Tag Archives: Temperature

Does Salt Water Boil Faster?

It seems like there’s an old wives’ tale for everything — from cleaning to maintaining your health, there’s always some trick to doing things better. The same goes for cooking, too! One of the classic examples is to add salt to your water in order to get it to boil faster. But is this actually true?

salt water cooking

Adding salt to water is going to do two things to water’s physical properties: it will raise the boiling point and it will lower the specific heat. These two changes actually work against each other. Raising the boiling point will make the water boil slower. We’ll need to get it to a higher temperature, which may mean a longer time on the stove. But lowering the water’s specific heat — AKA, the amount of energy needed to change an object’s temperature — will cause the salt water to heat up faster! So which effect is stronger?

salt water vs boiling

The answer isn’t an easy one. It will depend on how much salt you put into your water. In our experiment in the video above, a decent amount of salt raises the boiling point a significant amount, but does not speed up the process by much, if at all. But if we wanted to, we could speed it up more by adding more salt.However, if we look at this from a practical standpoint, you don’t actually want that much salt while you’re cooking. A little bit will go a long way to making your food taste good, but too much will ruin your dish.

salt water boiling pt

So it’s possible that the wives’ tale is correct about your food cooking faster, but perhaps for just the wrong reasons. If you’re making pasta, or a stew, or something that requires a pot of boiling water, a higher boiling point will actually mean your water is hotter and will cook the food quicker. It may take longer to get to the boiling point, but once you’re there, the cooking time is cut down considerably.

Written By: Scott Yarbrough

 

Thermochromic Slime Experiment

We’ve all experienced a mood ring change color when you put it on your finger. But have you ever seen slime do that? Thermochromism is the property of substances to change color due to a change in temperature. Here is a DIY to make your own thermochromic slime.

What you need:

  • ¼ cup white glue
  • 1 tablespoon water
  • 3 teaspoons thermochromic pigment
  • ¼ cup liquid starch
  • Food coloring

Mix all of the ingredients together and there you have it! (If it is very sticky, add more starch until it doesn’t stick anymore.) You just made your very own heat sensitive color changing slime. But how does it work?

experiment diy

 

There are two types of thermochromic materials. The first is liquid crystals. The temperature change causes a movement in the crystals which changes their spacing. The change in spacing causes light in the crystal to refract at different wavelengths than before. Refraction is the change in direction of propagation of any wave as a result of its traveling at different speeds at different points along the wave front.

The second type of thermochromic material is called a Leuco dye. A temperature change causes the dyes to change their molecular structures. A change in molecular structure will cause the dye to reflect different colored light. Reflection is the throwing back by a body or surface of light, heat, or sound without absorbing it.experiment

Your thermochromic slime uses Leuco dyes to show the difference in temperature when you play with it. Can you think of anything else to use to manipulate it’s color? What happens when there is an extreme temperature change?

Written by: Mimi Garai

Is it a Microwave or a Shrink Ray?

I’ll be honest: This one blew me away the first time I saw it! What is going on here? It all has to do with polymers, which are basically long chains of repeated molecules, which you can learn more about here.

Just to re-iterate: This is not dangerous to you, but it is dangerous to the microwave!

Microwave Magic

These polymers have some interesting properties that we need to understand, and then this will become much simpler. As we have seen before, temperature can be an important part of chemistry. That is also important here! Let’s start by talking about something a more familiar polymer: protein!

We all have lots of protein in our bodies which perform many different functions. These functions are determined by their shapes, which are in turn determined by the specific molecules that make them up. However, if proteins are exposed to high temperatures or acidic conditions, they lose their shape and just uncoil into a big mess. This is called denaturation, is actually a good thing for us. Human stomachs are highly acidic, and when you eat food with protein in it, this allows the protein to be broken down so that it can be used.

image002

A similar thing happens with the polymers in the bag*. When these huge chains are hot, they become very flexible. Left to their own devices, they will tangle up and clump into a polymer mess. Alternatively, in this flexible state they can be easily manipulated a different shape. When they are cooled down, they maintain this shape.

