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Introduction: Matter & Measurement AP Chemistry Chapter 1 (Day 2)

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Chemistry What is chemistry? composition of matter It is the study of the composition of matter and the changes that matter undergoes. What is matter? It is anything that takes up space and has mass.

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Elements, Compounds & Mixtures A substance is matter that has a definite composition and constant properties It can be an element or a compound

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Elements, Compounds & Mixtures An element is the simplest form of matter. It cannot be broken down further by chemical reactions

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Elements, Compounds & Mixtures A compound can be separated into simpler forms It is a combination of two or more elements

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Mixtures A mixture is a physical blend of two or more substances 1. Heterogeneous Mixtures –Not uniform in composition –Properties indefinite & vary –Can be separated by physical methods

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Mixtures 2. Homogeneous Mixtures –Completely uniform in composition –Properties constant for a given sample –Cannot be separated by physical methods (need distillation, chromatography, etc) –Sometimes called solutions

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Physical & Chemical Properties Physical property – characteristics of a pure substance that we can observe without changing the substance; the chemical composition of the substance does not change

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Physical & Chemical Properties Chemical property – describes the chemical reaction of a pure substance with another substance; chemical reaction is involved

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Physical & Chemical Properties Physical properties appearance odor melting point boiling point hardness density solubility conductivity Chemical properties reaction with oxygen (flammability) rxn with water rxn with acid Etc….

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Intensive & Extensive Properties Intensive properties Do not depend on the amount of sample being examined –temperature –odor –melting point –boiling point –hardness –density Extensive properties Depend on the quantity of the sample –mass –volume Etc….

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Physical & Chemical Changes Physical changes The composition of the substance doesn’t change Phase changes (like liquid to gas) –Evaporation, freezing, condensing, subliming, etc. Tearing or cutting the substance Chemical changes The substance is transformed into a chemically different substance All chemical reactions

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Signs of a Chemical Changes 1.permanent color change 2.gas produced (odor or bubbles) 3.precipitate (solid) produced 4.light given off 5.heat released (exothermic) or absorbed (endothermic)

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Making Measurements A measurement is a number with a unitA measurement is a number with a unit All measurements, MUST have unitsAll measurements, MUST have units

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Types of Units

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Measurements There are two types of measurements: Qualitative measurements are words, such as heavy or hot Quantitative measurements involve numbers (quantities), and depend on: 1) The reliability of the measuring instrument 2) The care with which it is read – this is determined by YOU!

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Accuracy & Precision Accuracy – how close a measurement is to the true value Precision – how close the measurements are to each other (reproducibility)

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Precision and Accuracy Neither accurate nor precise Precise, but not accurate Precise AND accurate Our goal!

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Which are Precise? Accurate?

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Uncertainty in Measurements Measurements are performed with instruments, and no instrument can read to an infinite number of decimal places Which of the balances below has the greatest uncertainty in measurement? 12 3

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Significant Figures in Measurements Significant figures in a measurement include all of the digits that are known, plus one more digit that is estimated. Sig figs help to account for the uncertainty in a measurement

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To how many significant figures can you measure this pencil? What is wrong with this ruler? What is it missing?

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Rules for Counting Significant Figures Non-zeros always count as significant figures: 3456 has 4 significant figures

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Rules for Counting Significant Figures Zeros Leading zeroes do not count as significant figures: 0.0486 has 3 significant figures

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Rules for Counting Significant Figures Zeros Captive zeroes always count as significant figures: 16.07 has 4 significant figures

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Rules for Counting Significant Figures Zeros Trailing zeros are significant only if the number contains a written decimal point: 9.300 has 4 significant figures

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Rules for Counting Significant Figures Two special situations have an unlimited (infinite) number of significant figures: 1.Counted items a)23 people, or 36 desks 2.Exactly defined quantities b)60 minutes = 1 hour

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Sig Fig Practice #1 How many significant figures in the following? 1.0070 m 5 sig figs 17.10 kg 4 sig figs 100,890 L 5 sig figs 3.29 x 10 3 s 3 sig figs 0.0054 cm 2 sig figs 3,200,000 mL 2 sig figs 3 cats infinite These all come from some measurements This is a counted value

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Significant Figures in Calculations In general a calculated answer cannot be more accurate than the least accurate measurement from which it was calculated. Sometimes, calculated values need to be rounded off.

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Rounding Calculated Answers Rounding Decide how many significant figures are needed Round to that many digits, counting from the left Is the next digit less than 5? Drop it. Next digit 5 or greater? Increase by 1

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Rules for Significant Figures in Mathematical Operations Addition and Subtraction The answer should be rounded to the same number of decimal places as the least number of decimal places in the problem.

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Rules for Significant Figures in Mathematical Operations Addition and Subtraction: The number of decimal places in the result equals the number of decimal places in the least accurate measurement.Addition and Subtraction: The number of decimal places in the result equals the number of decimal places in the least accurate measurement. 6.8 + 11.934 =6.8 + 11.934 = 18.734 18.7 (3 sig figs)18.734 18.7 (3 sig figs)

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Sig Fig Practice #2 3.24 m + 7.0 m CalculationCalculator says:Answer 10.24 m 10.2 m 100.0 g - 23.73 g 76.27 g 76.3 g 0.02 cm + 2.371 cm 2.391 cm 2.39 cm 713.1 L - 3.872 L 709.228 L709.2 L 1818 lb + 3.37 lb1821.37 lb 1821 lb 2.030 mL - 1.870 mL 0.16 mL 0.160 mL *Note the zero that has been added.

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Rounding Calculated Answers Multiplication and Division Round the answer to the same number of significant figures as the least number of significant figures in the problem.

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Rules for Significant Figures in Mathematical Operations Multiplication and Division: # sig figs in the result equals the number in the least accurate measurement used in the calculation. 6.38 x 2.0 = 12.76 13 (2 sig figs)

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Other Special Cases What if your answer has less significant figures than you are supposed to have?What if your answer has less significant figures than you are supposed to have? –Calculator Example: 100.00 / 5.00 = 20 Add zeros!Add zeros! –20 is 1 sf –20. is 2 sf –20.0 is 3 sf

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Sig Fig Practice #3 3.24 m x 7.0 m CalculationCalculator says:Answer 22.68 m 2 23 m 2 100.0 g ÷ 23.7 cm 3 4.219409283 g/cm 3 4.22 g/cm 3 0.02 cm x 2.371 cm 0.04742 cm 2 0.05 cm 2 710 m ÷ 3.0 s 236.6666667 m/s240 m/s 1818.2 lb x 3.23 ft5872.786 lb·ft 5870 lb·ft 1.030 g x 2.87 mL 2.9561 g/mL2.96 g/mL

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