Holiday How-to: Chocolate Leaves

My mom was a chemistry and home-ec teacher, so I grew up in a home where ingredients were carefully measured and food items were attractively arranged. While I got to help out in the kitchen as much as I wanted, I always liked being in the kitchen around the holidays. There were always new tricks or special touches added to dishes and along with these came short science lessons on why we were doing things that particular way.

One of my favorite things to help with in the kitchen were chocolate leaves. When done correctly, these are perfect little molds of the living leaf, just like the perfect molds and casts in the Morian Hall of Paleontology.

A chocolate leaf is made by smearing melted chocolate onto a leaf and putting it into the fridge to harden. Sounds easy, right? It is pretty easy. Read on!


Activity: Chocolate Leaves


Leaves (*See note in step 1.)

Chocolate candy melts

Parchment or wax paper

A cookie sheet or plate for your leaves to rest on as they cool


1. Pick your leaves. I like to use slightly waxy leaves so you don’t have to worry about fuzzy bits in your chocolate. NOTE: Learn about the plant you are picking leaves from before you decide to use them. Many household plants are decorative but poisonous.  Oleander is a great example of a plant that is pretty but poisonous. If you hate botany or don’t know about the Internet, getting pre-packaged basil or mint from the grocery store is a safe way to go. These leaves will be a little less firm, so you will need to be more careful with them.

2. Don’t pick leaves from poisonous plants. Seriously.

3. Wash your leaves with soap and water, rinse them thoroughly and then dry them completely. The chocolate won’t stick to wet leaves, so don’t rush this step. You will only be frustrated.

4. Put wax or parchment paper on a cookie sheet or plate. You want this to be something that will fit in the fridge with no problems.

5. Get out your candy melts. The melts come in a hundred colors. We are using chocolate colored ones in this tutorial. There will be instructions on the package on how to melt the specific brand of melts you purchased. In general, you will put the melts in a microwave safe bowl and microwave them a few seconds at a time stirring as you go. Don’t overheat the melts. They get gross and there is no coming back from that.

6. When you have everything melted and creamy, hold the leaf by its stem. I like pinching it between my thumb and index finger and then using my middle and ring finger to support the leaf. Do what feels comfortable to you.

7. Dip your stirring spoon into the chocolate. Use the BACK of the spoon to spread the chocolate on the leaf. Make sure the chocolate is thick enough that it won’t break when you try to peel it. Place the leaves on the parchment as you work, and don’t let them touch.


8. The side of the leaf you use is up to you. If you are using mint and you put the chocolate on the back of the leaf, you will have some crazy patterns.  If you want something more subtle, use the front of the leaf. Coat the leaf almost to the edges. If you go too far, you will get ugly edges that are hard to peel. But don’t worry! Those leaves are the best to eat.

9. Put the tray of leaves in the fridge and wait a few minutes.


10. When the chocolate is set, peel the leave off the chocolate. You should have a perfect little mold of your original leaf. This may take a little practice. Work quickly as you have something designed to melt with heat in your hot little hands.

11. Done! You can store the leaves in the fridge until you are ready to use them. If the leaves got soft when you were working with them, put them back in the fridge to firm them up. Once they are firm, you can toss them in a plastic container.


Okay! So what’s the science here?

The word “chocolate” comes from the Nahuatl word Xocolatl for “bitter water,” referring to its original incarnation as a hot, spiced beverage in the Mayan and Aztec traditions. Traditionally, chocolate is a mixture of cacao powder, cocoa butter, and a sweetener. To make chocolate palatable and stable, we now mix milk solids, added flavors, modifiers, and preservatives.

Those candy melts? NOT CHOCOLATE! In this example, they are sort of chocolate colored, so they have that going for them, but they also come in a bunch of colors that are not known to nature so… not chocolate. They are mostly made of sugar and vegetable fats – not cocoa butter – and depending on the brand, they may throw in a little wax for better melting. Mmmmm… wax.

