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Showing posts with label Chemistry. Show all posts
Showing posts with label Chemistry. Show all posts

April 6, 2019

SAT Chemistry Subject Test: The Best Prep Books

Update: I've added a section addressing True/False/CE questions.

As a credentialed teacher with an M.S. in chemistry, I've noticed that some SAT Chemistry study guides are great, some are so-so, and some must be taken deep into Mordor and cast back into the fiery chasm from whence they came.

Let's start with the great ones:

The Official SAT Subject Test in Chemistry Study Guide

If you can only afford one book, get this one. It has two official practice tests and answer explanations.

Neither of these tests is a copy of the one in The Official Study Guide for ALL SAT Subject Tests, so you should get both books if you can.

Pros
Official material is a true confidence builder. Every question you get wrong contains skills you need to practice.

Most prep books have poorly written questions, answer key errors, and questions that are unrealistically easy, difficult, or off-topic. If you get questions wrong or run out of time on unofficial tests, you'll have trouble figuring out whether the fault lies with you or with the book you're using.

Based on the raw-to-scaled score conversion tables in the book, raw scores of 80/85 and 76/85 will get you perfect 800's on the first and second practice tests,

Cons
There's no Kindle edition, so you'll have to plan ahead and order a physical copy from Amazon.


The Official Study Guide for ALL SAT Subject Tests

This book has an official chemistry practice test that isn't the same as the two in the dedicated chemistry guide (above).

Pros
If you're going to take several Subject Tests, you need this book anyway.

Cons
The test questions in this book are easier than the ones in the dedicated chemistry guide, and the curve reflects that difference: to get a perfect 800, you need a relatively high raw score of 82/85.


Strategy for True/False/CE Questions
  1. If either the first part or second part of the question is false, don't bubble in CE. For example, neither "All elephants have four legs BECAUSE elephants use their legs to eat peanuts" nor "All elephants have five legs BECAUSE elephants use their legs to walk" deserves the CE mark.
     
  2. Mark "CE" if the second part of the sentence is a good reason to believe that the first part is true. For example, "The back side of the moon never faces the Earth BECAUSE scientists have never observed the back side from the Earth's surface" should be marked CE. Strictly speaking, this is not the correct way to use the word because, but it will get you the right answer on SAT Chemistry tests.


Cracking the SAT Chemistry Subject Test

This is a good all-around study guide. It contains content review, useful strategies, and decent practice tests.

Pros
The first two practice tests are very similar to real College Board tests, and there are no answer key errors. The third one, however, contains a few poorly written questions (#104, 37, 38, 56, and 62). You may want to use that one for untimed practice.

The book's helpful content review chapters can keep you from feeling lost. The Chemistry Subject Test covers a broader range of topics than you're likely to learn in your high school class, so content review is a must.

Cons
You'll need a calculator to do some of the practice questions in the content review chapters. You're not allowed a calculator on the actual Subject Test, though, and the full practice tests included in the book are very doable using mental math.

The Princeton Review is all about giving you what you need and not one iota more. Since this book is meant for the Subject Test, you'll need to get an additional study guide if you're planning on taking the AP test, which goes into greater depth and has some additional topics you need to know, such as laboratory chemistry and reaction kinetics.


For the Love of SAT Chemistry (Chris Reddick and Michael Cerro)

This book is geared at about the same level as The Princeton Review's, but it focuses less on textbook-type content review and more on practice problems and answer explanations.

It's an excellent place to start if you like inductive learning. If you prefer to review content in an organized way before starting practice questions, go with The Princeton Review's book.

Pros
The material, including the four practice tests at the back of the book, closely mimics the content and feel of real College Board questions. The answer key is accurate.

Cons
After grading each practice test, you'll be left with a raw score (out of 85 total points) without any way to convert that into a scaled score (out of 800). The scoring instructions and conversion table are waaaaaay back on pages 9-10. Follow the directions carefully: you need to remember to deduct 1/4 of a point for each answer that's incorrect!



