Scenario: You are given two rocks. Rock A is labeled Sandstone. Rock B is labeled Limestone. According to the information you are given, sandstone will not make bubbles when put in a vial filled with vinegar. Limestone will make bubbles when in a vial filled with vinegar.
Vinegar-expected results Vinegar-actual observations
Rock A=Sandstone Does not bubble Bubbled vigorously
Rock B=Limestone Bubbles No bubbles seen
(The bubbles are carbon dioxide. They form when carbonates/calcites are exposed to acids. Vinegar is a weak acid. Note: Chemists would call limestone a carbonate, but geologists call it a calcite.)
Sandstone is a grainy rock composed of sand grains fused together. Other minerals are usually present between the sand grains. You can easily tell with even a simple magnifying lens (3-5 times magnification) that the rock is made up of small grains, and you can also feel the bumpiness of the grains with your fingers.
Limestone is a harder rock. In Indiana, it is usually light gray in color, but I've also seen darker gray samples with almost rust colored lines. The surface feels powdery and is dull (not shiny). Limestone is made of calcium carbonate, which is also what Tums is made of. Some forms of chalk are also made with limestone.
Both limestone and sandstone are used to cover the outside of buildings. Indiana limestone is sometimes used to make monuments because the color is consistent and it can be cut into large pieces.
The teachers at a workshop I attended thought that perhaps the samples of limestone and sandstone were mislabeled because both behaved the opposite of what they expected. I borrowed their samples to do some more testing. I was hoping to determine if the rocks had been mislabeled.
A bit about my own previous experiences with sandstone and limestone: The house that I grew up in was built on land that had a large sheet of sandstone just a foot or so below the surface. There were lots of places where you could see exposed sandstone, especially along the side of roads where hills were blasted through to make a flatter road. My dad worked for a quarry where they mined and crushed limestone for building roads, so I've seen a limestone quarry and limestone in lots of different sizes. Our house's driveway was covered with limestone. Large pieces of limestone were used for covering homes and businesses. We even had it on the bottom third of my house. So, I know a bit more about both rocks from my own observations growing up. (This is a good example of how each of us brings their own experiences in when doing science. Had I not known as much about sandstone and limestone, I might have approached these experiments differently.)
I'm not a geologist, but I did take a geology class in college. I really enjoyed rock and mineral identification. My chemistry background came in handy when I had to learn about the different tests that are used to identify rocks and minerals, especially the chemical test of dropping small amounts of acid on a sample to see if bubbles are formed.
A bit about my own previous experiences with sandstone and limestone: The house that I grew up in was built on land that had a large sheet of sandstone just a foot or so below the surface. There were lots of places where you could see exposed sandstone, especially along the side of roads where hills were blasted through to make a flatter road. My dad worked for a quarry where they mined and crushed limestone for building roads, so I've seen a limestone quarry and limestone in lots of different sizes. Our house's driveway was covered with limestone. Large pieces of limestone were used for covering homes and businesses. We even had it on the bottom third of my house. So, I know a bit more about both rocks from my own observations growing up. (This is a good example of how each of us brings their own experiences in when doing science. Had I not known as much about sandstone and limestone, I might have approached these experiments differently.)
I'm not a geologist, but I did take a geology class in college. I really enjoyed rock and mineral identification. My chemistry background came in handy when I had to learn about the different tests that are used to identify rocks and minerals, especially the chemical test of dropping small amounts of acid on a sample to see if bubbles are formed.
Before starting my experiments, I wanted to get some photos of what both samples look like close up.
Here is what the sandstone sample looks like. Notice how grainy it looks even in the photo. Can you see that there seems to be a lot of white between the grains? I wonder what that might be...hmmm.
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| Sandstone: Before testing |
This is what the limestone sample looks like. Notice the lines in the rock on the upper right hand corner of this photo. That is an interesting set of lines in the rock. They look like they might be tiny cracks.
So, without even doing any testing, I was able to make several observations about the rocks. I'm going to add these observations from my table above, but I'm going to change how my table is formatted so I can fit the other experiment data on the table more easily. |
Limestone: Before testing (Well, not quite. Notice the bottom center of the rock. That's where I did the streak test.) |
Rock A=Sandstone? Rock B=Limestone?
Vinegar-Expected Results No bubbles Bubbles
Vinegar-Actual observations Bubbbles No bubbles
Color Gray with white streaks Dark brown. A few cracks present.
Looks like a crystal No No
Surface Grainy Smooth
Luster (shine) dull dull
I decided to do the tests that would cause the least damage to the rocks first. The last experiment was going to use a hammer, and I didn't know what the rocks would look like after that!
