room. Everything tasted so good! After supper, it was time for cake and coffee. Jessica and Chris boiled some water and poured it into mugs. Instant coffee, cream and sugar were added to the mugs. The family enjoyed their coffee, Jell‐O and delicious chocolate cake for dessert. Later, as Jessica and Chris cleaned up, the scraps were placed in the garbage can. Their father washed the dishes in steaming hot soapy water and Chris dried them. Jessica worked on her oil painting, while their mother played the piano. They had all enjoyed their terrific evening, although Chris had a stomach ache from eating too much. His mom gave him a seltzer tablet and this helped ease the pain. Their parents thanked them for a fantastic evening and went to bed!