SAT Chemistry Subject Test Problems (Christopher Bozza and Dr. Steve Warner)

This bank of practice questions has the best answer explanations I've seen in any chemistry book. The questions target exactly what's on the Subject Test, and the answer explanations are about two pages long per question.

Pros
This book has the same format as Dr. Warner's SAT and ACT Math books. You can jump right in and start working without having to wade through preliminary reading.

The practice material is very similar to real SAT Chemistry Subject Tests.

The problems in this book are arranged by topic and difficulty level, so students who don't need any content review can jump straight to the chapters that contain what they want to work on.

Cons
Most of the content review is in the answer explanations, so you can't treat this book like a textbook. You really have to engage with the material to receive the maximum benefit.

Errata
#56 on page 81 is worded in an unclear way (and therefore not answerable).

The answer to #110 on page 153 is (B), not (A). The book's answer key is mistaken!

#151 on page 205 is unrealistically difficult. Although you'll need to know how to do unit conversions for SAT Chemistry, you won't have to convert between amperes, coulombs, and moles.

#39 on page 225 has two correct answers: (C) and (E).

#68 on page 233 expects students to (1) figure out that lanthanum has a larger radius than potassium, and (2) go against their intuition that potassium should actually be more reactive, since it's an akali metal that reacts violently with water. Those expectations go against students' experience with metals' reactivities and would not show up on a real test. #71 (below) has a similar problem.

The answer to #71 on page 234 is (B), not (E). Potassium is more metallic than barium based on its Mohs hardness and its reactivity with water.

#83 on page 238 is unrealistically difficult: vapor pressure is related to boiling point, since a liquid boils when its vapor pressure becomes equal to atmospheric pressure. Students shouldn't be expected to know which of the liquids in the list has the highest boiling point (and therefore the lowest vapor pressure). The book's answer is wrong anyway: bromine, octane, and nitrogen trichloride all have boiling points that are higher than 100°C and therefore have lower vapor pressures than water does.

#88 on page 240 is also unrealistically hard. The correct answer should be (C), not (D), since Mg(OH)2 is not a strong base due to its poor solubility in water (0.00064 g/100 mL at room temperature).

#94 on page 241 has two correct answers: (A) and (C).

#125 on page 250 is problematic because the nitrogen atoms in the NH2 groups have lone pairs that can be delocalized into the benzene-like rings through resonance. Those nitrogen atoms are likely to be either sp2-hybridized or somwhere between sp2 and sp3. For this reason, (B) is a better answer than (D).

The second sentence of #130 on page 251 should read "saturated hydrocarbon," not "unsaturated hydrocarbon."

The answer to #136 on page 253 is (B), not (D).

#145 on page 255 has two correct answers, (A) and (B). RbCl and RbF are both soluble in water, while PbO and PbS are insoluble. The soluble salts will produce equal numbers of ions, causing the light bulb to glow with equal intensity, while the insoluble salts will produce negligible concentrations of ions, making the light bulb very dim.

#149 on page 256 is slightly questionable: HI is larger molecule but should also be less polar than HCl, so strictly speaking, students would have to look up the boiling points of both compounds to know the answer. HCl does have a lower boiling point, so it has a higher vapor pressure. (Recall that something boils when its vapor pressure becomes equal to atmospheric pressure, so high-vapor-pressure compounds boil first.)

#159 on page 260 should say, "Absorption of a photon CAN [but doesn't have to] cause electrons to become excited to a higher energy level." Photon absorption can also result in a change in electron spin (radio waves), molecular vibrational states (infrared), or molecular rotational states (microwaves), so the problem needs to be clear that photon absorption can lead to consequences other than just electrons becoming excited (visible and ultraviolet radiation).


Sterling Test Prep SAT Chemistry Practice Questions: High Yield SAT Chemistry Questions with Detailed Explanations 

This is a huge bank of practice questions. It's useful if you're already scoring 800 and want to challenge yourself some more.

Pros
Sterling highlights the trickiest topics on SAT Chemistry, including amphoteric compounds, flame test colors, solubility rules, and unusual Lewis structures. If you like hard questions, this is the book to get.

