Exam Review Question #9

Question: Describe some of the physical and chemical changes involved in cooking.  What are the 5 main observations that indicate that a chemical change has occurred?  Explain them in terms of your cooking examples.


Answer:


Both physical and chemical changes can be easily observed while cooking.  An example of a physical change in cooking is when you mix two things together in a mixing bowl.  If I were to mix two or more things together (for example sugar, flour, and water), a physical change would have occurred because the chemical composition of those individual ingredients would not have changed.  Other examples of physical changes in cooking are melting chocolate and boiling water.  Again, these are physical changes because their chemical composition has not changed.


The five main observations that indicate that a chemical change has occurred are: 
1. Change in color
2. Change in odor
3. Formation of a gas
4. Formation of a precipitate
5. Change in temperature (can be observed in the form of light)

Examples in cooking:

Change in color - When an apple is left out, its chemical properties react with the oxygen in the air, resulting in a color change to brown.  When meat is cooked, its color turns darker because a chemical change has occurred.

Change in odor - When an egg goes rotten, it is very easy to detect a change in smell.  The oxygen in the air reacts with the egg's chemical composition, causing it to rot.  The resulting smell is proof that a chemical change has occurred.

Formation of a gas - When baking soda and vinegar are combined, bubbles are formed within the liquid solution.  This fizzing is actually gas bubbles being formed inside the composition, which indicates a chemical change has happened.  

Formation of a precipitate - When cooking an egg, the formation of a precipitate occurs when the gel around the yolk is transformed into egg whites (a solid).

Change in temperature - When baking a cake, energy is absorbed by the runny cake batter as it forms into a cake.




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Exam Review Questions #27:
Question:
What is the difference between isotopes and ions?

Answer:
Isotopes are atoms of an element with different numbers of neutrons in the nucleus, which in turn affects the atom's mass number.
Ions are the difference in number of electrons in the outer valence shell of the atom.  The more electrons an atom has, the more negatively charged it becomes.

Properties of Sugar

For my at-home lab, I decided to discover both the physical and chemical properties of pure cane sugar.

PHYSICAL PROPERTIES - Properties that can be observed without changing the chemical ID of the substance (cane sugar).


1.  COLOR

After examining the grains of sugar, I determined that they are clear with a slight white tint.  When looked at in a group of many grains, the overall color appears to be plain white.  This is a physical property because I can observe it without changing the chemical ID of the sugar.

2.  SOLUBILITY

I poured some of the sugar into a glass of water.  At first, it just sank and sat at the bottom.  However, after I stirred it, I began to see waves in the water appear and it was clear that the sugar was dissolving.  In conclusion, sugar is soluble because it dissolves when put into a liquid.  It does not change the substance of the sugar, though, so it is a physical property.  It can also be reversed by evaporating the water.

3.  CONDUCTIVITY

To measure the electrical conductivity of the sugar, I borrowed my dad's multimeter/ohm-meter which is able to measure conductivity in a substance.  By doing this, I discovered that sugar is not conductive. This means that it is not a good medium for electricity to use as a path to charge something.  Measuring the sugar's conductivity did not alter its chemical ID, so it is a physical property.

4.  MALLEABILITY

Measuring the malleability was a very simple process.  I tried bending or stretching out the grains of sugar by hammering them and applying pressure to them, but in every case they just broke down into smaller grains or turned into a fine powder.  I can conclude that sugar is brittle and not malleable because it is not able to bend in order to change its shape.  Even though I broke down the sugar into smaller pieces by doing this, the pure substance of the sugar still remained the same - making it a physical property.

5.  TASTE

'Sweet' is the only word I can think of to describe the taste of sugar.  It doesn't have a distinct taste except for sweetness.  The taste of the sugar in its solid form is a physical property because tasting it did not change its chemical ID.


CHEMICAL PROPERTIES- Reactions that a substance can have that alter its chemical ID.

1.  ADDING HEAT

When I heated a small pile of sugar with a blow torch, the sugar began to burn and melt into a dark brown carmel-liquid substance.  When it cooled, it turned almost black and was hardened.  This is a chemical change because it can not be reversed and the chemical makeup of the sugar was altered.

2.  FLAMMABILITY

While I was heating the sugar to discover it could turn into a liquid, I also discovered that it is slightly flammable.  Once the sugar began to liquify, it caught on fire for a brief amount of time.  This means that sugar reacts with oxygen (in the fire) by catching on fire.  The chemical ID was changed by burning the substance - making it a chemical change.

3.  VINEGAR REACTIVITY

I poured some sugar into a cup with vinegar to find out if a chemical reaction would occur.  To my disappointment, nothing really happened except for the sugar slowly dissolving into the solution of vinegar.  No fizzing or bubbling occurred that would indicate a chemical change had occurred.  Sugar is not reactive with vinegar.

4.  REACTION WITH SULFURIC ACID

I could not test this one at home, but I did discover by doing some research that sugar reacts with sulfuric acid.  When mixed with the sulfuric acid, the water is removed from the sugar - so only carbon is left.  This process essentially dehydrates the sugar.  The color also changes to a light yellow and then brown color.  The composition procedes to bubble and fizz.  Finally, the substance expands starts to create and emit smoke.  This is a chemical reaction because the chemical ID of the sugar would be altered.
(http://chemistry.about.com/b/2011/07/23/sulfuric-acid-and-sugar-reaction.htm) .  A video of the reaction can be observed here: http://www.youtube.com/watch?v=_gG0UAX3V7c


5.  REACTION WITH HYDROCHLORIC ACID

To my surprise, my dad happened to have a bottle of hydrochloric acid in the garage to clean electrical connections in his car.  When I poured hydrochloric acid over a small pile of sugar, nothing happened.  The sugar just floated to the bottom of the cup.  I can now conclude that sugar does not react with hydrochloric acid.

WORKS CITED:

Helmenstine, Anne. "Sulfuric Acid and Sugar Reaction." Chemistry - Periodic Table, Chemistry Projects, and Chemistry Homework Help. 23 July 2011. Web. 19 Sept. 2011. <http://chemistry.about.com/b/2011/07/23/sulfuric-acid-and-sugar-reaction.htm>.

"Sulfuric Acid in Sugar - YouTube." YouTube - Broadcast Yourself. Web. 19 Sept. 2011. <http://www.youtube.com/watch?v=_gG0UAX3V7c>.