Have you ever bumped your head or twisted your ankle, and had a nurse put a cold pack on your injury?  You may have noticed he or she did not take the pack out of the freezer—the pack had no ice in it...yet it was still cold!  Or, have you ever been outside on a cold day and used gel hot packs to keep your hands warm?  You may remember breaking a small disk inside the gel, and feeling it get hot to the touch.  Both the cold packs and the hot packs use chemistry to change their temperature! 

When chemical reactions or processes occur, there is always an exchange of energy.  Some of these reactions or processes give off energy as heat; these are called exothermic (‘exo’ meaning outside, ‘thermic’ meaning heat).  Other reactions and processes absorb energy, making the surroundings cooler; these are called endothermic (‘endo’ meaning inside). 

But why are some reactions exothermic while others are endothermic?  Can we predict if a reaction will give off or absorb heat?  As it turns out, we can! 

Chemical Reactions
First, we need to briefly discuss chemical reactions.  A chemical reaction is when one or more chemical compounds are changed into one or more different compounds.  In any chemical reaction, some bonds need to be broken, and others need to be formed—this is how the reaction produces new compounds.  If we know how much energy is required to break the bonds in the reactants (the compounds present before the reaction takes place), and we know how much energy is released on formation of the bonds in the products (the compounds present after the reaction takes place), we can compare them to see how much energy will be produced or consumed by the reaction.  Fortunately for us, there are tables we can use to figure out the energy of the reactants and products. These are called bond energy tables, similar to the one below (1). 

If the formation of the products releases more energy than it took to break the bonds in the reactants, the reaction must give off some of this energy as heat, and so is exothermic.  However, if the formation of the products releases less energy than it took to break the bonds in the reactants, the reaction must take in heat energy from the surroundings, making the reaction endothermic. 

Chemical Processes
The same is true for chemical processes.  A chemical process is what happens when there is a change in the state of one or more chemical compounds (like changing from a liquid to a gas, or dissolving in water), but there is no formation of a new compound.  If we know how much energy the compounds have before they undergo the process (such as melting, or dissolving in water), and how much energy they have after this process, we can discover if the process is endothermic or exothermic.  For example, if we have an ice cube sitting at room temperature, we know the ice cube will begin to melt.  The warmth of the room is melting the ice because the water molecules are absorbing the thermal energy from the air in the room, and this energy is making the molecules move faster and farther away from each other, bringing them from a solid state (ice) to a liquid state (water).  Because this process absorbs energy, it is endothermic.

However, if we put the ice cube back in the freezer, the liquid water will begin to turn back into solid ice.  In this freezing process, the water molecules are giving up thermal energy to their surroundings in the freezer, and are thus losing energy to change states.  This is therefore an exothermic process. 

One type of chemical process that can be either exothermic or endothermic is dissolving of salts in water.  A salt is a compound made up of positively charged ions and negatively charged ions which are held together in a solid state because the positive and negative charges attract one another.  The salt we put on our food is referred to as “table salt”, and is a salt compound made up of sodium ions (Na+) and chloride ions (Cl-). 

If we put salt in water and it fully dissolves (that is, the ions all become evenly dispersed within the water), two exchanges of energy need to happen:

1.       Energy is added to the solution to pull the ions away from each other:  in order to pull the positively and negatively charged ions apart, energy must be added.  This energy needed to pull the ions apart is called the Lattice Energy.

2.       Energy is released into solution when the water molecules surround the ions: as the water molecules are attracted to and surround the ions, energy is released into the solution. This energy released as water molecules surround the ions is called the Hydration Energy.

Whether the dissolving of a salt is exothermic or endothermic depends on which is greater, the Lattice Energy, or the Hydration Energy.  These are usually expressed in units describing the amount of energy released per set amount of salt, such as kilocalories per mole (kcal/mol) or kilojoules per mole (kJ/mol).  We can usually look up the values of the Lattice and Hydration Energy values for a particular salt in tables, such as the one below (2).

For example, if we dissolve table salt in water the Lattice Energy is 779 kJ/mol, and the Hydration Energy is 774 kJ/mol (1).  If we subtract the Hydration Energy from the Lattice Energy, we get a change of +5 kJ/mol:

779 kJ/mol – 774 kJ/mol = +5 kJ/mol

It takes just slightly more energy to separate the ions from one another than is released from the water molecules surrounding the ions.  This means just slightly more energy must be put into the solution than is released back into the solution; therefore dissolving table salt in water is endothermic. 