Cons
Despite the claim on the book's cover, most of the questions don't have answer explanations.

Since the questions are organized by topic, you have to work on one chapter at a time. There aren't any timed practice tests.

The book covers some topics that are so hard I doubt they'd ever show up on the Subject Test. For example, it expects you to know the exact role of each of the four quantum numbers. You also have to calculate a dipole moment given the size and distance of two separated charges. (!)

Don't use Sterling until your foundation is very solid. Be willing to Google the explanations for topics you don't understand.

Books to Avoid

I'm not sure how Kaplan's SAT Chemistry (2013-14 edition) got its four-star Amazon reviews. The practice questions in the content review chapters are very calculator-based, and the content review includes some difficult AP-only topics, such as zero, first, and second order reaction kinetics and complicated redox reaction balancing involving H+, OH-, and H2O.

The diagnostic test isn't any better. Out of 85 total questions, two are AP-level rate law questions (#107 and #37), one is an AP-level diffusion rate question (#44), and seven are written in a way that could legitimately make you think there's something wrong with the answer choices (#9, 30, 31, 58-60, and 64). In addition, some of the diagnostic questions are hard to do without a calculator. #69, for example, makes you do a proportion involving the ratio 2/7. (2/7 is about 0.29, in case you're wondering.) Official tests stick to easy fractions like 5/2 or 88/44.

Barron's SAT Chemistry (2009 edition) is even worse: out of 85 questions, the diagnostic test has ten unrealistically tricky questions (#4, 9, 113, 34, 38, 45, 49, 57, 67, and 70) and eight unclearly worded questions (#14, 17, 106, 107, 43, 46, 52, and 56). The 2016 edition fixes questions 4, 17, 113, 45, 52, 56, and 67, but questions 9, 14, 106, 107, 34, 38, 43, 46, 49, 57, and 70 remain problematic.

If you're planning to take the AP test, know that McGraw-Hilll's 5 Steps to a 5 on AP Chemistry (2017 edition) is also really bad. There were so many incorrectly drawn diagrams and poorly written questions on Practice Test #1 alone that I had to quit before I got to the free-response section. I know this book has a four-star Amazon review average, but pay close attention to the negative reviews!


Suggested Study Schedule

Unlike Math Level 2, SAT Chemistry doesn't have many quality prep books. I suggest following the study schedule below.
  1. Take the first Princeton Review practice test and read the answer explanations.
  2. Go through all of the content review in the Princeton Review's Cracking the SAT: Chemistry. Alternatively, you can go through the first thirteen chapters of For the Love of SAT Chemistry.
  3. Read the articles on my Web site about flame test colorssolubility rules, saturation/unsaturation, and avoiding small calculation mistakes.
  4. Take the second Princeton Review practice test and read the answer explanations.
  5. Go through Dr. Warner's SAT Chemistry practice book. Read the answer explanations carefully.
  6. Borrow an AP Chemistry textbook and review your weakest topics. Keep reviewing and re-taking the original two practice tests until your score is 800.
  7. Take the official practice test in the Official Study Guide for ALL SAT Subject Tests. Go over the test with a fine-toothed comb. Make sure you understand every question so well that you could stand up and teach it in a classroom.
  8. Do the same for the two official practice tests in The Official SAT Subject Test in Chemistry Study Guide. At this point, you should be scoring solid 800's.
  9. If you need more practice tests, use the four at the back of For the Love of SAT Chemistry.
  10. The week before the real test, re-take one practice test a day. Your goal at this point is to increase confidence, not to learn new material. You should receive an 800 on each of the five re-takes. If you don't, you have a good idea of what to review.

Going for a Perfect Score

A raw score of 80/85 will usually get you a perfect scaled 800 on SAT Chemistry. Even after the test deducts a quarter of a point for every question you get wrong, you can afford to miss four of the eighty-five problems. That's like getting a 95% on a comprehensive high school chemistry final.

The books above contain everything you need to get an awesome score, but if you'd like personalized help, you can sign up for in-home or online tutoring.

February 21, 2019

AP Chemistry Practice Tests

Update: I've added a link to the 2018 free-response questions.

