SNC1D CHEMISTRY ATOMS, ELEMENTS, & COMPOUNDS L (P.152-155) When we watch a fireworks display we are, in scientific terminology, seeing the chemical properties of the fireworks. A chemical property is a property of a substance that describes its ability to undergo changes to its composition to produce one or more new substances. February 8, 2013 1DCHEM - 1 CHEMICAL PROPERTY describes the ability of a substance to change and produce one or more new substances February 8, 2013 1DCHEM - 2 1
Observing For example, when a match burns, a chemical change occurs because the matchstick is coated with a substance that burns easily. This ability to burn, or its combustibility, is a chemical property of the match. Chemical properties can be observed only when a chemical change occurs. February 8, 2013 1DCHEM - 3 Observing 1. Classify each as a physical or chemical property: (a) wood burns (b) clay is brown (c) steel wool rusts (d) a paper clip is 3 cm long chemical physical chemical physical February 8, 2013 1DCHEM - 4 The usefulness of many substances is determined by their chemical properties. Many substances, such as wood and coal, are useful because they burn easily. Other substances, such as liquid/powder fire retardants used to make fire resistant materials, are useful because they do not burn easily. February 8, 2013 1DCHEM - 5 2
2. (a) What chemical properties of nail polish remover makes it useful? (b) What chemical property makes nail polish remover dangerous? (a) removes nail polish (b) flammable/poisonous February 8, 2013 1DCHEM - 6 Metals are used to make jewellery, but only some metals, such as stainless steel, titanium, and some types of gold are used for body piercings. This is because these metals have a chemical property that make them useful for piercings these metals, unlike copper and silver, do not react easily with substances in the body. February 8, 2013 1DCHEM - 7 3. Surgeons sometimes use metal plates and screws to hold broken bones together. What chemical properties do you think the metal should have? the metal should not react with substances in the body February 8, 2013 1DCHEM - 8 3
A change that results in the formation of a new substance is called a chemical change. The original substances do not disappear though. Instead, the components of the original substances are rearranged when the new substance is formed. February 8, 2013 1DCHEM - 9 CHEMICAL CHANGE change in which one or more new substances is formed components of original substance(s) (i.e. the atoms) are rearranged February 8, 2013 1DCHEM - 10 For example, roasting a marshmallow on a campfire produces a brown, flaky substance on the surface of the marshmallow. This new substance is clearly different from the gooey centre of the marshmallow. Many chemical changes are easy to observe and occur all around you. February 8, 2013 1DCHEM - 11 4
4. Classify each as a physical or chemical change: (a) folding a paper airplane P (b) water freezing on a pond P (c) a car rusting C (d) making a campfire C (e) melting a marshmallow P (f) dissolving sugar in coffee P February 8, 2013 1DCHEM - 12 Electrolysis is another example of a chemical change. Electrical energy is used to break water molecules apart into oxygen and hydrogen. But how do we know when a chemical change has occurred? February 8, 2013 1DCHEM - 13 Chemical Change Clues 5. There are 6 distinctive clues (either alone or together) that indicate a chemical change has occurred. What are they? CHEMICAL CHANGE CLUES Î a new colour appears Ï a new odour appears Ð heat/light is given off/absorbed Ñ bubbles of gas are formed Ò a solid material (precipitate) forms Ó the change is difficult to reverse February 8, 2013 1DCHEM - 14 5
Chemical Change Clues These chemical change clues can help you decide whether a chemical or physical change has occurred. But it is important that you do not come to a conclusion too quickly. While all of these clues suggest that a new substance has been produced, any one of them could also accompany a physical change. You must consider several clues in order to determine what type of change has taken place. February 8, 2013 1DCHEM - 15 Chemical Change Clues 6. What evidence suggests that these changes are chemical changes? (a) Bubbles form when baking soda is mixed with lemon juice. (b) Cookies baking in the oven give off a delicious aroma. (c) A match is struck and ignites. (d) When two liquids are mixed a solid red substance appears. (e) A banana tastes sweeter as it ripens. (a) bubbles of gas are formed (& change is difficult to reverse) (b) new odour appears (& change is difficult to reverse) (c) heat/light is given off (& change is difficult to reverse) (d) a solid precipitate forms (& change is difficult to reverse) (e) the change is difficult to reverse February 8, 2013 1DCHEM - 16 Physical or Chemical? 7. When a frozen pizza is placed in the oven, the cheese begins to melt and then darken. The crust becomes brown on the edges. Are these physical or chemical changes? How do you know? melts physical (easy to reverse) crust/cheese browns/darkens chemical (change is difficult to reverse) February 8, 2013 1DCHEM - 17 6
Physical or Chemical? 8. A candle burns for 15 minutes before it is extinguished. Some wax melts, drips down the side, collects at the base of the candle, and then hardens again. The candle becomes shorter. (a) What changes were physical changes? Explain. (b) Was there any evidence of a chemical change? Explain. (a) wax melting/hardening and candle getting shorter chemical properties did not change (b) burns heat/light is given off and the change is difficult to reverse February 8, 2013 1DCHEM - 18 Physical or Chemical? 9. Classify each of the following as a physical change or a chemical change. For each chemical change, explain how you can tell that a new substance has been formed. (a) Water boils and turns into steam. (b) Wood is sawed and made into a toy box. (c) Firewood burns and ashes remain. (d) Orange drink crystals are stirred into a pitcher of water. (e) Sugar, eggs, and flour are mixed and baked into cookies. P P C P C February 8, 2013 1DCHEM - 19 Activity: Using Properties to (B7/P.156) INSTRUCTIONS A. Read the activity B7: Using Properties to Identify Pure Substances. B. Follow the instructions given (i.e. procedure 1 to 19). C. Answer the questions given (i.e. analysis 15 to 19). D. Submit a formal lab report. NOTE! This is a formal lab report. Use your practice formal lab report as a guide. Make sure you do your own work! February 8, 2013 1DCHEM - 20 7
TEXTBOOK P.161 Q.4 U Check Your Learning February 8, 2013 1DCHEM - 21 8