My big toe has been dipped into the murky, to me, waters of chemistry measurements. Don’t really know much about metrics and the International System of Units (SI). I know the USA or imperial system of miles, gallons, cups, etc. So to understand worldwide science units, follow me and let’s jump in together!

**SI Base Units**: There are seven SI base units that are traditionally used in all chemistry and these are the basic mathematical language of chemistry and other sciences. Being able to understand and use the units are important to communicating as they set the standard measure for comparing, evaluating, etc. The seven units are length, mass, temperature, electric current, time, the amount of a substance, and its light.

**Length base unit and meters:** The meter base unit with its prefixes such as centimeter and millimeter and kilometer (which I have seen on signs when driving in Canada) is critical to understanding. This base unit is 1 meter, which is equivalent to 1.0936 yards or 39.370 inches. Visually, this is a little longer than a yardstick, about the width of our dining table, and about the distance from a door handle in my house to the floor. Here are some other examples to help visualize the metric system:

A decimeter (dm) is about the size of a large apple that you are ready to eat, minus any worm.

A centimeter (cm) is about the size of that button that popped off your shirt. 100 centimeters = 1 meter.

A millimeter (mm) is about the thickness of a small coin’s thickness. 1000 millimeters in 1 meter and 100 centimeters in 1 meter.

A micrometer (μm) is about the size of a bacteria cell. It is 1 millionth of a meter. Tiny little thing!

A nanometer (nm) is the thickness of an RNA molecule. It is 1 billionth of a meter. We are now entering the dynamic world of atoms and quantum physics and computer nanotechnology!

A kilometer is larger than a meter; actually 1000 meters equals 1 km and 1 mile = 1.609 km.

**Volume and meters:** Volume is length x width x height and measures the space occupied by the matter being tested or evaluated. 1 liter (L) is the liquid base unit. Last time you bought soda it may have been the 2L bottle which is also 2000mL. That cube of sugar for your coffee is 1 cm3 or 1 mL; cm3 is the symbol for a cubic centimeter. A milliliter is about 20 droplets of a liquid such as water and is 1 mL. Aha, we can interchangeably use mL and cm3. The salt we use at the table is a microliter or 1 μL. When I walk into a lab, I see many containers that will contain substances which must be measured for volume. These include pipettes, syringes, cylinders and such. Volumes will normally be taken at room temperature because volumes vary with temperature changes, especially for gases.

**Mass and the kilogram base unit:** Mass is measured in kilograms and abbreviated as kg. Mass is the same on Earth and Mars, but weight is different due to the two differing gravities. Weight is the pull on the mass due to gravity. A kilogram (1000 grams) is about the size of a dictionary and 1 gram is about the size of a paper clip. A thousand kilograms gets renamed as 1 tonne or metric ton.

**Temperature is kelvin or K:** When a hot object is in contact with a cold object, the heat will move towards the colder object and cool down. Typically, when something heats up, it expands and when cooled, it contracts. I like the Fahrenheit measure, but scientists love Kelvin and Celsius. Kelvin is known as the absolute scale where the freezing point of water is 273.15 Kelvins (K); the boiling point is 373.15 K. But, on the Celsius scale the freezing point of water is 0o C and the boiling point is 100o C. I much prefer water boiling at 212 degrees Fahrenheit. Three different ways to measure the same object’s temperature! Why must it be so.

**Time**‘s base unit is a second or s. Enough said.

**Electrical current** has a base unit of ampere or A.

**The amount of a substance** that we are measuring is a mole or mol.

**Light or luminous intensity** is measured in candela or cd.

**Mass and Volume = Density:** Density is determined by a calculation involving an item’s ratio of mass to (divided by) volume. Density is an intensive property of a substance, and the composition, not the size of the substance, is what matters. Aha, I hear the need to some chemistry math clomping down our road.

My grandson did a science project a few years ago involving copper pennies. Let’s figure out what the density of one penny is. A post-1982 penny is inhomogeneous as it is copper clad upon zinc. The standard for copper density is 8.96 g/mL and for zinc it is 7.13 g/mL. Let’s assume (since we don’t have a handy lab available) that the penny is 25% copper and 75% zinc and this will provide us with information on the weighted average of zinc and copper density.

Density (g/mL) = Mass/Volume. In this case we have the density of each element and just need the proportionate density calculation so we will calculate known data for a weighted average density:

25% copper and 75% zinc:

Density = (0.25)( 8.96 g/mL) + (0.75)( 7.13 g/mL) = Weighted Average Density

Density = 2.24 g/mL + 5.3475 g/mL = 7.5875 g/mL

Weighted Average Density = 7.5875 g/mL

**Density of a Federal Reserve gold (Au) bullion bar that measures 7 x 3.625 x 1.75 inches:** We know that Gold (Au) density at room temp is 19.3 g/cc or 19.3 grams per cubic centimeter. We could convert the bullion measurements into centimeters to figure out the volume. Knowing that 1″=2.54 cm: 7″x2.54cm = 17.78 cm; 3.625″ = 9.21 cm; 1.75″ = 4.45cm. This means that the volume is 17.78cm x 9.21cm x 4.45cm = 727.9 cm.

To figure out the mass, we can then multiply the volume 727.9 cm using known density as a conversion factor (19.3g=1cc), or 19.3g/cc (density) / 1cc to get 14,048.47g, the mass.

m = d x v = 19.3 g/cc x 727.9 cm = 14,048.47g.

d=m/v: d = 14048.47g mass /727.0cm volume = density of 19.3g/cc

**Making Chemistry Real:**

A helium balloon is less dense than the density of air, so it will rise and float about.

Corn oil has less density than water and will float on water. If you pour water into a container of hot corn oil, stand back now! The water and corn oil fight each other and spit hot oil everywhere. A big no-no.

My husband’s favorite pancake syrup is plain old corn syrup (sorry, Aunt Jemima). Corn syrup is more dense than water and sinks below a layer of water.

Mass does not change when heated, but volume likely will and density must be recalculated.

Density of solids is normally stated in grams per cubic centimeter (g/cc) and grams per milliliter (g/mL) for liquids.

If I do any more of this now, my heart will just clutch. See you again soon.

SOURCES:

https://www.boundless.com/chemistry/introduction-to-chemistry/units-measurement/standard-units-si-units/

http://www.metric-conversions.org/length/meters-to-feet.htm (Convert online)

http://www.diffen.com/difference/Kilometer_vs_Mile

http://www.eformulae.com/physics/meter.php

http://www.shodor.org/unchem/math/units/

http://www.elmhurst.edu/~chm/vchembook/125Adensitygold.html

http://www.ehow.com/how_7778653_change-density-mass.html – ixzz2SestC6k5