Chemical Tests

A workshop for Dutch Neck School Teachers March 28, 2000

by David Reibstein, Outreach Director

Outline

• The terms acid and base refer to behaviors in aqueous solution
• Certain substances yield acidic or basic solutions
• Properties of acidic and basic solutions
• acidic solutions
• basic solutions
• reactions with each other
• About water
• About atoms
• Chemical bonds
• How water molecules come apart, or dissociate
• What makes a solution acidic or basic?
• Some common acids and bases
• How acids and bases combine with each other
• The strengths of acids and bases; the concentrations of acidic and basic solutions
• One application: acid rain

• The concept of acids and bases has reference to the behavior of certain substances in water solution ("aqueous" solutions)
• Certain substances have the ability, when dissolved in water, to produce acidic or basic solutions
• How do we recognize acidic and basic solutions? By their properties
• Acidic solutions
• react with certain metals (e.g., zinc, copper) but not others (e.g. gold, silver), producing bubbles. These bubbles turn out to be composed of hydrogen gas
• have a sour taste
• are caustic, i.e. they "eat through" many materials
• turn indicators certain distinctive colors. e.g., turn litmus red.
• Basic solutions
• react with certain other metals
• have a bitter taste
• are also caustic
• turn indicators other distinctive colors. e.g., turn litmus blue.
• Acidic and basic solutions react with each other, to produce compounds known as salts, which are neither acids nor bases. Thus, the reaction of acid with base is called neutralization.
• When a group of substances has similar properties, we suspect they have something in common.
• An acid is a substance that produces an acidic solution in water
• A base is a substance that produces a basic solution in water
• All acids have in common that they change something about the water they are dissolved in. Likewise, bases.
• About water:
• Water = H₂O. 1 molecule water = 2 hydrogen atoms and 1 oxygen atom.
• Structure of a water molecule: H-O-H
• The dark lines - represent chemical bonds. A chemical bond is formed when two atoms share a pair of electrons.
• About atoms:
• All atoms are composed of 3 types of particle:
• proton: heavy, has a positive (+) charge
• electron: very light, has an equal negative (-) charge
• neutron: heavy, has no charge (is believed to be composed of an electron plus a neutron)
• The protons and neutrons are found tightly packed at the center of the atom, in the nucleus.
• The electrons occupy a cloud (relatively) far away. Most of the atom is empty space!
• The identity of an atom is determined by the number of protons in its nucleus.
• Hydrogen atoms have 1 proton, oxygen atoms have 8 protons. This never changes in chemical reactions.
• Free atoms are electrically neutral because they have equal numbers of protons and electrons.
• Hydrogen atom has 1 proton, 1 electron. Oxygen atom has 8 protons, 8 electrons.
• Chemical bonds
• Since the electrons are out on the edges of the atom, it is the electrons that are involved in bonding atoms together.
• A chemical bond is formed by sharing a pair of electrons between two atoms. Sometimes we represent this with dots standing for the electrons. For a water molecule, it would look like this:
H ŸŸ O ŸŸ H
• To a small extent, water molecules come apart, like this:

One of the hydrogens leaves. But the whole atom doesn't leave, just the proton. It leaves the electron behind. So, what's left behind is OH with an extra negative charge, or OH^-.   And what's left of the H is just a proton, which has a + charge, written as H⁺. The whole process is written like this:    H₂O  H⁺ + OH^- (Molecules with charges, such as these, are called ions.)

• Since each water molecule produces 1 H⁺ and 1 OH^- ion, in pure water the concentrations of H⁺ and 1 OH^- are equal. Such a solution is neutral.
• In pure water, this "coming apart" (we call it dissociation) occurs to only a tiny extent. So, not only are there equal amounts of H⁺ and 1 OH^- , there are also only tiny amounts of them. (In pure water only 1 out of ten million molecules are dissociated.)
• What makes a solution acidic or basic?
• Acids are substances that contribute extra H⁺ to a solution, so that
concentration of H⁺ > concentration of OH^-.
• Bases are substances that either contribute extra OH^- or take away H⁺ , so that
concentration of H⁺ < concentration of OH^-.
• H+ and OH- are very reactive, so solutions with high amounts of these have some interesting chemical properties. Strongly acidic or basic solutions can dissolve many otherwise insoluble substances, so they can eat away at things. Also, acids and bases can promote other reactions, serving as catalysts.
• What kinds of things add extra H⁺ to a solution (what sorts of things are acids)?
• How can you tell if something is an acid or base or neither?
• You can cheat: memorize a short list of common acids and bases:
• Some common acids:

• Some common bases:

• Some bases do not contain OH, but nevertheless increase the concentration of OH^- in water. Examples:
• Ammonia is a gas, formula NH₃. When dissolved in water, it "steals" an H⁺ from water as follows:

NH₃ + H₂O  NH₄⁺ + OH^-
thus increasing the concentration of OH^-and making the solution basic.

• Most soaps and some detergents show the same behavior.
  
• Acids and bases combine with each other
• A water molecule can be thought of as a combination of an H⁺ and an OH^-
• Since acids supply H⁺ and bases supply OH^-, it is not surprising that acids and bases combine to produce H₂O.
• An example: if we mix a solution of hydrochloric acid with a solution of sodium hydroxide:

Note: this yields water + a salt. In fact, a salt may be defined as the product of a reaction between an acid and a base.

Water is such a stable molecule that such reactions are quite favorable, occur rapidly, and release large amounts of energy in the form of heat. (In fact, one needs to be careful when mixing acids with bases not to mix them too quickly if the solutions are concentrated.)

• The strengths of acids and bases; the concentrations of acidic and basic solutions.
• Two factors determine how acidic a solution is: 1. the strength of the acid; and 2. how concentrated the acid is. It is important to not confuse these two concepts.

• A strong acid is one that dissociates completely into H+ and another ion.
An example: hydrochloric acid is a strong acid. The reaction

HCl  H⁺ + Cl^- goes 100% from left to right. That is, in the solution there are no more molecules of HCl left, only H⁺ + Cl^-. Consequently, there is a very high amount of H⁺ in a hydrochloric acid solution.

• A weak acid is one that dissociates less than completely. An example: acetic acid is a weak acid. The reaction

CH₃COOH  H⁺ + CH₃COO^- goes only to a small extent from left to right. Consequently, there is a smaller amount of H⁺ in an acetic acid solution compared to a hydrochloric acid solution of the same concentration. 2. The concentration of a solution effects its acidity: the more of an acid dissolved in a given amount of water, the more acidic the solution is.

• Examples:
• a concentrated solution of HCl is very acidic, but a dilute solution of HCl would be not very acidic.
• a highly concentrated solution of acetic acid is very acidic, even though acetic acid is called a weak acid.
• Of course, for a given concentration, the stronger acid gives the more acidic solution.
• The same holds true for basic solutions.
• An application: acid rain and snow
• Acid rain and snow are caused by industrial and automotive pollution combining with water in the air to produce an acidic solution.
• The chief culprits are nitrogen oxides and sulfur oxides, by-products of combustion.
• Nitrogen oxides combine with water in the air to yield nitric acid, HNO₃, which dissociates as follows:
  HNO₃  H⁺ + NO₃^-
• Sulfur oxides combine with water in the air to yield sulfuric acid, H₂SO₄, which dissociates as follows:
  H₂SO₄ H⁺ + HSO₄^-
• These acids have harmful effects on life.
• Note: pure rain water is slightly acidic. This comes from carbon dioxide in the air combining with water in the air to yield the weak acid carbonic acid, H₂CO₃. But acid rain/snow is significantly more acidic than this.