As you may already know, the AP Chemistry test changed in 2014. It's now more like a college final exam than a high school test, and you can't study for it as you'd prepare for SAT Chemistry, which largely tests memorization.

The questions have become more conceptual, and there's a large focus on lab chemistry. They remind me of the SAT's Critical Reading section: if you're not extremely careful, you'll misread the something without realizing it.

These changes aren't fully reflected in the prep books, not even the 2017 Princeton Review book I recently looked at. Of the students who took the AP exam, only 12.6% received a 5 in 2018, as opposed to 18.2% in 2013.

The good news is that you can train yourself to be one of the top 9% who gets a perfect score. I recommend taking released exams and free-response questions two months before the AP test. Prepare a list of questions you'd like to review in each tutoring session.

If you need more practice material, do the hardest problems at the end of every chapter of your AP Chemistry textbook. Treat them like free-response questions: write out a paragraph-long explanation for explaining how you arrived at each answer. Since your book won't have free-response-style answers in the back, ask your tutor to check your explanations for completeness.

Practice Tests

Here are some official practice questions. Start at page 117 of the booklet, which is PDF page 126.
AP Chemistry sample multiple-choice questions and answers

The College Board has also released the free-response questions from actual AP exams:
2014 Free-response questions and answers
2015 Free-response questions and answers
2016 Free-response questions and answers
2017 Free-response questions and answers
2018 Free-response questions and answers

AP Chemistry is my favorite subject to tutor. If I didn't like the chemistry, I wouldn't have bothered to get two degrees in it! Contact me if you'd like to schedule a session.


September 8, 2018

Chemistry: Solubility Rules in Five Words

We're back today with the easiest way ever to remember the solubility rules. This three-minute video will save you hours of time with flash cards.



Here's a summary of the mnemonic from the video.

Always Soluble: NAG SAG (Exceptions: PMS, Castro Bear)

Always Soluble:
  • NAG (Nitrates, Acetates, Group 1 alkali metal ions)
  • SAG (Sulfates, Ammonium ion, and Group 17 halide ions)
Exceptions:
  • PMS (Pb2+, Mercury(I) ion, and Silver ions are insoluble when combined with sulfate and group 17 anions.)
  • Castro Bear (Ca2+, Sr2+, and Ba2+ are insoluble when combined with the sulfate anion.)
  • Note that Castro Bear is still soluble when combined with Group 17 anions. For example, ice melt (calcium chloride) works because it's highly soluble in water, its dissolution is exothermic, and it lowers water's melting point. (Melting/freezing point depression is directly proportional to the number of particles dissolved in a given volume of water. CaCl2 contains three particles per mole and is very soluble, so it lowers water's melting point a lot.)
No one knew where calcium chloride was at the hardware store. I actually had to ask for ice melt. Imagine that!
If you still want to use flash cards, check out the NAG SAG Quizlet.

Fun facts
Aqua regia, a mixture of nitric and hydrochloric acids, can actually dissolve gold. The nitric acid oxidizes the gold to Au3+, which then pairs with chloride ions from HCl. Since gold(III) chloride is soluble, the oxidized gold moves into solution, exposing more gold metal to be oxidized. A Jewish scientist used aqua regia to hide two solid gold Nobel Prize medallions from the Nazis.

P.S. Did you know that Fidel Castro actually owned a bear?
“Fidel Castro with a bear cub Baikal that was given to him by Siberian geologists. The bear went with his new master to Cuba but, unfortunately, could not get accustomed to the local tropic climate.”

August 23, 2018

Stanford Thesis: The Relevance of the Composition of Earth’s Early Atmosphere for Current Origin of Life Theories

A student requested that I post my Stanford master's thesis online. Enjoy!

The Relevance of the Composition of Earth’s Early Atmosphere for Current Origin of Life Theories

INTRODUCTION

Scientific research is able to proceed most quickly when the practitioners in a given field share the same paradigm: a common set of definitions, theories, and model experiments.  The distinction between observation and inference is not a sharp one because experimental results are interpreted through the paradigm, and such interpretations are usually consistent with the paradigm’s assumptions.

