Chemistry - Chemical Formulas

For words you are unfamiliar with, please reference the Chemistry Glossary related to this material but clicking HERE.

Let's begin by reviewing a chemical formula.

Remember, a chemical formula is like a recipe for a compound. These recipes include the elements that make up the compound and the specific ratios for each element in the compound. Those ratios are indicated by the coefficients (large numbers in front of the chemical symbols) and the subscripts (the little numbers at the "foot" or the lower left of the chemical symbols).

See these demonstrated in the image below:

 

On the left hand side, we see the reactants, as Hydrogen and Oxygen are used to make water. The "red" indicates coefficients - how many molecules we have, which pertains also to the entire compound of water as well.

On the right hand side, we see the product - water. The subscripts in "blue" represent the number of individual element molecules in the recipe. Consider what a recipe is: it is the instructions for making a desired product - change it, even a little, and we might get something else. So, the subscripts never change for any given formula.

Adding up the molecules is important when balancing equations to determine the needed coefficients. 

Notice, the recipe for water is H₂O. There are 2 hydrogen, and just one oxygen. But, why then, is there a coefficient of 2 in the product above? 

Except for in rare occasions, the noble gasses do not exist alone; they are instead paired. Notice the H₂  and the O₂ on the reactants side of the equation. Thus, this reaction must move forward in such a way that 2 molecules of oxygen are consumed. Well, we do not want to get H₂ O₂(hydrogen peroxide), because that is hydrogen peroxide.

Notice, too, that the hydrogen reactant has a coefficient of 2. In order to get the correct ratio for our recipe, we need to balance our equations.

A simple table makes this task much quicker.

2H₂ +  O₂ ---> 2H₂O

 

Reactants			Products
Element	Amount		Element	Amount
H	4		H	4
O	2		O	2

Because the coefficient of 2 in the product applies to the entire water molecule ( H₂O) , there are then 2 oxygen and 4 hydrogen in the product. We must have the 2 oxygen molecules in order to balance that oxygen is a paired gas. Therefore, we have to make sure that there is a coefficient of 2, to designate that 2 paired molecules of hydrogen are necessary for this reaction to progress.

If you need more review, you can look more into balancing equations here with a very informative youtube video.

Chemistry Glossary for Chemical Formulas and Naming

• Chemical composition (or formula) – the elements that make up a material and the specific ratio the those elements combine in to make that particular compound

• Coefficients (the large number before the element or molecule) - indicates how many moles (a mass unit) of a particular product or reactant there are in the compound. This number serves as a multiplier for the entire molecule without changing the actual chemical formula 

• Common name – a name typically in everyday use that does not necessarily indicated the chemicals composition ex: salt

• Monatomic Ions - ions formed from a single atom

• Product (in chemistry) - what is created in the reaction as the reactants are used up

• Reactants (also called reagents) - in chemistry are the substances that are used in a chemical reaction

• Subscripts (small number following the element or molecule) indicate how many molecules of the particular element or molecule it follows that exist within the compound

Science Fair Proposal Blog Links

Click the group number for the link to their Proposal blog.

Chemistry A 2

• Group 1
• Group 2
• Group 3
• Group 4

Chemistry B 2

• Group 1
• Group 2
• Group 3
• Group 4 
• Group 5

Biology B 3

• Group 1
• Group 2

Biology B 4 

• Group 1
• Group 2
• Group 3
• Group 4

Anatomy of the pig

There are 2 really great websites already prepared for instructions on the fetal pig lab.

Because I have a small class, each pair of students actually has their own pig to work with.

This site, from Whitman College, is really comprehensive with pictures and details about each system and the different structures.

This site, by Mr. Dougherty at Penncrest High School, has great use of vocabulary terms, simple instructions, and expandable photos to help students understand the dissection steps. However, in following these steps, you will likely have students lose some structures (the cavity membranes and abdominal muscles) that are carefully preserved in the Whitman Site. I like this one slightly better (despite lacking some details that the Whitman site has), because the whole lab is on one page, you do not have to keep scrolling through to get through each organ system.

ACT Practice Problem 2 Passage (Relating ACT and Science Fair)

Practice Passage 3 at the ACT practice site includes a test that is similar to expectations for this science fair of Applications of Science in Daily Life, even though this set up takes more than our 6 weeks.

Here is the Passage:

Questions
Question Discussion

ACT Practice Problem 2 Questions (Relating ACT and Science Fair)

Below are the Questions for ACT Practice Problem 2 

Question Discussion

ACT Practice Problem 2 Discussion (Relating ACT and Science Fair)

Below are the answers and discussion for ACT Practice Problem 2 Questions  from ACT Practice Problem 2 Passage.

Before you even read these questions, try to think about what is going on. Think about their research question - what are they trying to figure out? Notice, they are looking at a few different variables here.

Basically, they are looking at whether or not these seeds will experience different rates of germination when the seeds are are stored at 5 degrees Celcius for different lengths of time (4, 6, 8, and 10 weeks).
- First variable?  Length of time the seeds were stored at 5 degrees C.

