1. Two students measure the liquid at right. Zoe says 20.3 ml, Brian says 21.50 ml. Describe why EACH student is incorrect. 2. Why do objects float? (tell me what you think) 3. Predict: If I put two cans of pop in an aquarium filled with water, one regular and another diet soda, what will happen to each can? Objectives: I will know (knowledge): 1) significant figures 2) DENSITY and how it s used in science I will be able to (skill): 3) identify & calculate with Sig Figs 4) how to calculate density, mass & volume Agenda: Any questions over homework? SIG FIGS Identify & manipulate Density Concepts & practice How do SF & density work together? Significant Figures a.k.a. Significant DIGITS Sig. Figs. Atlantic/Pacific method All known digits of a measurement plus one estimated digit. To Recognize Significant Figures: Example #S.D. 1. Nonzero numbers are always significant. 3539 = 2. Zeros to the right with a decimal are significant. 9.20 = 3. Zeros between non-zero digits are significant. 10,045 = 4. Zeros for placeholders are NOT significant. Placeholders to make a # smaller Placeholders to make a # bigger 5. A line over a zero (left of an implied decimal point) will make it significant. 0.000362 = 730 = 450,000 = 1
How Many Sig. Figs.? Start counting when you meet the first non-zero digit TEXT Time 34.607 4.500 630200 0.00021 700 0.00430 Find Section 2.3 in your text Sig Figs!! Shoulder Partners: A You do problem #35 B You do problem #36 Timed Share take turns back and forth, explaining 35a, 36a, 35b, 36b, 35c, 36c, 35d, and 36d. HOW TO: Round Significant Figures Math & Rounding w/sig Figs 1. Determine the number of sig figs in the final answer and count from LEFT TO RIGHT. 2. Check the Number(#) to the RIGHT of the last sig fig: #<5, last sig fig stays the same # 5, last sig fig rounds up by 1 3. Drop any leftover digits Round to 3 sig figs: A. 0.001232 B. 20.3711 C. 345,560 THIS is what we need to be PRECISE FOR!!! This is the entire reason we are careful about counting sig figs & measuring appropriately, because This Photo by Unknown Author is licensed under CC BY-SA D. 0.420267 2
Math Rules w/sig Figs Turn to page 53 Adding/Subtracting answer rounds to the same # decimal places as the measurement with the least # of decimal places. May only go to the RIGHT as far as the least precise #. Multiplying/Dividing answer rounds to the same # of sig figs as the measurement with the least # of sig figs. Homework#2 Catch up on a good BOOK Chemistry Matter & Change! Read Ch. 2 Section 3. DO #s 38-39, 41, 43, 47-48 Due next class. What weighs more, a pound of marbles or a pound of ping-pong balls? Neither, it was a pound of each: 3
1Marble has more mass than 1ping pong ball. MASS is the amount of matter in an object. VOLUME is the amount of 3 dimensional space (L x W x H) matter takes up The metric unit for Mass is? Grams! When you have the same mass of both marbles and ping-pong balls, you have MORE of the lighter object. The ping-pong balls have more VOLUME. This can be measured in ml or by measuring 3 lengths and multiplying. 1 ml = 1 cm 3 So when we had a pound each of marbles and ping-pong balls, the volume of the ping pong balls was larger in order to get the mass equal. Water Displacement Method Final volume initial volume = volume of object 4
Density is an intensive physical property. Density (g/cm 3 ) is how much matter is contained in a certain area. (how much STUFF per unit SPACE) Equation: Density = mass/volume It s INFORMATIONAL The density of water is 1 g/ml. That means if you have 1 ml of water, it weighs 1 gram. The density of mercury is 13.6 g/ml. So, 1 ml of mercury weighs 13.6 g. Use densities to compare between substances. Higher densities mean MORE STUFF per unit SPACE. (MASS) (VOLUME) You made some predictions let me hear them! 1. Why do objects float? (tell me what you think) 2. Predict: If I put two cans of pop in an aquarium filled with water, one regular and another diet soda, what will happen to each can? Metal sinks and wood floats in water- why? Knowing the density of water is 1 g/ml, what can we say about the density of wood and the density of metal? 5
Which of these items would float? Just like solids, liquids each have a unique density. When 2 or more liquids share space but do not mix (due to chemical properties), they will form layers within their container. Density Tower Try to list these solids from LEAST dense to MOST dense LEAST Density Objects beads bolt cherry tomato die ping-pong ball popcorn kernel soda cap MOST Density The Density formula can be manipulated to solve for Density, Mass, or Volume. You should end up with these 3 equations: D = M / V V = M / D M = D V The Density formula can be manipulated to solve for Density, Mass, or Volume. You should end up with these 3 equations: D = M / V V = M / D M = D V 6
A girl buys a medallion at a pawn shop that the shop owner swears is pure platinum but she is suspicious that it might be silver (which is much less valuable) and decides to test her theory, knowing: Platinum density = 21.4 g/cm 3 Silver density = 10.5 g/cm 3 She weighs the medallion and finds mass = 55.64 g She places water in a grad. cylinder and reads the volume as 75.2 ml. Then she drops the medallion in the cylinder and reads the new volume as 77.8 ml. Water Displacement Method! What is the density of the medallion? Given: Mass = 55.64 g Volume = final initial volume = 77.8 ml 75.2 ml = 2.6 ml Unknown: Density =? Equation: D = M / V Substitute: D = 55.64g/2.6mL Solve: D = 21 g/ml Add these to the book assignment: Ch. 2 problems Density & Sig Figs: #67-67, 103-105 So is the medallion platinum or silver? 7