Fundamentals of Chemistry 1030

Elements, Compounds, & Mixtures

WEAR SAFETY GLASSES

Elements, Compounds & Mixtures

There are 116 known elements in  arranged in groups and periods on the Periodic Table. Elements are the primary substance from which all other things are made. Elements can not be broken down into simpler substances.

When two or more different elements are chemically combined, they form compounds. Just about everything you see around you is made up of compounds. In a compound, each element is present in a fixed whole-number ratio. This ratio is represented by the chemical formula of a compound. For example, water has the chemical formula H₂O. This tells us that each water molecule always has 2 atoms of hydrogen and 1 atom of oxygen.

When two or more substances (elements or compounds) are physically combined, they form a mixture. The components of a mixture retain their individual physical properties and they can be separated by physical means.

Properties of Elements & Compounds

Observation of a substance allows us to describe physical properties such as shape, state, color and luster. Other physical properties can be measured such as density, melting point, and boiling point. The physical properties of an element or compound are characteristics that do not change unless the element or compound undergoes a chemical change to form a new substance. Therefore, physical properties for many substances can be found in reference books such as the CRC Handbook of Chemistry and Physics.

Types of Bonding in Compounds

The elements in compounds are held together by chemical bonds. Two types of chemical bonds are ionic bonds and covalent bonds. Ionic bonds form when a metal element combines with a nonmetal element. The metal loses electrons to form a positive ion and the nonmetal gains electrons to form a negative ion. Since opposites attract, the negative and positive ions are attracted to each other to form the ionic bond. When two nonmetals form a compound, they share electrons and form covalent bonds.

Separation Techniques

In this lab several techniques are used to separate components of a mixture. These techniques employ physical processes to carry out the separations. In this lab evaporation, extraction,  filtration and sublimation are used.

Evaporation is a method by which a liquid vaporizes at a certain temperature, but other substances in a mixture do not and are left behind as a solid residue.

Extraction utilizes the differing solubility of a substance in different solvents to permit separation.  For instance, if salt is added to a bottle of salad dressing and the bottle is shaken and then allowed to stand until two layers form again, the salt will be in the water layer rather than the oil layer because salt is more soluble in water than in oil.

Decantation is the techniques of pouring a liquid off of a solid which rests at the bottom of the container.  If muddy water is allowed to settle, then clear water may be decanted from the container.  The clear liquid phase poured off is called the decantate.

Filtration, like decantation, is used to separate a solid from a liquid.  In this case, the mixture is poured through a porous medium (often filter paper), in which the size of the pores allows liquids and dissolved solutes to pass through but retains solid particles.  This is essentially like using a strainer, but with much smaller openings.  The clear liquid phase obtained is called the filtrate.

In this experiment you will be given an unknown mixture containing mostly sand (which is silicon dioxide, SiO₂). Also in the mixture are small amounts of ammonium chloride (NH₄Cl) and sodium chloride (NaCl, which is commonly called salt). You are to separate the mixture into its component substances and determine the mass percentage of each component.  Carefully examine the table below which details the different physical properties of the species you will be analyzing.

Physical Properties of Substances in the Mixture

Laboratory Procedures

Part A.  Separation of Components of a Mixture.

1. Obtain a sample of “unknown mixture” from your instructor; record its identity   number or letter on the report form.

2. Weigh a clean, dry crystallizing dish (Petri dish) and record the mass to the nearest 0.001 g.  Measure approximately 2.0 g mixture on a weighing paper and transfer it into the dry crystallizing dish (Petri dish). Weigh the dish, with the mixture to the nearest 0.001g.

3. Heat the dry crystallizing dish (Petri dish) and its contents on a hot plate in the Fume Hood. Ammonium chloride, NH₄Cl, will sublime from the mixture and produces a white gas. Continue to heat until no white gas is produced from the mixture. Allow the crystallizing dish (Petri dish) to cool to room temperature. Weigh the cool crystallizing dish (Petri dish) and residue to the nearest 0.001 g and record this mass.

4. Quantitatively transfer the remaining mixture to a 150 mL beaker. With the distilled water squirt bottle, remove any remaining solid from the dish into the beaker. Add about 20 ml of distilled water to the residue in beaker to dissolve (extract) the NaCl. Heat gently on the small hot plate at your desk (at a setting of 3 or 4 to warm the solution- DO NOT boil) and stir for at least 5 minutes to loosen and extract salt completely in with water.

5. Obtain a piece of filtration paper and weigh to the nearest 0.001 g. Fold the filter paper in to a cone and place into a glass funnel. Support the funnel with a clamp attached to a stand about 12 inches above the desk top. Place a 250 mL beaker under the funnel to collect the liquid.

6. Slowly decant (or filter) the liquid (salt water) into the funnel with the pre-weighed filter paper to separate the salt and sand. Wash the sand residue on the filter paper several times with small aliquots of distilled water.

7. Remove the filter paper and place in a watch glass and heat the with sand residue in the oven for 20 minutes. (While the filter paper and sand is in the oven proceed to Part B) Allow the watch glass to cool to room temperature; then weigh the filter paper and residue to the nearest 0.001 g and record this mass.

Calculations and Results.

See this link for a table form to collect data. This form also aids in the calculations. Calculate the total percentage of mixture recovered in this experiment. The accuracy of the experiment is such that the total percentage of your three components should be in the neighborhood of 99 percent. If your percentage recovery is less than hundred percent or more than hundred percent, give an explanation for your errors.

Determine the mass and calculate the percentage NH₄Cl, SiO₂ and of NaCl.

Part B. Physical Properties of Iron sulfide and Its Elements.

(Note: This part is done while the sand and filter paper from Part A is in the oven).

1. Obtain samples of iron (Fe), sulfur (S), a mixture of iron and sulfur (Fe + S), and iron(II) sulfide (FeS). Describe the observable physical properties of each sample in Table 2 (color, state, luster).

2. Using a chemistry handbook (CRC), look up the density, melting point, and boiling point of Fe, S, and FeS. Record these in Table 2.

Table 2. Physical Properties Data

3. Test each sample for magnetic attraction by running a bar magnet under each sample. (Do not place the magnet directly into the samples.) If there is magnetic attraction, the particles will follow the magnet. Record your observations in Table 3.

4. Describe each sample as a metal or a nonmetal in Table 3. (Metals will be attracted to the magnet and nonmetals will not be attracted to the magnet.)

5. Describe each sample as an element, mixture, or compound in Table 3.

Table 3. Elements, Compound & Mixtures

Issues to be addressed in your conclusion...

Why do the physical properties of iron and sulfur differ from those of iron(II) sulfide?

Could the elements in the Fe + S mixture and the compound FeS be separated using the same methods? Why or why not?

How does the magnetic attraction differ for the elements, mixture, and compound?

Explain the following statement in your own words: “In a compound, there is a definite composition of the elements. “

Give the percentage composition of sodium chloride, ammonium chloride and sand in the mixture.

Explain what is meant by the terms decant and evaporate.

(updated 1-28-08  P. Powers)