Presentation topic idearelaxer: what is it, how does it work (get a little technical with

images and chemical explanation)

Good HairWed, September 15, 8pm DIGS

Is this an element, a compound, or a mixture? This is a common mineral.

Calcium carbonate,CaCO3 - calcite.

Balanced formula (recipe) Always a neutral charge

Sodium carbonateNa2(CO3 )

INVESTIGATING CHEMISTRY A FORENSIC SCIENCE PERSPECTIVE second

edition

CHAPTER 2:

EVIDENCE COLLECTION

AND PRESERVATION

CASE STUDY: GRAVE EVIDENCE

Soil on a shovel

2.1 PRESERVING EVIDENCE: REACTIONS, PROPERTIES, AND CHANGES

• It is vital to preserve evidence in its original state.

• This is so crucial to prosecuting a case successfully that it must be done by:– specially trained officers,

or– forensic lab personnel

themselves.If evidence is not preserved properly, it can be damaged through chemical changes it undergoes over time.

2.1 PRESERVING EVIDENCE: REACTIONS, PROPERTIES, AND CHANGES

It requires good judgment and critical thinking to know which evidence is going to be key in getting a conviction, thus solving the case. Chemical and Physical changes must be prevented of minimized.

• As soon as it is gathered, evidence must be: Photographed, packaged separately to prevent cross-contamination , labeled very carefully

• It is vital to preserve evidence in its original state.– If evidence is not preserved properly, it can be

damaged through chemical changes it undergoes over time.

2.1 CHEMICAL CHANGES AND CHEMICAL PROPERTIES

• Chemical changes through reactions involve matter producing new substances.

• Evidence preservation involves preventing these kinds of changes or at least minimizing them. Air, moisture, and light can cause problems (chemical changes).– Chemical properties (Flammability, Explosiveness, Corrosiveness, Caustic properties)

relate to a substance’s potential to react with oxygen, H2O, or other agents in the environment.

• If the evidence is an acid, for example, it can’t be stored in most metallic containers.

• Keeping evidence cold or frozen slows down any chemical reactions/decomposition.

• Why is evidence normally dried before storage?

2.1 PHYSICAL CHANGES AND PHYSICAL PROPERTIES

• Physical properties are those properties which can be measured without changing the chemical identity of the substance. In contrast to chemical changes, physical changes involve no new substances being formed.– These include: color, melting point, boiling point, odor, refractive index,

hardness, texture, solubility, electrical properties, and density of the substance, for example.

• The chemical composition does not change when ice, H2O(solid), melts to become water, H2O(liquid)-a physical change.

• Likewise, when the water turns to steam or just water vapor, H2O(gas), there is no change in the chemical composition.

INTENSIVE AND EXTENSIVE PHYSICAL PROPERTIES

• Intensive physical properties: Do not depend on the size of the sample– color, melting point, boiling point, odor, refractive index,

hardness, texture, solubility, electrical properties, and density– Density is mass/volume.

– Refractive index (r. i.) is a measure of how much a beam of light is bent as it passes from air into the sample

• useful for identifying clear liquids and solids such as glass. Its value is 1.333 for water at room temperature, 23oC. nD

23 = 1.333

• Extensive properties do depend on the size of the sample– mass, volume

2.2 PHYSICAL EVIDENCE COLLECTION: MASS, WEIGHT, AND UNITS

The universe is composed of matter and energy.

• Matter is that which has mass and occupies space (has volume).– Mass is a measure of the amount of

matter in the sample.– On the moon, your mass is the same,

but your weight is only 1/6th. – Weight is related to mass, but unlike

mass, it depends on how gravity affects the mass.

• Energy is kinetic or potential– Kinetic energy relates to motion.

Potential energy is stored energy, as in chemical bonds.

Density is mass/volume.Pumice is a rock that floats.

• Unit ConversionsUnit Conversions may be needed.may be needed.• It is important to select the proper size units for It is important to select the proper size units for

reporting scientific data.reporting scientific data.• The best approach is to always do one The best approach is to always do one

conversion involvingconversion involving the base unitthe base unit..• Base unitsBase units include: theinclude: the gramgram, the, the literliter, the, the metermeter, ,

and so on.and so on.

2.2 UNITS

1 kg = 1000 grams 1 mm = 103 meter1 L = 1000 mL 1 cm = 102 meterOne pound is 454 g. One quart is 0.9464 L.One mile is 1.6093 km. One inch is 2.54 cm.

2.2 UNITS

2.3 THE MATH OF UNIT CONVERSIONS

• Always begin with an equation that involves the base unit.• If you want to change centimeters (cm) to millimeters

(mm), go through meters (m).• Change 29 cm to mm.• First, write an equation relating cm to m.

1 cm = 102 m or 1/100 m or 0.01 m• Next, make a conversion factor from the equation. This is

a fraction.• If you multiply this conversion factor (CF) times the

original data (number and its unit), the units must cancel.

2.3 UNIT CONVERSIONS• 29 cm (102 m / 1 cm) = 29 102 m• Step two involves another equation:

1 mm = 103 meter (m)• We will use the equation to form another conversion

factor, a fraction we can use to cancel the base unit, m.• 29 102 m (1 mm / 103 m) = 29 101 mm

• In scientific notation the pre-exponential factor must be in the range of 1 to 9.999…

• So our final answer is 2.9 102 mm or 290 mm. Making the 29 smaller (2.9) means we must make the exponent of 10 bigger (102) to keep the same value or size.