During periods of normal science, experimental results are easily made to fit the paradigm model, and research is fruitful because little effort is expended on the fundamental questions with which almost all of the workers are in agreement.

However, normal science is regularly punctuated by periods of crisis in which it is difficult to interpret a field’s experimental results in a way that is consistent with the paradigm.  During these times, the paradigm will be modified by various workers to attempt to explain anomalous data.  Different versions of the paradigm can become so numerous that the paradigm ceases to function as such, giving way to different schools of thought that each advocates its particular modifications.

The period of crisis continues until a satisfactory way is found to explain the anomalous results by one of the modified versions of the existing paradigm or by shifting to another paradigm altogether.  When a large number of practitioners has accepted the solution, normal science can resume because the fundamental assumptions are no longer being debated.

The origin of life field may currently be in a state of crisis. The rapid proliferation of competing theories combined with recent changes in our understanding of the atmosphere of the early earth have generated different, incompatible models of how life originated on the early earth. A brief review of these models is followed by a discussion of their dependence on our knowledge of earth’s early atmosphere.

Download the rest...

This is the Miller-Urey experiment that shows up in every high school and college biology textbook.
Is it an accurate model of the conditions that existed early in the history of our planet?

December 16, 2017

How to Avoid Small Calculation Mistakes in Chemistry and Physics

One of the best ways to avoid small mistakes is to include units when you plug your numbers in and to carry those units all the way through without skipping any steps.

Take a look at the problem below for an example. Here, we're given the kinetic energy and mass of an object and need to solve for its velocity:




I wrote all of the units into the solution, didn't skip any steps, and plugged my answer (including the units) back into the original equation to make sure the answer was right. I converted joules into simpler units (kg * m2 / s2) in order to make the units canceled and caught a potential mistake where I might have forgotten to convert grams to kilograms.

If you do this consistently, most of your small mistakes should go away on their own. Showing your work takes discipline, but it's a lot better than having to solve problems over and over again on homework, or, worse, get them wrong on tests without realizing it.

Chemistry problems in upper-division college classes involve calculus and statistics and are much harder than the one I just solved. Ironically, as a problem gets harder, it's more important to show every step of the solution, as a single mistake causes you to have to do the entire problem over again.

If you want me to write up a solution for a problem you're working on right now, please contact me. I'd be happy to do one for you for free!

October 23, 2017

Chemistry: What Does the Word 'Saturated' Mean?

In chemistry, the word saturated can be used to describe individual molecules or entire solute/solvent mixtures, and it has a different meaning in each of those situations.

Generally speaking, something that's saturated is completely full, as in a saturated sponge (one that's filled with water) or saturation bombing (using hundreds of bombs to destroy an entire area).

Saturated and Unsaturated Molecules

When the word is applied to an individual molecule, it means that the molecule doesn't have any carbon-carbon double bonds or rings. In the molecules below, notice how changing a single bond to a double bond means that you have to remove two hydrogen atoms:


Because carbon can only have four bonds, adding C=C double bond uses up two places that hydrogens could have been bonded two instead.

The first molecule has as many hydrogen atoms as it can hold (no double bonds), so it's saturated, or full of hydrogens. Because the second molecule has a double bond, it doesn't have all the hydrogens it could potentially hold, and it's unsaturated.

Important: Fatty acids are considered saturated if the hydrocarbon chain part of the molecule contains no double bonds. The C=O in the COOH group at the left end of the molecule isn't considered.

Note that the presence of a ring will also make a molecule unsaturated. Straight-chain and branched pentane both have the formula C5H12, while the ring-shaped molecule cyclopentane has the formula C5H10:


Forming the ring shape involves joining the two carbons at the ends of the chain, using up two places to bond where hydrogen atoms would have been. Straight-chain and branched-chain hydrocarbons are saturated, while ring-shaped hydrocarbons are unsaturated.

By the way, there's no such thing as a supersaturated molecule. If you see that option on a test, you're looking at a trick question.