However, if we dissolve sodium hydroxide (NaOH) in water, it separates into Na+ and OH- ions.  The Lattice Energy for this process is 737 kJ/mol, and the Hydration Energy is 779 kJ/mol.  Subtracting as before, we get a change of -42 kJ/mol.

737 kJ/mol – 779 kJ/mol = -42 kJ/mol

More energy is released into the solution than is required to pull apart the ions; therefore dissolving sodium hydroxide in water is exothermic.  If you dissolve sodium hydroxide in a small amount of water, be careful—the container may get hot enough to burn your hand!

TRY THIS!!

Here’s what you’ll need:

1.       Two small jars or drinking glasses

2.       Two teaspoons

3.       Two cups of distilled water

4.       One half-cup of magnesium sulfate (MgSO4).  You can purchase this online (click here for options).

5.       One half-cup of ammonium chloride (NH4Cl).  You can purchase this online (click here for options).

6.       One thermometer that will measure temperatures from 70-150°F

7.       Safety goggles, one pair for each person participating

8.       Latex or nitrile gloves (you can get these in grocery or hardware stores)

Here’s what you need to do:

NOTE:  Be very careful with the magnesium sulfate and ammonium chloride—they can cause irritation to the skin, lungs and eyes.  Do not breathe them in or get them in your eyes!  You should do this procedure in a well ventilated area, and wear the goggles and gloves to make sure your eyes and skin are protected.

1.       Put on your goggles and gloves!

2.       Pour one cup of distilled water into each of the small jars.

3.       Measure and record the temperature of the water in each jar.

4.       Pour one half cup of the magnesium sulfate into one of the jars.  Stir carefully with a spoon for 20 seconds (don’t worry if not all the magnesium sulfate dissolves).  Measure and record the temperature of the solution.

5.       Pour one half cup of the ammonium chloride into one of the jars.  Stir carefully with a spoon for 20 seconds (don’t worry if not all the magnesium sulfate dissolves).  Measure and record the temperature of the solution.

Did the temperature of the water change each time?  How much did it change?  Did it get hotter or colder?  Are these processes endothermic or exothermic?  Did you observe anything else?  BE SURE TO WRITE EVERYTHING DOWN IN YOUR JOURNAL!


References:

(1)    “Bond Enthalpy/Bond Energy”.  Mr. Kent’s Chemistry Page.  www.kentchemistry.com Accessed 8/28/14.

(2)    “Chapter 13: Solutions”. intro.chem.okstate.edu. Accessed 8/29/14.

We hear the word “normal” all the time, referring to everything from our homes to our health.  We use the word carelessly, loosely referring to anything that seems the same as any other, such as a “normal” house, on a “normal” street, in a “normal” town.  It seems to imply that they are very much like most others...in other words they are very average.  In fact, that is what the word normal means, but there’s a little bit more to it.

In mathematics—or more specifically, statistics—the word normal refers to a type of probability distribution: a way to calculate the chances that a specific event will occur using mathematical equations (click here for more information about probability).  It also describes the way these events look on a graph.  Here is a simple example:  imagine a 10 mile stretch of a very busy road.  All 10 miles of road was constructed at the same time and is therefore the same age, and gets the same wear.  The engineers who built this road know that at some point in the next year, the road will develop a crack. 

For our purposes, we assume that the probability of the crack forming is the same at any point in the road.  Let’s look at what this looks like if we turn this information into a graph, with the probability of a crack forming on the vertical axis and miles on the horizontal axis:

We can see this graph is just a horizontal line, representing the equal probability of the crack forming at any point along the 10 mile stretch of road.  Because the probability is uniform across the whole range of possible values—that is, along the whole stretch of road that we are considering—we call this a uniform distribution. 