This gives us all the tools we need to understand the behavior of these bags in the microwave. These bags contain a polymer skeleton surrounded with foil and paint. When the bags are manufactured, they are heated up and stretched out before being cooled off. The result is that the long polymers in the bag are locked into long straight lines forming the structure of the bag.

In the microwave, they get HOT. Without anything to hold them in position, they collapse into their natural shape: a big clump. As the bag’s polymer skeleton collapses, the bag itself shrinks dramatically. The results are astounding!

Written By: Scott Alton

Microwave Magic Display

*Note: If you are reading carefully, it may seem like this is the exact opposite of the protein. This is because proteins are heated up to the point where they collapse to the appropriate shape at body temperature, whereas for these polymers that temperature is much higher. If these polymers were to get even hotter, they would exhibit similar denaturation behavior. Similarly, cooling the protein well below body temperature would lock them into their current shape. The same thing is happening, it is just on different temperature scales.

Discovering Density with Liquid Nitrogen

Why does water form lakes and oceans underneath a vast expanse of airy sky? The answer to that, and many more questions, is density! Density is most easily described mathematically by the following equation:
Screen Shot 2016-03-09 at 2.42.42 PM
This means that density depends on only these two quantities: Mass, or the amount of matter that something is made up of, and volume, a measure of how much space it takes up.

Mass

Most of us have heard of density in relation to things floating in water, but it also works for air. Seeing whether or not something floats in another thing is a great way to compare the density of two substances. Here, we can see that a balloon filled with nitrogen is more dense than air. Air is mostly made up of nitrogen, so it is mostly the additional weight of the balloon itself that causes it to fall to the ground. Alternatively, the helium balloon floats upwards as it is so much less dense than the surrounding air that the comparatively heavy rubber balloon is dragged up along with it. These two objects have the same volume, but their densities differ because of their mass.

Volume

The other quantity that we can look at is volume. If the volume becomes smaller, but contains the same mass, the density will increase. Alternatively, increasing the volume will decrease the density. In this demonstration, we take advantage of a property of gases described by Charles’s Law, which states that if we change the temperature of a gas, its volume will change proportionally. By lowering the temperature of the helium balloon with liquid nitrogen, we decrease its volume. This raises its density to be above that of air…until it warms up and expands back to a low enough density to again take flight!

This concept can help to explain many things, even if they seem odd at first. Clouds are made of water, but hang in the sky! It might seem like this is just because clouds are light, but the average cumulus cloud weighs over 1 million pounds! That is definitely not light, but they are light for how big they are, and they are big. Clouds are made up of water, but this water is very spread out. The volume is so big that this 1 million pounds of cloud actually weighs less than an equal amount of air would, even though we always think of air as being pretty light!

However, as this air rises it cools and gets closer together just like the balloon dunked in liquid nitrogen. Eventually, these water molecules band together as they get cold. When enough of the molecules stick together, they get to be more dense than the cloud around it, and plummet back to earth as precipitation.

clouds1

By the way, the average person is a little less than a thousand times more dense than air. This means if you were 10 times taller, wider, and thicker but not any heavier, you would float like a balloon! Please don’t try this at home.

Written By: Scott Alton

Diffusion, Pizza, and You

If you were to take a freshly baked pizza and put you and it at opposite ends of a vault with no airflow at all, would you ever be tempted by its delicious aroma? The answer, it turns out, is yes! Even without the aid of air currents, the smell would spread. Nothing in the room appears to be moving, but at the molecular level, things are very different.

At room temperature, the molecules in stagnant air whiz around at insane speeds– over 1000 miles per hour! The air seems still because molecules are so light that nobody can feel them individually, and there are approximately equal amounts of them going in every direction. However, tiny particles (like the delicious pizza odor) can still get knocked around by these randomly moving molecules. The process of stuff spreading out due to the movement of tiny molecules is called diffusion.

Molecule TempThe experiment you see in the video has one obvious difference: we are looking at a liquid instead of a gas. The molecules in water are much more tightly packed, and unlike in a gas, there isn’t as much room for them to zoom around. This means the molecules aren’t moving at such incredible speeds, but they also aren’t sitting still. They bump into one another, rotate, and vibrate like crazy!