The advantage to the melts over the regular chocolate is that they do have the wax and the vegetable oil in them, which makes melting easier since the chocolate doesn’t need to be tempered. It hardens pretty quickly and sticks to whatever you dip in it, so it makes a great coating for cake pops or whatever crazy things show up on Pinterest this month.

Want to get super nerdy about your chocolate?  (I assume you do…) MIT has these tidbits available.

What’s in typical chocolate?

  • 10-20% cacao
  • 8-16% milk solids
  • 32-60% sugar
  • 10-20% cocoa butter
  • 2% theobromine and polyphenols

Cocoa Butter Chemistry

Fats and oils are organic molecules made up of three fatty acids chemically linked by an ester bond to glycerol. Fats are solid at room temperature, while oils are liquid.

Cocoa butter fats are made up predominantly by three major fatty acid molecules: Palmitic Acid, Stearic acid, and Oleic acid.

Oleic acid is unsaturated (has a double bond on its carbon chain), making it kinked and unable to pack well with other molecules. Because of this, a greater portion of oleic acid in the fat results in a lower melting temperature for the cocoa butter.

Chocolate makers can adjust the amounts of each fatty acid to produce a chocolate that melts only in the mouth, giving it a superior quality.

Tempering chocolate

The cocoa butter in chocolate can have several different crystal structures (three-dimensional patterns in which the fat molecules pack). There are six known chocolate crystal forms, or polymorphs. You can obtain each form by varying the fatty acid ratios and the temperature at which the chocolate is tempered (cooled).

Only a few of the polymorphs are considered good for gourmet chocolate because they give the right blend of snap (when you bite into the chocolate) and melting (when it warms up in your mouth). Melting is especially important because it controls how well the chocolate disperses and releases flavor onto your tongue.

Whether you will be constructing culinary masterpieces this fall or sitting back and enjoying the kitchen creations of others, we hope you have a happy holiday with you and yours!  (And when you’ve had a little too much togetherness, we will be open on Friday…)

With Soil, Make Me Wine: The Dirt on Growing Great Grapes

I like wine. And I make my own. Not huge batches, mind you. Just about 30 bottles per month in the winter months. I learned the hard way the chemistry of wine. If you let the wine get too hot while it’s fermenting, it can radically alter the taste.  I let one of my batches get above 95 degrees a few times this summer. I was making a port and the flavor was ruined. The entire batch came out tasting like welches grape juice. Flat, tasteless, 20 percent alcohol-by-volume grape juice. I only inflicted a few bottles on my friends.


Good wine is a combination of science and art. There is the botany of the grapes. The meteorology of the climate. And the pedology. What’s pedology you ask? It’s the study of soil.  And since it is the International Year of Soils, we are going to get down and dirty with the effect of soil on one of my favorite drinks.

The ground beneath us is incredibly active. There are millions of different types of bacteria, fungi, and arthropods that give dirt everywhere its characteristics. If you’ve been taking the museum’s class on gardening and landscaping, you’ll understand the importance of the health of soil for plants. To briefly sum it up, good soil makes good crops. A shocking concept. But beyond that, what effects can the soil have on wine?


The effect of soil and climate on wine is called terroir. Wine tasters with a good palates say they can discern the flavor of the soil in the wine. Scientists have begun to examine a comparison of terroir to wines in an attempt to explain this phenomenon but so far have not been able to. That doesn’t mean that the flavor of the soil isn’t in the wine; it just means more scientists will have to drink more good wines. That’s a study I want to be a part of!

Good soils will encourage the vines to produce grapes instead of growing more vine. So the best soils need to provide lots of water at just the right time and then be able to drain it away. And the soil needs to keep the right nutrients such as nitrogen and potassium available to the vine, which can help intensify the flavors in the grape.


Tasting wine is about more than just “good” or “bad.” With an entire family of varietals out there in the world, it’s about what gives the wine its identity. Fans of wine, like me, like to get closer to the wine and the wine-making process through the quality of its flavor. And, oddly enough, tasting isn’t just about the taste. Wine Folly offers a five-step process to tasting wine, and explains a few things to be aware of. Here’s the basic process outlined in their blog.