The normal distribution is also a probability distribution, but in this case not all of the possible outcomes have equal probability.  In the normal distribution, there is one outcome value which is the average of all the possible values, and this value has the highest probability.  All values higher and lower than this value have lower and lower probabilities the farther they are from the average.  For example, think about the height of men in the United States.  McDowell reports that the average height for men over 20 years old in the US is 5 feet, 9.5 inches (1).  However, we know there are men that are shorter and men that are taller: currently the tallest man in the world is 8 feet, 3 inches tall (2); the shortest man in the world is only 1 foot, 9.5 inches tall (3)!  The probability of anyone being this tall (or this short) is very, very low.  If we could measure the height of 1000 men from all over the US, and we could plot this information on a graph with number of men on the vertical axis and height measured (in feet) on the horizontal axis, it would look like this:

The peak of this curve occurs at a mean (that is, at an average) of 5.79 feet which is 5 feet, 9.5 inches—the average height for men over 20 years old in the US.  This means men from our sample have a high probability of being around this height.  As the heights get taller or shorter, the probability of finding a man in our sample of this height goes down.  This makes the curve look like a bell, which is why it is often called the “bell curve”. So when we say something is “normal”, it really does mean that it has a high probability of being similar to the average!

TRY THIS!

Here’s what you’ll need:

1)      A pen or pencil

2)      A piece of paper

3)      A computer with a spreadsheet program installed, such as Microsoft Excel®

4)      At least 30 individuals of around the same age (50 is even better).  If you are in school now, you can use your classmates!

5)      A tape measure at least 10 feet long.

Here’s what you need to do:

1)       Make a table on your piece of paper with height (in feet) in one column, and number of individuals in another.  In the height column, write numbers ranging from 3-7 in increments of 0.5, as in the following example:

2)      Ask each of the individuals of about the same age how tall they are.  If they are not sure, measure them with the tape measure!

3)      As each individual tells you their height, make a mark in the other column across from the height range they fall into.  Here is some example data:

4)      When you are finished, open your spreadsheet program and make one column with your numbers for how many people were in each height range.

5)      Use these numbers to make a column chart.  In Excel, you can do this by selecting the data you wish to graph, and selecting the insert tab.  Select the column chart under the ‘chart’ options, and choose the type on the top left corner. 

When we use our sample numbers, this is what our graph looks like:

Notice it looks a lot like the graph of the normal distribution we saw earlier. 

What does your chart look like?  If you connect the tops of all the columns, what shape does it look like?  Is the highest point roughly in the middle?  Print out this graph and save it with your notes!

CHALLENGE YOURSELF!

What else do you think might have a normal distribution?  Try measuring the weight of each nail in a box, or count the number of seeds in a bunch of identical seed packages.  Be sure to use at least 30 nails or seed packages, or more if you can; the more you use for your testing (that is, the larger your sample size) the better you will be able to see the probability distribution.  Be sure you keep track of you numbers the same way as before, and use a spreadsheet program to graph all the numbers (weight or number of seeds on the horizontal axis, and number of observations within a certain range on the vertical axis).  This way you can get an idea of what the probability distribution might look like. 

References:

1)      McDowell, Margaret A. et al. (October 22, 2008). "Anthropometric Reference Data for Children and Adults: United States, 2003–2006". National Health Statistics Reports, 10.

2)      “World’s Tallest Man—Living”. Guinness Book of World Records .  Accessed 8/21/14.  Last updated 2/8/2011.

3)      Sheridan, Michael. (February 26, 2012).  “Chandra Bahadur Dangi is world’s shortest living man: Guinness World Records.”  New York Daily News.  www.nydailynews.com.

Think about the last time you washed dishes, cleaned your clothes, or watched a car go through a car wash.  What did you do?  What did you see?  Did you just use water to wash the dishes, or did you add some special liquid to the water?  Did the car ever look like it was covered in foam?  The dishes, the car, and your clothes all need a little more than just water when you clean them; they need detergents! 

A detergent is a substance whose molecules (that is, its smallest building blocks) dissolve in water (see our blog on solubility), and also helps dissolve dirt and stains that would otherwise not dissolve in water.  They can do this because they are amphiphilic (AM-fi-FILL-ick), which means that part of the molecule is hydrophilic, or water loving (likes to dissolve in water), and part of the molecule is hydrophobic, or water hating (does not like to dissolve in water).  Because part of the molecule likes water, you can dissolve the detergent in water.  But because the other part of the molecule doesn’t like water, the detergent would also like to grab onto something else that doesn’t like water.

