Introduction: Matter & Measurement. AP Chemistry Chapter 1 (Day 2). Chemistry. What is chemistry? 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 . Elements, Compounds & Mixtures.
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Introduction: Matter & Measurement
What is chemistry?
It is the study of the composition of matter and the changes that matter undergoes.
What is matter?
It is anything that takes up spaceand has mass.
A substance is matter that has a definite composition and constant properties
It can be an element or a compound
An element is the simplest form of matter.
It cannot be broken down further by chemical reactions
A compound can be separated into simpler forms
It is a combination of two or more elements
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
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
Physical property – characteristics of a pure substance that we can observe without changing the substance; the chemical composition of the substance does not change
Chemical property –describes the chemical reaction of a pure substance with another substance; chemical reaction is involved
reaction with oxygen (flammability)
rxn with water
rxn with acid
Do not depend on the amount of sample being examined
Depend on the quantity of the sample
The composition of the substance doesn’t change
Phase changes (like liquid to gas)
Evaporation, freezing, condensing, subliming, etc.
Tearing or cutting the substance
The substance is transformed into a chemically different substance
All chemical reactions
permanent color change
gas produced (odor or bubbles)
precipitate (solid) produced
light given off
heat released (exothermic) or absorbed (endothermic)
A measurement is a number with a unit
All measurements, MUST have units
There are two types of measurements:
Qualitative measurements are words, such as heavy or hot
Quantitative measurements involve numbers (quantities), and depend on:
The reliability of the measuring instrument
The care with which it is read – this is determined by YOU!
Accuracy – how close a measurement is to the true value
Precision – how close the measurements are to each other (reproducibility)
Precise, but not accurate
Neither accurate nor precise
Precise AND accurate
Measurements are performed with instruments, and no instrument can read to an infinite number of decimal places
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
What is wrong with this ruler? What is it missing?
Non-zerosalways count as significant figures:
Leading zeroes do not count as significant figures:
3 significant figures
Captive zeroes always count as significant figures:
4 significant figures
Trailing zerosare significant only if the number contains a written decimal point:
4 significant figures
Two special situationshave an unlimited (infinite) number of significant figures:
23 people, or 36 desks
Exactly defined quantities
60 minutes = 1 hour
How many significant figures in the following?
5 sig figs
4 sig figs
These all come from some measurements
5 sig figs
3.29 x 103 s
3 sig figs
2 sig figs
2 sig figs
This is a counted value
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.
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
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.
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 =
18.734 18.7 (3 sig figs)
3.24 m + 7.0 m
100.0 g - 23.73 g
0.02 cm + 2.371 cm
713.1 L - 3.872 L
1818 lb + 3.37 lb
2.030 mL - 1.870 mL
*Note the zero that has been added.
Multiplication and Division
Round the answer to the same number of significant figures as the least number of significant figures in the problem.
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)
What if your answer has less significant figures than you are supposed to have?
Calculator Example: 100.00 / 5.00 = 20
20 is 1 sf
20. is 2 sf
20.0 is 3 sf
3.24 m x 7.0 m
100.0 g ÷ 23.7 cm3
0.02 cm x 2.371 cm
710 m ÷ 3.0 s
1818.2 lb x 3.23 ft
1.030 g x 2.87 mL