Physci text 14 Chemistry

14 Chemistry

An introduction the periodic table

Of hydrogen, isotopes and helium

Elements are materials that consist only of atoms with the same atomic number. The atomic number for an atom is the number of positively charged protons in an atom.

The number of electrons is equal to the number of protons. Electrons orbit the protons. This is an oversimplification of reality, but the reality is that the electron is actually a probability wave function. The mathematics to get the electron right are beyond most people on the planet. Best to think of the electron as zipping around the proton. The drawing below left is an abstract image of a hydrogen atom with its single proton and single electron.

The atomic number is determined by the number of protons. Hydrogen has an atomic number of one. The atomic mass is determined by the number of protons and neutrons. While hydrogen always has one proton in the nucleus, the number of neutrons can vary from none to two. Hydrogen with one neutron is called deuterium and has an atomic of mass of two, hydrogen with two neutrons is called tritium and has an atomic mass of three. Water is comprised primarily of hydrogen and oxygen, but some of the hydrogen atoms have an extra neutron. Occasionally two extra. This is what causes the numbers after the decimal place in the atomic mass. The atomic mass on a periodic table is the average of all of the isotopes weighted by their proportion in the element.

Electrons not only go around the proton, but electrons also spin around their own center. Some spin "clockwise" and some spin "counter-clockwise." Spinning is one of the fundamental movements of the universe. Electrons tend to pair up. Electrons are particles, not thinking beings. If electrons could think, we would say electrons like to be in pairs, one spinning one way and the other spinning the other way. Hence, to continue to anthropomorphize, a single electron is lonely and wants an opposite spin partner. The result is that hydrogen's electron will either try to snatch a partner electron from another element or simply run away to an element in which it can become a pair. Two hydrogen atoms will remain bonded together, sharing their electrons, as this puts two electrons together.

Helium is atomic number two because of the two protons in the nucleus. Along with two protons are two neutrons, thus the atomic mass is four.

Laboratory 14: Floral pigments as litmus tests for acids and bases

Chemistry from the flower garden into the kitchen

Questions

What floral pigments make the best detectors of acids and bases? What substances are acids, bases, or neutral?

This laboratory explores whether flower petal pigment solutions are good indicators of acids and bases. The flower petal pigment solutions are produced by boiling flower petals in water. If a flower petal solution is found that is a good detector of both acids and bases, then that solution will be used to determine whether unknown substances are an acid, a base, or neutral. Flower pigment solutions are also called floral pigments in this laboratory.

    Hypothesis: Flower pigment solutions can be used to detect acids and bases.

    Analysis: Use a flower pigment solution to determine whether unknown substances are an acid or base.

In the English language acids are said to be sour or tart. Bases are often said to be bitter.

Phases

There are three phases to this laboratory.

    In the first phase you will gather flowers to use as tests for determining whether a substance is an acid or a base. Pick common flowers. Do not pick flowers that belong to someone. Do not pick the orchids in front of the library or other rare flowers. You might also try colorful leaves, again, avoiding rare plants.

    In the second phase you will determine which flower produces the best floral fluid for determining whether something is an acid, a base, or neutral. After boiling the petals, the fluid will be tested with a known acid and a known base. You are looking for a flower fluid which shows a distinct and different color change for the known acid and the known base. In this lab lime juice is usually our known acid and baking soda is usually our known base.

    In phase three you will use that one "best" flower fluid to test liquids to determine if they are an acid, a base, or neutral. When working with test tubes, be sure to rinse them well between tests. Be careful, some unknowns are poisonous, others can burn your skin.

Equipment

•     flowers

•     hot plate

•     tea pot to heat water

•     many small and medium sized beakers

•     numerous small test tubes

•     spatulas for placing powder into test tubes

•     eyedroppers

•     mortar and pestle for grinding aspirin, anti-acid tablets (not every term)

•     paper towels

•     transparent tape

•     Gloves for handling hot beakers

•     Sheets labeling knowns and unknowns

•     x11 color chart [optional]

•     Fruit picker pole for flowers

Phase I: Collecting

Collect the flowers and leaves, boil them.

Phase II: Finding a litmus solution

Test each floral solution for color changes, including changing to clear or clear changing to a color.

Make a table of your results for the flower that produced a color change for both the known acid and the known base. Specifically record the color change, if any, caused by the known acid or the known base. The table should include:

•     flower name

•     original color of solution

•     color change, if any, when known acid is added

•     color change, if any, when known base is added

Look for a change in color as in a change in hue, of color angle, not a change in saturation or luminosity.   

Note that you are not testing to determine whether the floral litmus solution is acidic or basic. The floral solution is being used as an indicator fluid. Whether the fluid itself is acidic or basic is not determined by this laboratory, but in general the floral litmus solutions are near neutral.

In the laboratory the known acid is citric acid provided by key lime fruit. The known base is usually baking soda.

Instructional note: some flowers can detect bases but are not always reactive to baking soda. Hibiscus rosa-sinensis often displays this behavior. A more consistent floral litmus solution here in the tropics is the purple flowered Philippine ground orchid Spathoglottis plicata.

Phase III: Testing household substances

Use flower petal solutions that change color in a consistent way for acids or bases. If you find one that changes color for both acids and bases, use that one. If you find that one changes colors for acids and another changes color for bases, use both of those two floral pigment solutions. These floral solutions can also be called "litmus" solutions.

Use the solution to analyze whether the following household substances are acids or bases (different compounds might be made available on lab day):

•     vinegar

•     ammonia

•     isopropyl alcohol

•     aspirin

•     cream of tartar

•     diluted bleach

•     household cleaning products

•     mineral lime (coralline lime)

•     hand soap

•     shampoo

•     detergent

Make a table of your results for the household substances noting the color change and whether the substance is an acid, a base, or neutral. Only put the substances present in the laboratory on the day of the laboratory. Not all substances will be available.

Discussion

How was the hue angle affected by an acid for your flower? Did that change appear to be similar to what other students were seeing? How was the hue angle affected by a base for your flower? Did that change appear to be similar to what other students were seeing? Was there a pattern to the unknown substances? For example, are cleaning products usually basic or acidic? Discuss difficulties or complications you encountered during the laboratory. Write the discussion as a paragraph of text, of prose, not a series of questions and answers. Explain to the reader what you did and what you found to be true.

Further instructional notes

Label the "known" substances clearly. Make clear that we do not know if the floral litmus solution is an acid or a base. We cannot know this. We can only know how the floral litmus solution reacts to a known acid and a known base. "Lime. Citrus fruit. Sour. Known acidic solution. Excess H ions." "Baking soda. Bitter. Known basic solution. Excess OH- ions."

Red Hibiscus rosa-sinenis and dark red leafed coleus works well. Spathoglottis plicata, Philippine ground orchid, also works well. Early morning Hibiscus tiliaceus does not work well, but by noon the flowers do react consistently.

Students tend to be deeply confused about the conceptual underpinnings of the laboratory. The concept of a litmus fluid eluded them. The students often consider the floral solutions to be acidic or basic, misunderstanding their role as indicator solutions. Introducing the laboratory using litmus paper to test the knowns appears to help, but the point of the laboratory is to be able to determine whether a solution is acidic or basic without having to resort to litmus paper.