Let’s look at what happens when you wash the dishes:  When you begin, your dishes are dirty with lots of leftover food particles that may not come off easily.  This is because they may not like water very much:  things like egg yolks, butter, and mayonnaise are more hydrophobic than hydrophilic because they are mostly made of fats, and fats don’t like water! 

If you place these dishes in a sink full of water, the fats will stick to the dishes, and usually won’t come off easily.  They do this because they would rather be stuck to the dishes than float away into the water.

However, if you place the dishes in a sink full of water with added detergent—such as a typical dish washing liquid—the fats will slowly come off the dishes.  The hydrophobic part of the detergent is slowly grabbing onto the fats, while the hydrophilic part still wants to be dissolved in water.  This way, the detergent lifts the fat off the dishes, and allows it to be dissolved in water. 

Finally, remember the car in the car wash covered with foam?  The foam is caused by the detergent; in fact you probably see this type of foam when you wash dishes too.  Detergents foam in this way when they are mixed with air—the hydrophobic part of the molecule would rather touch the air than the water, so the hydrophobic parts of the detergent line themselves up so they all touch the air, keeping them away from water.  This is how the bubbles are formed!

TRY IT!!

Here’s what you’ll need:

1.       2 small drinking glasses, or 2 small glass bowls

2.       2 teaspoons butter

3.       ½ teaspoon dish detergent

4.       Water from a kitchen sink

Here’s what to do:

1.       Measure one teaspoon of butter into each glass.

2.       Smear the butter all around the walls of the glass with your fingers.

3.       Fill one glass up to the rim with water only.

4.       Measure ½ teaspoon of dish detergent into the other glass, then fill it with cool water up to the rim also. 

5.       Allow both glasses to sit on your kitchen counter for 20 minutes

6.       After 20 minutes, pour out the water from each glass, and rinse both glasses gently with cool water.

What do the glasses look like after you rinse them?  Do they still have butter in them?  Do they look the same, or different?  Try to sketch in your notebook what the glasses look like, and make notes about the differences between them.  Be sure to write down what you did!

DETERGENT CHALLENGE!

Try this experiment again, but use several different dish detergents to see which one works best!

·         Be sure to have an identical glass for each different detergent

·         Be sure to use the same amount of butter in each glass

·         Be sure to use the same amount of detergent and water in each glass

·         If you have a scale, record the weight of each glass and butter before and after soaking in water or water and detergent.  Then you can compare detergents by determining how much weight (butter) the glasses lost after rinsing.  LET THEM DRY BEFORE YOU WEIGH THEM THE SECOND TIME!


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Engineering is something we hear a lot about these days.  There are engineers for buildings and bridges, cars and air planes, medicines and computers.  Engineering is one of the oldest professions; we hear about the engineers that built the pyramids of ancient Egypt, and the Pantheon in Rome.  Clearly it is a diverse and important field...but what exactly is engineering?

The American Engineers’ Council for Professional Development (ECPD) defines engineering thus:

“The creative application of scientific principles to design or develop structures, machines, apparatus, or manufacturing processes, or works utilizing them singly or in combination; or to construct or operate the same with full cognizance of their design; or to forecast their behavior under specific operating conditions; all as respects an intended function, economics of operation or safety to life and property”

~Engineers’ Council for Professional Development (1,2)

That’s a pretty complex definition!  What this really means is that engineers design, build, and operate many different things in our world, and try to find ways to make those things better.  Some of these things include buildings, roads, machines, and even processes (like the process for building a car). 

Because there are so many different things an engineer could do, engineering as a field is divided into many different types.  Below are just a few of the different types of engineering. 

Because engineers are always designing and building new and progressive things, and because they are always trying to improve what we already have, they often face great challenges.  In fact, the National Science Foundation announced in 2008 what are considered the 14 greatest engineering challenges for the 21st century (3).  One common thread between many of these challenges is finding the right resources and tools; some of the tools needed probably haven’t been invented yet, but others are simply too expensive or too rare to be economical.  For instance, solar energy has great potential to solve much of the world’s energy needs, but many of the materials needed to build the cells that capture solar energy are hard to come by (3). Because getting the right materials for a job is sometimes challenging, engineers frequently must build with what they have available.

CHALLENGE: BUILD A BOAT OUT OF PAPER!

Engineers use not only scientific information to design things, but also their practical knowledge and understanding of the society they work in.  So, what do we already know a boat needs?