For the second study, researchers stored the seeds for 10 weeks at varying temperatures: 0, 5, 10, 15, and 20 degrees Celcius.
- Second variable? Temperature of seed storage for 10 weeks

The researchers prepared petri dishes for each of the 1- different conditions that prepared seeds at (5 different lengths of time at 5 degrees C, and 5 different temperatures for 10 weeks). After the petri dishes were prepared, 25 seeds from those stored at 5 degrees C for 4 weeks were put into teach of 4 different dishes (experiment 1). One of each dish was placed maintained at a temperature of 13, 18, 23, or 28 degrees Celcius for 30 days. This was repeated for the seeds stored at 5 degrees C for the different time lengths as well as for the seeds stored at the 5 different temperature for 10 weeks. At the end of 30 days, researchers counted how many of the seeds had germinated in each dish.
-Third Variable? - Temperature at which the seeds were maintained for 30 days

So we have our three variables:

1. Length of storing the seeds at 5 degrees C
2. Temperature of storage for 10 weeks
3. Exposing the seeds to different temperatures during germination

So, let's think about the science fair. Why did they pick these temperatures? Does this apply to our life? 

Well certainly! A lot of times, we have extra seeds or we buy them on sale after the garden season is over. There is some confusion as to whether the seeds can be stored in our homes, or if they should be kept in the fridge or other cool place for longer term storage.

Why so many variables? Well, really, they want to know if the temperature and length of storage time of the seeds will hurt their ability to germinate. 

But, why keep the seeds at different temperatures for germination. They are looking for ideal germination temperatures over all, but if they only set their seeds in a temperature of 23 degrees, are they going to know if storing seeds for 4 weeks still produces some sprouts? - Nope. 

Additionally, what of seeds stored for 10 weeks need a higher temperature for successful germination as compared to the seeds stored for 8 weeks? Well, that does almost seemed to be the case. The data from table on show that the 8 week stored seeds germinated better at 18 degrees than 23. And, the 10 week stored seeds had a better germination rate at 23 degrees than the 8 week stored seeds did. See:

Ok, so on to the questions. 

Question 1 give you a key bit of information to narrow your search for the asnwer - look at study one. So then, across all of the seeds, is there one temperature where more seeds germinated? YEP! 18 degrees had 11 more total seeds germinate than the runner up of 23 degrees. So B is the answer.

Question 2 asks you do decide if it is even worth trying to store seeds at 25 degrees C for 10 weeks (slightly warmer than room temperature). Because we are looking at a varied storage temperature, we need to look at the data for study 2. Note that seeds stored for 10 weeks at 15 and 20 degrees C displayed no germination at any temperature. So, we can infer that temperatures of 15 degrees or higher are not good for long term storage. Thus, a higher temperature of 25 would likely yield no germination as well, choice F.

Question 3 Says to look at the seeds kept at 5 degrees in study 2 and describe the trend of seed germination at the different temperatures as the temp increases. For 13 degrees, 16 seeds germinated, as temperature increased, 23 of the 25 seeds germinated. At 23 degrees, 21 of the 25 seeds germinated, and at 28 degrees, just one of the 25 seeds germinated. So, the germination first increased my temp, by decreased with higher temps. This is describe by choice D. 

Question 4 Requires you to compare the two studies to determin which set of seeds were actually stored in teh same conditions. Remember, the control in study 1 is that the seeds were all kept at 5 degrees. Only the length of time varied from 4 - 10 weeks. The second study dealt with a changing variable of temperature from 0-20 degrees, where all seeds were stored for 10 weeks. The only common variables were a temp of 5 degrees and a storage duration of 10 weeks. So, in study one, the seeds kept for 10 weeks were kept at 5 degrees, the same as the seeds kept in 5 degrees for 10 weeks in study 2. Notice too that the germination data are VERY similar in these two cases. The correct answer, then, is choice H. 

Question 5 wants you to look at the data in the question and compare it to the data collected in study 2 to see what the unknown temperature of which the seeds ion the question were stored. 

Notice, the sample size of 100 seeds, thus 25 seed per dish is the SAME as the sample sizes for each temperature in study 2. Pay attention to that detail. Sometimes they might test you further by doubling the sample size or even cutting it in half. What would you do then in those cases? You would need to "normalize" the data, meaning make it all equal. So, if your sample size in study 1 is 100, then you need to make your sample size for the question equal to 100 also through division and multiplication. If they give you a sample size of 200 in the problem, just cut all your numbers in half. If they give you a sample size of 50, then you would double all the numbers in the problem. Then, you will be able to directly compare the data in the study to the data in the problem. 

Back to the problem. The seed germination numbers are 1, 6, 3, and 0. Usually, in since, data are similar if they are only off my a few numbers. Notice the seeds in study 2 that were stored at 10 degrees had rates of 0, 6, 4, 0 - just 1 sprouts different than the data in the question. So, the seeds were most likely stored at a temperature of 10 degrees C.

Question 6 asks you to state how the 2 studies were different. Remember, we mentioned that the fixed variable in study one was the temperature (5 degrees C for all of the seeds), but the length of time the seeds were stored did change. In study 2, all of the seeds were stored for 10 weeks, but they were stored at different temperatures. This question has you answering in the context of what happened in study 2, and there are 2 possible answers to look for either the temperature varied, or the duration of storage was the same. The other variables (germination time and temperature) were the same in both studies. So, choice G is correct.