A hardness test is a great way to start identifying a rock after you've described the color(s) in the rock, the luster of the surface (Is it shiny or dull?), and identified any special fractures or layers in the rock. To test hardness, you "scratch" or rub the rock against something with a known hardness. Geologists use a scale called a Mohs scale that lists materials and their hardness on a scale of 1-10, with 10 being a diamond and 1 being talc. Here's a link to a step-by-step guide for testing hardness from the Children's Museum of Indianapolis. http://www.childrensmuseum.org/geomysteries/cube/b2.html
I first tried to scratch the rocks with my thumbnail. My nail was worn down by the rocks, so the rocks are both harder than my fingernail. Nails have a hardness of 2.5, so I knew that the rocks were harder than 2.5. I then tried to scratch the rocks with a paperclip, which I read had a hardness of 3.5. In both cases, the paperclip scratched the surface of the rocks. I forgot to take photos of the rocks after they had been scratched by the paperclip, but it'll be on the video.
From the two hardness tests, I determined that the rock was harder than 2.5 but less than 3.5. That eliminates a lot of possibilities of what the rocks were, but it's not enough to identify them.
Link to mineral hardness of many common minerals:
http://en.wikipedia.org/wiki/Mohs_scale_of_mineral_hardness
http://www.minsocam.org/msa/collectors_corner/article/mohs.htm
In a streak test, you rub the rock against a rough surface. The surface must be harder than the rock. Otherwise, the rock will get scratched by the surface. I had a unglazed tile left over from some Christmas craft projects last year, so I rubbed the rocks against the tile. This is what rubbed off the rocks onto the tile.
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Streak test (limestone on left; sandstone on right) This was really interesting to me. The limestone sample left a white powder on the tile, but the sandstone left a dark gray mark on the tile. When I rubbed the limestone powder between my fingers, it felt chalky. The sandstone didn't feel chalky but it wasn't really grainy either. I wasn't expecting either rock to feel chalky, so that was an interesting observation. (If you look at the bottom of the post, you'll see my notes that I took during my experiments...but you may want to wait if you don't want to know what I discovered just yet!) Rock A=Sandstone? Rock B=Limestone? Vinegar-Expected Results No bubbles Bubbles Vinegar-Actual observations Bubbbles No bubbles Color Gray with white streaks Dark brown. A few cracks present. Looks like a crystal No No Surface Grainy Smooth Luster (shine) dull dull Hardness test >2.5 but <3.5 >2.5 but <3.5 Streak test Gray White. Powder feels like chalk. Next I wanted to try a variation on the vinegar test that the teachers did. Normally the bubble test is done with dilute hydrochloric acid, but since this experiment is for 3rd graders vinegar is used because it's safer. I wanted to put just a few drops of dilute hydrochloric acid (0.5M for any chemistry geeks out there) onto each rock and see what happens. Based upon what the teachers saw, I expected the limestone sample to not bubble and the sandstone sample to bubble. Here are the photos of what I saw. |
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| Hydrochloric acid on Sandstone-You can see tiny bubbles at the top and bottom edges where the rock is touching the liquid. I also noticed what appeared to be powder (white?) forming on the surface of the rock. I then immersed the bottom of the Sandstone in a small puddle of hydrochloric acid, since that was similar to the immersion test the teachers did. |
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| Sandstone surface immersed in a small puddle of hydrochloric acid. You can see bubbles where the liquid and the rock are touching. |
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Large carbon dioxide bubble forms on sandstone after several minutes. Can you see the big bubble?
I did the same experiments with the Limestone. I first put a few drops of Hydrochloric acid on the surface.  |
| Hydrochloric acid on Limestone If you look closely, you can see bubbles forming. |
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| Several minutes later-- One surface touching a puddle of Hydrochloric acid on Limestone Lots and lots of bubbles! |
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| Limestone: Large bubbles forming in hydrochloric acid several minutes after starting the immersion test. Rock A=Sandstone? Rock B=Limestone? Vinegar-Expected Results No bubbles Bubbles Vinegar-Actual observations Bubbbles No bubbles Color Gray with white streaks Dark brown. A few cracks present. Looks like a crystal No No Surface Grainy Smooth Luster (shine) dull dull Hardness test >2.5 but <3.5 >2.5 but <3.5 Streak test Gray White. Powder feels like chalk. Hydrochloric acid Small bubbles Many large bubbles, lots of bubbling More experiments to be posted soon..... |
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