Maya Ellison
Rocks and plastics are two very different materials, and there are a variety of ways to test for their properties. Rocks are made of one or more minerals, which are inorganic materials found in the Earth's crust. Geologists use tests to identify rocks and minerals, including hardness, colour, streak, luster, cleavage, and chemical reaction. For example, a scratch test can be used to determine mineral hardness, and a colour streak test can identify the colour of the mineral in its original, powdered form. On the other hand, plastics are synthetic materials made from organic polymers. To differentiate between rocks and plastics, one can perform tests such as the scratch test, the magnetism test, the acidity test, and observing physical properties such as transparency, light reflection, and electrical conductivity.
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What You'll Learn
Hardness test
The Mohs scale of mineral hardness is a qualitative ordinal scale that ranks materials on a scale from 1 to 10 based on their scratch resistance. The scale was introduced in 1812 by German geologist and mineralogist Friedrich Mohs. The Mohs scale is based on the ability of a harder material to scratch a softer material. The Mohs scale ranks talc as the softest mineral at 1, while diamond is the hardest at 10.
The Mohs hardness test can be used to test the hardness of rocks. This can be done by scratching the rock with a fingernail, a copper penny, a glass plate, a nail, or a ceramic plate. Each of these items corresponds to a certain hardness level on the Mohs scale. For example, if a fingernail can scratch the rock, the rock has a hardness of less than 2.5.
The Mohs hardness test can also be used to test the hardness of industrial materials, including plastic. The Mohs hardness test kit for industrial use includes four double-ended picks, colour-coded, with eight points comprising 2, 3, 4, 5, 6, 7, 8, and 9 on the Mohs hardness scale. The kit also includes two hardness plates, a grinding stone, a rugged plastic case, a hardness table, and instructions.
The hardness of plastics can also be determined using a loaded ball indenter in accordance with standard ISO 2039-1 or ISO 2039-2. The measured indentation depth is used to calculate the surface area of the indent and the hardness of the plastic. The Rockwell method is a similar test that uses a steel ball indenter and applies the total test force in two stages.
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Colour and streak test
The colour and streak test is a valuable method for identifying rocks and minerals. While some minerals have a single colour, like malachite, which is green, others, like quartz, can be blue, brown, pink, red, purple, or even colourless. This makes it difficult to identify them based on colour alone.
The streak test is a more reliable way to identify rocks and minerals. A streak is the name given to the coloured residue or trail left by a mineral when it is dragged across an unweathered, unglazed surface, such as a porcelain tile, the back of a glazed tile, or smooth cement. This powder is the mineral in its original, powdered form, and its colour is often a very important property for identifying the mineral.
To perform the streak test, first break the rock to expose a fresh surface. Then, select a representative protrusion on the rock and drag it firmly across the surface of the streak plate. If the rock is too hard and does not leave a streak, scrape a nail across the rock to obtain its powder. Finally, compare the colour of the streak or powder to an identification chart to determine the mineral's identity.
The streak test is particularly useful because, unlike the apparent colour of a mineral, which can vary due to impurities or disturbances in its crystal structure, the streak colour is generally more consistent. For example, hematite can be black, red, brown, or silver, but it will always produce a reddish streak.
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Magnetism test
The magnetism test is a simple and basic method to determine whether an object is magnetic or not. This test can be performed by using a magnet or a compass.
Using a Magnet
Hold a magnet close to the object you are testing. If the object is magnetic, it will be attracted to the magnet. If the object is non-magnetic, it will not be attracted to the magnet.
Using a Compass
Place the object you are testing on a flat surface and then place a compass close to it. If the object is magnetic, the compass needle will move and point toward the object. If the object is non-magnetic, the compass needle will not move. This method is more scientific and accurate as it measures the magnetic susceptibility of a material.
Safety Precautions
It is important to handle magnets with care and keep them away from small children. Neodymium magnets, in particular, have a strong pull force and can cause sharp metal objects like forceps, scissors, knives, and screwdrivers to quickly move towards them. It is recommended to keep these objects away from your workspace. Additionally, keep other magnetic equipment away from your work area unless you have tested them first to ensure they are not ferromagnetic.
Magnetic Wand
You can also assemble your own magnetic wand for testing. A neodymium magnet can be purchased online and attached to a handle, such as a hex bolt from a hardware store. This wand can be easily carried and transported in a plastic container, such as a pill bottle or a lipstick tube.