• Not allNot all the digits in a calculated numberthe digits in a calculated number are significantare significant..• The The last digitlast digit that should be retained in any that should be retained in any

measurementmeasurement is estimatedis estimated..• TheThe rulerule in recording data is toin recording data is to keep all the digits keep all the digits

known exactlyknown exactly plusplus oneone that is estimated and can vary that is estimated and can vary from one reading to the next.from one reading to the next.

• These digits are known collectively as significant figures.

• It is an error to record more digits or fewer digits than are actually significant.

2.4 ERRORS AND ESTIMATES IN LAB MEASUREMENTS: SIGNIFICANT FIGURES

This bloody footprint is between 11 and 12

cm wide. So recording the width

as 11.5 cm is appropriate. In fact,

recording 11.4, 11.5, or 11.6 would all be

correct.

ZERO AND NONZERO NUMBERS IN MEASUREMENTS

1. Zeros located between nonzero digits in a valid measurement are significant.

2. Zeros at the beginning of a number with decimals are not significant.

3. Zeros at the end of a number containing a decimal are significant.

4. Zeros at the end of a number without a decimal are ambiguous.

• How many digits are significant in 12.030 cm?

– Answer: All five. Rules 1 and 3 apply.

• How many digits are significant in 0.0347 inches?

– Answer: The last three. Rule 2 applies.

2.5 MATH OF SIGNIFICANT FIGURE 2.5 MATH OF SIGNIFICANT FIGURE CALCULATIONSCALCULATIONS

Significant Figures Math Rules 1. For addition or subtraction: The answer can have only as many decimal places as the measured number with the fewest digits after the decimal place.

• For example: 4.05 + 2.2 + 3.981 =not 10.231 but 10.22. For multiplication or division: The answer can have only as many significant figures as the measured number with the fewest significant figures.

• For example: (2.3 1.007)/23.4 = 0.098 and not 0.098978632!

RULES FOR ROUNDING

1. If the number being dropped is greater than 5, increase the last saved digit by 1. Example: 3.47 cm becomes 3.5 cm.

2. If the number being dropped is 5 or less, leave the last digit saved as is.Example: 3.93 mm becomes 3.9 mm.

EXAMPLES

• Problem: Add 0.347 + 10.22 + 4.9.• Solution: Align the decimals 10.22?

0.347

4.9??

Add 15.467

We can only keep 15.5?? or 15.5,

which has one decimal place.

Adding an unknown(?) to a known gives an unknown quantity.

MULTIPICATION/DIVISION OF MEASURED DATA: EXAMPLE

• Problem: What is the answer if 12.5 is multiplied by 4.309 and then the product is divided by 52.70?

• Answer: Before doing the math we can tell that the 12.5, which has the fewest number of sig. figs. will control the outcome. The answer may have three sig. figures. The answer is 1.02.

2.6 EXPERIMENTAL RESULTS: ACCURACY AND PRECISION

• Accuracy corresponds to proximity of a value to the true or accepted value.

• Precision relates to the internal consistency of a group of measured data on a single quantity.

• The more significant digits in a piece of data, the more precisely its value is known.

Figure 2.2, pg. 38

Investigating Chemistry, 2nd Edition

© 2009 W.H. Freeman & Company

2.7 HOW TO ANALYZE EVIDENCE: DENSITY MEASUREMENTS

• Density is defined as the ratio of the mass of an object to its volume.

• The lab units for solids and liquids are:– grams per cubic centimeter for solids (g/cm3)– grams per milliliter for liquids (g/mL)

• Density is useful in identifying different types of broken glass at a crime scene.

Volume measurements involve estimating between the lines. Volume change from glass dropped in DI water provides volume

Measuring Temperatures also involves estimating between the lines.

There areThere are many kinds of glassmany kinds of glass. This fact can be. This fact can be very very helpful helpful in solving a crime.

2.8 THE MATH OF DENSITY MEASUREMENTS

• Problem: A glass mirror fragment had a mass of 7.891 grams and a volume of 2.587 mL or c.c.’s. Find its density.

• Solution: D = m / VThe density equals 7.891 g / 2.587 mL or 3.050 g/mL or g/c.c. or g/cm3

• Note that 1 mL = 1 c.c. = 1 cm3 • Note that the answer has 4 sig. figs.

2.9 HOW TO ANALYZE GLASS AND SOIL EVIDENCE: USING PHYSICAL

PROPERTIES• Densities are useful physical properties for

identifying unknown materials.• The sink-float method involves placing the

sample in a liquid of known density. If it floats it is less dense than the liquid. If it sinks, it is more dense.

• Because soil is heterogeneous, the density gradient method is used. This uses a tall cylinder with a low-density liquid at the top and a high-density liquid at the bottom.

Measuring Temperatures also involves estimating

between the lines.

Unnumbered Figure, pg. 49

Investigating Chemistry, 2nd Edition

© 2009 W.H. Freeman & Company

The less dense solids and liquids are at the top in the density gradient method.

2.9 HOW TO ANALYZE GLASS AND SOIL EVIDENCE: USING PHYSICAL

PROPERTIES• The heterogeneous soil particles are The heterogeneous soil particles are

suspended at the pointsuspended at the point where their where their densities match densities match those of the liquid.those of the liquid.

• If If soilsoil from the suspects clothing from the suspects clothing matches, it onlymatches, it only indicates indicates he or she he or she waswas presentpresent..

• ColorColor is another importantis another important physical physical propertyproperty used by forensic geologists.used by forensic geologists.

• The finer solids pack more tightly. Hence they are more dense. Soil texture is also important evidence

This crime was solved using forensic geology and the properties of soils.

Do soil samples tend to be heterogeneous mixtures or homogeneous mixtures?