Diffusion in ActiojnThe amount of spinning, bouncing, and vibrating that they do depends on one thing: temperature. In fact, temperature is really just a way to measure how energetic the molecules are. In the container of warmer water, the food coloring spreads more quickly. This is because at higher temperatures, the more vigorous vibrating and bouncing of the molecules pushes the green food coloring around faster. The cooler water has calmer molecules, which do less to disturb the blue food coloring.

Notice that the food coloring never seems to regroup together. Diffusion is entirely based on random movements of huge numbers of molecules, so it always results in concentrated areas of stuff like the food coloring spreading out to fill the available area. This trend towards uniform distribution is called entropy.

DiffusionDiagram

Diffusion causes molecules to spread out from areas of high concentration to areas of low concentration. Image credit: UC Davis

Random movement of tiny things may seem inconsequential, but it’s actually incredibly important. It’s how plants hydrate, hot chocolate cools down, spaghetti noodles absorb water, and even how oxygen gets to the bloodstream. Even more important than all of that however, is that it lets you know when there is pizza nearby!

Tabletop Rockets Science

Temperature is a measure of energy. Adding energy to a substance makes it hotter; removing energy makes it colder. Warm, energetic molecules move faster and farther, spreading out over a larger volume of space.

TeaRocket1

This balloon has been cooled to hundreds of degrees below zero (Fahrenheit), condensing the gas molecules inside. At room temperature, the condensed gas spreads out and expands, stretching the balloon back out to its original size!

We can make a gas less dense by heating it up. Less dense substances float in denser substances. This is how hot air balloons work! The warm gas inside is thinner and lighter than the air outside, so the balloon rises up through the thicker, heavier air around it. In this experiment, we’ll harness the temperature-dependence of density to turn ordinary tea bags into miniature rockets.

TeaRocket2

Step 1: Cut the staple, string, & folded paper away from the top of the tea bag. Step 2: Empty & unfold the bag to form a cylinder. Step 3: Ignite the rocket from the top.

Tea bags work well for this demonstration because they’re light, flammable, and conveniently shaped.  Emptying and unfolding the bag yields an open-ended cylinder. As the delicate paper burns, the air inside the cylinder heats up and becomes less dense. At the same time, some of the tea bag is converted to smoke, leaving a super-light skeleton of ash behind. Takeoff occurs when the structure becomes so light– and the air inside so thin– that the rocket is, overall, less dense than the air around it.

TeaRocket3
WARNING: flaming tea bags follow unpredictable flight patterns. If you try this experiment at home, be sure to choose a non-flammable setting, and keep a fire extinguisher handy.

Thomas Edison and the Bright Idea

Thomas Edison famously invented the incandescent light bulb. These light bulbs are pretty simple things. They are really just a glass casing around a tiny metal resistor called the filament. As electricity runs through it, the filament heats up until it produces light. But how much does it have to heat up? It turns out to be a pretty interesting question.

Britannica Light Bulb

Incandescent light bulb. The filament is the horizontal piece in the center of the bulb. Image credit: Encyclopædia Britannica

Everything gives off light based on its temperature, but not all light is the kind we can see. That part, known as the visible spectrum, is a tiny part of the larger electromagnetic spectrum. These different kinds of light are separated by their energies, which can be described with wavelength.

IMG_0004

Where is a simple equation that allows us to find out what wavelength is given off by an object at a given temperature. It’s called Wien’s Displacement Law.

Screen Shot 2015-10-21 at 2.10.42 PM

Humans are about 90 degrees Fahrenheit, which is about 305K. To find the wavelength emitted by humans, all we have to do is divide .002898 by 305. This gives about 9500 nanometers (nm) which is what we measure wavelength in for visible light. Visible light is between 400nm and 700nm, so 9500nm is pretty far away from that. This puts it squarely in the infrared.