Rock Magnetism
The study of rock magnetism involves analyzing the magnetic properties of rocks, sediments, and soils. By heating rocks to high temperatures in a magnetic field, scientists can investigate the thermoremanent magnetization (TRM) of these materials. This helps paleomagnetists understand the ancient magnetic field of the Earth and develop methods to correct for effects like sediment compaction and metamorphism.
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Acidity test
The acid test is a great activity for rock collectors and enthusiasts who want to learn more about the rocks they find in nature. It is also used by geologists to identify carbonate minerals. The most commonly used acid for rock testing is hydrochloric acid. However, acetic acid in the form of vinegar can also be used.
To perform the test, place each rock in a glass jar and fill it with vinegar, ensuring that the rocks are at least half-covered. Observe any reactions that occur. Alternatively, a drop of dilute hydrochloric acid can be placed on the rock and observed for bubbles of carbon dioxide, indicating the presence of carbonate minerals. Different carbonate minerals have different responses to hydrochloric acid, with some reacting more vigorously than others.
It is important to note that not all rocks react to acid. For example, quartz and most igneous rocks are acid-resistant. Always exercise caution when handling acids, and adult supervision is recommended.
The acid test is just one method of rock identification and should be used in conjunction with other tests for a more accurate analysis. It can be a fun and interactive way to explore geology and create your own testing kit using safe household materials.
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Electrical conductivity test
Electrical conductivity is a physical property that describes how easily electric currents can flow through a material. It is defined as the ratio of the density of the current to the strength of the electric field. Conductivity is the inverse of resistivity.
To test the electrical conductivity of a rock, a cylindrical core sample is taken and placed in a sample holder between two copper/graphite electrodes. This sample then acts as a resistive element for a circuit. A direct current is driven through the core sample, and Ohm's law is used to determine the circuit resistance caused by the rock. The rock's conductivity can then be calculated.
The electrical conductivity of a rock is influenced by its pore-space network. Rocks with low tortuosities have an efficient conduction of electrical charges, while those with high tortuosities are more resistive. Additionally, the presence of certain minerals, such as metal-oxides, metal-sulphides, and native metals, can affect the current's path and make the rock more conductive.
In contrast, plastics are typically non-conductive materials, with electrical currents unable to flow through them easily. However, some plastics may have conductive additives or fillers, which could enhance their conductivity.
To test the electrical conductivity of a material, such as plastic, a common method is to use a four-point probe, also known as a Kelvin probe. This involves placing four equally spaced probes in contact with the surface of the material. A known current is passed through the outer two probes, and the voltage drop is measured between the inner two probes. This allows for the calculation of the material's resistivity and, subsequently, its conductivity.
Other techniques for measuring electrical conductivity include the eddy current method and the use of specialised micro-ohmmeters or benchtop laboratory instruments like conductivity meters. These methods provide valuable insights into the uniformity, thickness, and purity of the material being tested.
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Frequently asked questions
Geologists use various tests to identify rocks and minerals, including:
- Hardness: A scratch test developed by German mineralogist Fredriech Mohs in 1822 determines mineral hardness on a scale of 1-10. Common objects of known hardness, like a fingernail, penny, glass, or knife blade, can be used to scratch the rock's surface and determine its relative hardness.
- Color: While some minerals have various colors, like quartz, the presence of specific colors can indicate certain minerals.
- Streak: This is the colored residue left by scratching a mineral across an abrasive surface. A mineral with multiple colors will always produce a characteristic streak.
- Luster: The way light reflects off a mineral's surface can be used to identify it.
Rocks are made of one or more minerals, which are solid, inorganic materials found in the Earth's crust. Plastic, on the other hand, is a synthetic material made from organic polymers. While rocks are typically hard and brittle, plastic tends to be more flexible and softer. You can also try to differentiate them by using the various rock tests mentioned above; plastic will likely not react in the same way as rocks.
You can test for the presence of metal minerals in rocks by using a magnet. If there is a magnetic pull, the rock contains metal minerals. Additionally, you can test the rock's ability to conduct electricity by completing a circuit with an LED and a battery. Rocks like metallic elements and graphite are good conductors.
An acidity test can be performed by dripping vinegar or a weak acid on the rock. If the rock fizzes, it contains carbonate minerals like calcite and aragonite.