1. It needs to be waterproof, so they don’t leak.
2. It needs to float.  This sounds simple, but it means the boat must displace (push away) an amount of water equal to its weight (please visit our blog on buoyancy for more information!).  This is what makes the shape of a boat so important. 
3. It may need to transport cargo.  This could be people, or objects, or both!  This means they must be durable, and they must displace even more water when something else is placed in them. 

Most boats are made of aluminum; some very large boats, like huge cruise ships, are made of steel.  But if you put a solid piece of aluminum or steel in water, it will sink!  So how does a boat float?  Think about the shape of a typical boat.  What do you notice?  It is this shape that allows the boat to displace its weight in water, so it can stay afloat.

Here’s what you need:

1.       Paper!  Any kind of paper will do—newspaper, magazines, scratch paper from a copier; just make sure that no one needs the information on the paper!  Getting some from a recycling bin would be best!

2.       Other things to build your boat with.  Be creative!  Think about what your boat needs to have in order to float and not leak.  Here are some suggestions as to what else you might want to use:

            a.      Waterproof tape.

            b.      Glue

            c.      Thin wire or flexible sticks

3.       Some small object that is heavy for its size, like a small rock.

4.       A bathtub or sink full of water.

Here’s what to do:

1. Plan!  Write down how you plan to build your boat using the materials provided.  ONE OF THE MATERIALS YOU USE MUST BE A PAPER PRODUCT!
2. Assemble your boat . 

   3.       Put your boat in the bathtub or sink full of water for 20 minutes, with the heavy object inside.  The boat should be able to stay afloat for 20 minutes without any water getting in.

What happened?  Did your boat float?  Did it keep water out for the whole 20 minutes?  Could it hold the heavy object?  Do you think you could improve your design?  Write down what happened and how you might improve your boat in your journal.  Try building your boat again, and see if you can make those improvements!  See if you can make your boat float longer!

For further reading, check out these sites!

Ancient Egyptian Engineers: http://www.history.com/images/media/pdf/engineering_empire_egypt_study_guide.pdf

Ancient Roman Engineers:  http://www.historylearningsite.co.uk/roman_engineering.htm

Types of Engineering:   http://www.aboriginalaccess.ca/adults/types-of-engineering


References:

1)      Engineers' Council for Professional Development. (1947). Canons of ethics for engineers

2)      Engineers' Council for Professional Development definition on Encyclopædia Britannica (Includes Britannica article on Engineering)

3)      Cooney, Michael (2008).  “What are the 14 Greatest Engineering Challenges for the 21st Century?”, Network World, www.networkworld.com.

4)      Maehlum, Mathias Aarre (2014). “Solar Energy Pros and Cons.”, Energy Informative, www.energyinformative.org.


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We’ve discussed refrigeration and canning as ways of preserving our food from spoiling.  We now know that refrigeration works by keeping food cold, thus slowing down the growth of bacteria and other microbes.  We also know that when we can food we heat it first, thus killing microbes and preventing new ones from entering the food.  However, we know there are foods that are not refrigerated, and not canned either, yet they still seem to remain safe to eat for long periods without spoiling.  Some examples are dried foods like fruits and jerky, cereals, and seasonings.  Although these foods may not taste as good after an extended time, they are still usually safe to eat.  Why don’t these foods spoil?

To understand why these foods never seem to develop bacterial or mold growth, we must ask what it is that these microorganisms need to grow.  If microorganisms (also called microbes) are present on our food, they will need four basic things to cause food spoilage:

1.       Food—the sugars and proteins in our food provide food for the microbes to multiply

2.       Warmth—refrigerator temperatures (20°F) are cold enough to slow bacteria down considerably

3.       Time—microbes require some time to grow, however under ideal conditions some can produce a new generation (that is, they can be multiplied by two) every 20 minutes!

4.       Water—most food has some water in it, which allows bacteria to digest the food

If any one of these things is not available, microbes will not grow, or at least will grow very slowly.  Anything that prevents the microbes from getting these basic needs will prevent them from growing, and will thus preserve our food. 

Water is extremely important to microorganisms, as it is to all life!  Water makes up an average of 70% of all living things, and it has many necessary roles.  Water makes up most of the interior fluid of the microorganisms, their food is dissolved in water, and all the processes that take place inside them to keep them alive must have water to happen.  Without water, all these processes would grind to a halt, and the organism would die. 