NASA EM

The full electromagnetic spectrum, including the very small visible light portion. Credit NASA

So how hot does that filament need to be to give off visible light? Lets aim for 500nm, which is definitely in the visible light range. By using algebra, Wien’s Displacement Law can be rearranged to T=b/. Dividing b by 500nm gives a temperature of 9973℉, which is about the temperature of the surface of the sun!

This actually makes total sense too! This means that the sun’s primary light emission is smack dab in the middle of our visible spectrum. It definitely makes sense that if we were to gain the ability to see, it would be to see the kind of light that was the most common. However, pretty much everything in the sun is a plasma due to the immense heat. There is no way that a light bulb could survive such insane temperatures.

Light bulb filament comes in at a relatively cool to the sun but still insanely hot 4500℉. Throwing this back into the equation, we get 1052nm, which is much closer to the 400nm to 700nm range. The primary emission is still in the infrared. However, this only gives the peak wavelength of light given off. The actual light is a mix of light more like this:

Screen Shot 2015-10-21 at 12.39.16 PM

For a light bulb, only about 5% of the energy is given off as light that we can see. The rest of it is mostly given off as infrared light, or heat which is why these bulbs feel hot. The reason they don’t feel like 4500℉ is that the inside of the light bulb is a vacuum–meaning it contains no air–and the heat thankfully doesn’t conduct through it very well. They actually use this vacuum insulation for water bottles, thermoses, and even the Dewars that hold things like liquid nitrogen!

This inefficiency is the reason behind moving to much more eco-friendly fluorescent light bulbs. These bulbs don’t use Wien’s Displacement Law to produce light, but instead are filled with atoms that naturally release light when hit with a bit of electricity. Even many new bulbs that look like incandescent bulbs actually contain several small Light Emitting Diodes, or LED’s inside which share the energy efficient qualities of fluorescent bulbs. What a bright idea!

LED Bulb

Exploring Chemistry with Fire & Ice


Can you change the speed of a reaction just by changing the temperature? That’s what we’re going to find out! First, we need to understand the reaction:

see

This balloon is filled with hydrogen, and then it is lit on fire. This is known as a combustion reaction, where hydrogen combines with oxygen. This means that the result of this fireball is actually….H2O, or water vapor! In chemistry, we would write this equation like this:

2 H2   +   O2    →   H2O   +   Heat

While heat may not be a molecule, it is a very important part of the reaction. This all has to do with energy. While hydrogen and oxygen are stable molecules, they store more energy than water. Nature tends to push towards having molecules in the lowest available energy scenario. That seems to mean that hydrogen and oxygen should always be combining and wood should always be on fire (since if fire gives off heat, the result of fire has to be lower energy than the wood), but fortunately this isn’t what actually happens! This is because of the graph below:

Energy Reaction

The curvy red line shows what the energy looks like throughout the reaction. The reactants, as noted above, have higher energy, and the products have lower energy, and the difference between them is the heat. There is also the weird little bump, which is known as activation energy. For the reaction to start at all, some energy needs to be added, which is why we have to light a fire. Touching the lighter to the balloon supplies enough heat to start the chain reaction of combustion of all of the hydrogen.

The hydrogen in the balloon is about 70℉. What happens if we change that temperature? What does temperature mean and why should it matter? When we measure how hot something is, what we are really measuring is how fast the atoms and molecules are moving. For the hydrogen to react with oxygen, it has to be close enough to the oxygen and have heat to start the reaction. Adding liquid nitrogen to the situation cools it down by about 400℉, to -321℉. You can see this immediately by the size of the balloon. It appears to crumple under the weight of the liquid nitrogen, but really the gas inside is just slowing down so much that it can’t push out the sides of the balloon. With the molecules slowed down, the reaction should slow down too! Here they are side by side:

Smaller Fast Smaller Slow

 

If it looks like the one on the right has been slowed down, it has…by the dramatic change in temperature of the reaction! This may seem exotic, but there are practical examples of this. Refrigerators are used to keep the temperature of your food low to slow down spoiling. Lizards and other ectotherms move more slowly in cold weather as they lack a way to keep up their body temperatures.

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