Particle of Sweetness Lab Activities Chapter 20 Natural World

Composition of Carbohydrates The first part of the experiment in Ch 20 was intended to show two things. First, the heating of carbohydrates. Large molecules can be broken down into smaller molecules. Sucrose is gently heated. As more heat is added the bonds in the carbohydrate are broken. Water vapor forms on the rim of the test tube.

This is how the proof works This is how the proof works. Using an equation (not balanced): C12H22O11 + heat ----> H2O + C you should be able to see that sucrose, a molecule with 45 atoms, was broken down, or decomposed into water (3 atoms big) and carbon. Large to small.

Water vapor condensing proves that two of the atoms in a carbohydrate are oxygen and hydrogen. (Later this year water will be broken down with electricity into hydrogen and oxygen). In addition to water forming, carbon remains at the bottom of the test tube. So the three atoms that make up carbohydrates are carbon, hydrogen, and oxygen. This is NOT proof for the ratio, just that these three atoms comprise sugar.

Heating of the sugar also showed that large molecules can be broken down into smaller molecules, catabolism. It is important to also note that as atoms are removed from the sucrose the properties of the sucrose change. Both the color and taste change as the substance is heated. The initial heating results in only a phase change, so there is no change in the physical properties of sucrose.

Sucrose is a Disaccharide Sucrose is a sugar formed by the combining of the monosaccharides of glucose and fructose. The bond between them can be broken and sucrose can be converted back to these monosaccharides. To test for this you tested for glucose using the Benedict’s Test.

Benedict’s Test Benedict’s test is useful in forming an operational definition for glucose. It does two things, it tells you if glucose is present with a color change, and depending on the color it forms, it tells you the amount of glucose that has formed.

Benedict’s Test Benedict’s solution plus glucose will form a color change. If the concentration is low, a light green color will form. This color will change to yellow, then brown, and finally a bright orange-red color. Orange- red is the greatest concentration of glucose.

Sucrose being broken down into glucose again illustrates catabolism Sucrose being broken down into glucose again illustrates catabolism. Sucrose, a disaccharide, was broken down into a monosaccharide, and fructose. Although you do not have a chemical test for fructose, it was formed. (You can see that Fructose is formed with the Selwinoff reaction).

Proof for Glucose from Sucrose

Results from Lab The results from the table show the effects of various chemicals with Benedict’s solution. The various combinations of chemicals rule out that the Benedict’s could have reacted to any chemical except the glucose. Therefore the glucose formed had to have come from the sucrose. Again, large molecules can be catabolized to form smaller molecules.

Here you can see the positive test for GLUCOSE with the Benedict’s Test. Note the change in color from yellow to dark brown. Benedict’s shows not only that glucose is present, but in what concentration.

In addition to this, sucrose, a disaccharide is composed of two monosaccharides. Glucose, which you proved present with Benedict’s and fructose. The presence of Fructose is shown by reacting with Seliwanoff Reagent.

Positive Selawanoff reaction for Fructose.

Same atoms, but not isomers A second heating of a sugar is done. Glucose is heated in a test tube in the same manner that sucrose was heated. Like with sucrose, gentle heating resulted in a phase change but not a chemical change. Continued heating formed water vapor and after intense heating, carbon remained at the bottom of the test tube.

This means that both glucose and sucrose contain similar atoms This means that both glucose and sucrose contain similar atoms. Similar atoms does not mean isomers. In order to be isomers you need to have the same chemical formula and different structural formulas. Sucrose and glucose do not have the same molecular formulas.

Molecules vs. Ions From earlier labs you learned that ions are electrolytes. Ions are atoms that have an unequal number of protons and electrons so they have a charge. Molecules are compounds that are bonded covalently, so ions cannot form. This is why you hear of a molecule of water, not a compound of water.

Crystal Formation Crystals form from a repeated internal arrangement of atoms. Although sugar molecules are neutral, the covalent bonds do not result in equal sharing of electrons. This results in the molecules becoming DIPOLAR. This means that the molecules have poles, like magnets. These opposite poles attract and allow the molecules to attract to each other in the crystal.

The bonds formed when these dipolar substances form is not one of the stronger bonds in chemistry. That is why the molecules of sugar will separate so easily in water to form a solution. It is important to note that dissolving does not determine whether or not a substance contains ions, it is whether or not the substance forms electrolytes when it dissolves.

So why did the sugar crystals melt when earlier this year the KI crystals did not break down with intense heat? The difference is the type of bond formed in the crystals. Sugar is organic and the molecules form weak polar bonds. KI is ionic and the individual atoms make more bonds and stronger bonds between each other.