Saturated, Unsaturated, and Supersaturated Solutions

The word saturated has a different meaning when it's used to describe solutions.

For the definitions below, remember that the solvent is the chemical that the solute dissolves in. There's always more solvent than solute.

For example, salt water is a solution of sodium chloride (the solute) in water (the solvent).

The solvent is usually a liquid, but it doesn't have to be: air is a solution of oxygen, carbon dioxide, and water vapor (solutes) in nitrogen (the solvent). Steel is a solution of carbon (the solute) in iron (the solvent).

Saturated solutions contain as much solute as the solvent can hold.

The Dead Sea is saturated: it contains all the salt and minerals it can hold. It has so much salt that the water is more dense than the human body, and people can float easily in it. In the picture to the right, the white solid bits are the pieces of salt that aren't able to dissolve because the water is already saturated.

Unsaturated solutions contain less solute than the solvent can hold.

Unlike the Dead Sea, normal ocean water is unsaturated. Even though it's really salty, it doesn't contain as much salt as it can hold.

Supersaturated solutions contain more solute than the solvent is supposed to be able to hold.

You can make rock candy by boiling sugar water and then cooling it, leading to supersaturation: the sugar is supposed to come out of solution, but it won’t do so until you drop a seed crystal in. (The lack of a seed crystal is also one of the reasons that trees don't freeze in the winter.)

The video below shows how you can make your own supersaturated solution of sodium acetate and use it to make sculptures of "hot ice":



You'll also need to know the terms below:

Dilute solutions contain very little solute compared to the amount of solvent.

Concentrated solutions contain a lot of solute relative to the amount of solvent.

Trick Questions

A common trick question involves asking you to make the correct distinctions between the terms saturated, unsaturated, supersatured, dilute, and concentrated. 

A solution can be both saturated and dilute: When solubility is low, such as when iodine crystals are added to lake water to kill germs, the solution is saturated (full of solute) and dilute (not very concentrated) at the same time. It's a good thing that Iis nonpolar and not very water-soluble: if the solution were concentrated, you'd be drinking large quantities of an elemental halogen!

Ocean water, on the other hand, is both unsaturated and concentrated.

The Dead Sea is both saturated and concentrated.

Comment below if you have any questions!

March 28, 2017

Chemistry: Flame Tests in Seven Words


I've scoured the Internet for an easy way to memorize flame test colors (the ones that show up when you place a metal salt over a Bunsen burner).

The best mnemonic came from Reddit. I've added a couple of elements to the original list.
Li (lithium) + C(rimson) = Lick
Na (sodium) + Y(ellow) = Nay
K (potassium) + L(ilac) = Kill
Ca (calcium) + R(ed) = Car
Ba (barium) + G(reen) = Bag
Cu (copper) + B(lue) = Cub
Sr (strontium) + R(ed) = ShreddeR
The pictures below will help sear the mnemonic into your memory. (Bunsen burners are available upon request.)


Li (lithium) + C(rimson) = Lick
Crimson is a deep, purplish red. Think of the Alabama Crimson Tide.
Crimson is Harvard University's school color, and The Harvard Crimson is the name of its daily newspaper.

Na (sodium) + Y(ellow) = Nay
Street lights contain sodium vapor: that's what makes them yellow. Nay, not white. Yellow!

K (potassium) + L(ilac) = Kill
Lilac is a pale pinkish-violet color.
The more you cut lilac bushes, the faster they grow. They're definitely hard to Kill.

Ca (calcium) + R(ed) = Car
This Car will get you pulled over in style.

Ba (barium) + G(reen) = Bag
Some Bags are "green" because they claim to be environmentally friendly.

Cu (copper) + B(lue) = Cub
Isn't that bear CuCute?
Copper sulfate crystals are blue.

Sr (strontium) + R(ed) = Shredder
Pretend his arm blades are made from elemental strontium, which has an amazing Mohs hardness of 1.5.

Watch the video above to see what the colors look like in real life.

Then take the flame test quiz to apply what you've learned!

I have only one question left: 
Which of the objects in my pictures would you most like to set on fire?