  1. Look at the color. This goes deeper than just red and white. Ask yourself how it compares to other reds or whites in color. Gauge whether you can see through it. With practice, you can gauge whether the wine is bold, rich or viscous.
  2. Smell the wine, but swirl it around first to aerate it. Put the wine on the table and move the base in little circles, then shove your nose into the glass and take a big whiff. What do you smell?
  3. Taste the wine. Get enough of the wine to coat your entire tongue and roll it around in your mouth to maximize contact with all your taste buds. Don’t just think about flavor; think about texture and body, how it feels in your mouth. Does it have an alcoholic burn? Do the flavors match the smell?
  4. Decide whether to spit or swallow. You may have to drive later, or you may have 20 wines to taste and want to stay sober enough to think about all of them. If you hate the wine, spit it out. If you don’t want to waste it, swallow it. There’s no right or wrong choice.
  5. Think about the wine and formulate your own conclusions. Wine Folly states, “Wine tasting is a head game. Confidence and bold assertion can often make someone look like a pro.”


Join us for a Periscope wine tasting with local experts, curators, and myself on Wednesday, November 18 at 3 p.m. You’ll see some live wine tasting where we’ll talk about terroir and suggest some wine pairings for Thanksgiving. And to celebrate the International Year of Soils, join us for a film screening of the Symphony of the Soil at the Wortham Giant Screen Theatre Dec. 1 at 6 p.m.

Ever wonder how fireworks… work wonders?

The Fourth of July just isn’t the same without pyrotechnics. And while the inevitable giant fireball from Dad lighting up the grill may be exciting in the moment, I’m actually referring to the giant chemistry demonstration we watch at night.


Fireworks are basically a bunch of combustion reactions, which are rapid chemical reactions involving oxygen gas (O2) combining with another substance. These combustion reactions are exothermic, which means energy is released during the reaction in the form of heat, light, and sound.

A firecracker explosion is essentially one large combustion reaction involving black powder or gunpowder, which is made up of potassium nitrate (KNO3), charcoal, and sulfur. Potassium nitrate will provide oxygen to the reaction, while charcoal and sulfur will act as fuel. This reaction produces a lot of gas and heat in very little time, and all of that gas produced needs a place to go. When too much of it builds up in an enclosed space and the pressure becomes too great, you get an explosion.

The basic components of a firework are a fuse, tiny explosives called stars, and a burst charge that triggers the explosion. Precise timing is also helpful.


First, you need an entirely separate explosion to get the firecracker up into the air. Typically to get the whole package airborne, you need what’s called a mortar, a long tube that directs the firecracker onward and upward away from bystanders. This explosion needs to be very controlled so you don’t set off the second firecracker inside, yet strong enough to get the whole package off the ground. A malfunction can have disastrous consequences. You can search for “fireworks fails” on YouTube for some disaster action.

When you light a firework, it’s not just one fuse; it’s two: the fuse that sends the firework up, and a time-delay fuse that is longer and burns more slowly, allowing the firecracker to gain some altitude before the second reaction begins. If the fuse is too short and the firecracker doesn’t fly high enough before exploding, it can get noisy (not to mention dangerous.)


Once the time-delay fuse expires, the stars begin to explode. A burst charge will explode and expel the stars, spreading them out. The stars themselves may have different chemical components within, but the basic idea is still a combustion reaction. There is some sort of fuel reacting with oxygen and producing a lot of gas and heat.


All those colors you see come from burning metals, which produce different wavelengths of light when heated. I don’t know how many of you have tried to burn metal before, but I can tell you from experience, it’s not easy.

We model this particular combustion reaction in one of our ConocoPhillips Science On Stage Outreach programs! Since lighting a firecracker in a school is a terrible idea, in Cool Chemistry, we use a fuel and some granular chloride salts in a beaker. When I light the fuel, I am beginning a combustion reaction that releases a lot of heat and will burn the metal salts.


The red/pink flame is from the metal lithium, sometimes used in batteries. You’ll notice that in this photo, there is a large Nalgene beaker covering the beaker that used to be full of green flames. That Nalgene beaker is airtight and cuts off the flow of air in and out of the beaker. When this happens, no new oxygen is allowed to enter; once the combustion reaction has used all of the oxygen inside the beaker, the flame will be put out.