While canning works by removing the microbes themselves from the food and refrigeration works by removing warmth, other preservation methods work by removing water.  Some foods by themselves are dry enough to last in our pantry without needing to be canned or put in the refrigerator; dried pastas are a good example, as are some ingredients like flour and cornmeal (provided they stay dry).  However other foods need help—this is where dehydration comes in!

When it comes to dehydrating foods, there are essentially two methods:

1.       Air drying

2.       Freeze drying

Let’s start with air drying.  If you have a food dehydrator at home, you already have a device that will air dry food!  These machines are very readily available in department and kitchen stores, and they work by using a heat source and fan to blow warm air over whatever food is inside.  This air flow removes moisture, leaving a dry piece of food that will resist microbial growth.  This process may also be done in an oven, or by simply putting food out in the sun. 

Next, freeze drying.  If you have ever tried “astronaut food”, you’ve had freeze dried food!  Freeze drying works by taking food that is frozen at a very low temperature (around -40°F!) and placing it in a vacuum while maintaining its frozen state (for more information about vacuums, see our blog on the subject).  This causes the water molecules to go directly from their frozen state to the gas state, a process called sublimation.  What is left is a dry, airy food product that also resists microbial growth.  This works well on foods that have lots of liquid in them, like ice cream or fruit (astronaut ice cream is delicious, and can be purchased on astronauticecreamshop.com).

While removing water from food does prevent microbial growth, many dehydrated foods have a preservative added to them also to ensure that no bacteria can grow in any water that may remain, and sometimes to protect the food’s flavor.  Preservatives commonly added to dried foods include antioxidants (such as ascorbic acid and tocopherols) (Source: Bhat), various salts, and sugar, as well as some synthetic compounds. 

Antioxidants work by preventing loss of electrons to oxidizing agents, which can cause fruits to discolor and fats in meat to turn rancid (for more information, please see our lesson on antioxidants).  As an example, let’s look at what happens when the flesh of an apple starts to oxidize, and then what happens when we add an antioxidant—ascorbic acid from lemon juice:

Both salt and sugar work by pulling water out of the microbes.  This happens through a process called osmosis.  During osmosis, water flows from areas with fewer solutes (salts or sugar) to areas with more solute through the outer membrane surrounding the microbe, causing them to shrivel and die.  Traditional sugar plums are soaked in sugar water before drying for this reason, and it made the fruit sweeter! 

TRY PRESERVING SOME FRUIT THROUGH DRYING!

Here’s what you’ll need:

1.       Two cups of a fruit of your choice.  Small fruits like cherries should be cut in half and the pit removed.  Fruit larger than ½ inch in diameter should be sliced evenly into about ¼ inch slices. 

2.       One large baking sheet

3.       One large cooling rack

4.       An oven set to the lowest possible temperature, 130°F to 200°F

5.       One extra piece of fruit, the same kind as you dehydrate

6.       Two small, sealable containers

Here’s what to do:

1.       Place the cooling rack on top of the baking sheet.

2.       Spread the fruit on the cooling rack in a single layer.  Don’t let any of the pieces touch each other.

3.       Place the baking sheet with rack in the preheated oven

4.       Allow the fruit to dry for at least 6 hours, or until the fruit feels like soft leather

Store this fruit at room temperature for five days in an open container to allow any excess moisture to evaporate, stirring it every day.  Then cover the container and store for up to 10 months!

To demonstrate the ability of this dried fruit to withstand spoilage:

1.       Take one piece of fruit you dried, and place it in one of your sealable containers. 

2.       Take one ¼ inch slice of the same kind of fruit that has not been dried, and seal it in the other container

3.       Allow these containers to sit at room temperature for one to two weeks, observing them daily for changes and microbial growth.  Make sure they are kept in the same place to ensure they both experience the same conditions.

What did you observe?  How long did it take for you to see changes in the fresh fruit?  Were there any changes in the dried fruit?  Be sure to write down all your observations!

References for further reading:

Bhat, Rajeev; Alias, Abd Karim; Paliyath, Gopinadham (2011). Progress in Food Preservation. Retrieved from http://www.eblib.com

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