Our beaker simulation doesn’t produce the loud bang we often associate with fireworks because it is open to the air around it. The boom heard is actually all of the gas building up inside of the firecracker being expelled all at once, moving faster than the speed of sound, just like the pop heard when a balloon bursts.

One prevalent legend says fireworks were invented accidentally by a Chinese cook some 2,000 years ago, and the basic concept has remained the same over the years. If anything, precise timing of explosions in fireworks shows has made the spectacle all the more enjoyable.

So grab some apple pie, pull out a lawn chair, relax and enjoy the world’s most famous combustion reaction, celebrating America’s birthday in style!

Bring the wonders of the Houston Museum of Natural Science straight to you with HMNS Outreach! To book a presentation of Cool Chemistry, email or call (713) 639-4758!

Educator How-To: Crystals, Geometry and Chemistry

Math is beautiful and inescapable. Especially in nature, patterns and equations just keep showing up.  The path of an orbiting planet, the growth of a nautilus, arrangements of leaves on a stem, the efficient packing of a honeycomb; we can find rules and algorithms and make predictions from them.

Crystals, with their obediently repeating structure, are an elegant manifestation of the ‘rules.’  To be a crystal, your building blocks (atoms, molecules, or ions) must follow patterns over and over and over and over and over.  Atoms, being predictable, simply do what their chemical properties and the conditions (temperature, pressure, etc.) indicate.  So what exactly does it take to go from a mess of elements and compounds to this example from the Crystals of India exhibit at HMNS Sugar Land?

If you’ve ever tried making rock candy from sugar water or ornaments from borax solution, then you have some idea what it entails: something dissolved that is capable of making crystals has to slowly come out of solution – usually the longer you give it, the bigger it can grow and the slower it grows, the more perfect the crystals.

Freezing water into ice also gives you crystals; they just don’t stick around and let you handle them conveniently at room temperature. Water and solutions in water aren’t the only way to get crystals; molten rock cooling (slowly) can also give crystals, but that’s a little tricky for home experimentation.

So time is your friend for crystal growth, pressure is a factor, and it needs to be easier for atoms to attach to the forming crystal than to stay in solution.  Having a solution that is saturated or supersaturated so it can barely hold all of the dissolved material helps. It also helps to have places for the crystals to start forming; a tiny ‘seed’ crystal or sometimes even just a rough spot on a surface can provide the nucleation sites to kick off crystal growth. Are there other ways crystals and the things we consider ‘gems’ can form? Yes!

For those of us with shorter attention spans, a cool way so see the process is with crystallizing hand warmers – a pouch holds a saturated solution of sodium acetate. When you flex a metal disk inside the pouch, you kick off a chain of crystallization and end up with solid material (and released heat energy).  Because the process is so fast in the hand warmer, the individual crystals are very small and jumbled up (polycrystalline); oriented in all different directions, and as a mass they are opaque (light is refracting all over the place) and relatively dull rather than shiny and smooth as slower-forming large crystal faces can be.  The structure of most metals is also polycrystalline, and things like plastic and glass (even the kinds misleadingly labeled “crystal!”) are amorphous.

The external crystal shapes we see are related to the internal structure – there are a lot of different ways atoms can pack together.

Practically, there will always be some disruption in a crystal structure, no matter how perfect it may appear, which allows for some very cool effects – crystals “twinning,” impurities that alter the color; the reason ruby and sapphire (both corundum crystals) appear different.

Crystals aren’t always pretty! Sometimes we want to prevent crystallization to avoid things like kidney stones, but crystals are useful for all kinds of things; optical equipment and lasers, X-ray crystallography to figure out structures of proteins (and once upon a time, DNA), and silicon chips used in electronic devices. 

Whether you prefer your crystals practical or decorative, they are amazing!

Can’t get enough crystals? Check out the Crystals of India exhibit at HMNS Sugar Land (free for members!)