Experimental Classification of Matter

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Ruth Gilbert
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1 Experimental Classification of Matter Sample Heterogeneous? Heterogeneous Can sample be separated by physical means? Homogeneous Substance (pure) Can sample be separated by chemical means? Element Does compound contain metals? Ionic NH4 in formula or ammonium In name? Molecular

2 Classification of Matter by Chemical Entity What Chemical Entities are Present? Sample One (1) Does the entity have a charge? How many different chemical entities are present? Two (2) 3 or more Probably a mixture Ion t a sample! Are all entities Neutral? Element Is there atoms of only one element present? Molecular 2 elements or 2 molecular cmpds or 1 element/1 molec cmpd Ionic 1 type cation and 1 type anion? Homogeneous All entities Distributed Evenly Through out? Heterogeneous t a Sample!

3 Classification of Matter Definitions Edited 8/19/2014 Experimental Need to classify properties and changes into two categories to distinguish between substances and mixtures experimentally: Physical Chemical Macroscopic (Experimental) Definition A change that does not result in original substance(s) being converted into new (substances). A change that results in transformation to new substances. chemical change = chemical reaction Change Property Submicroscopic* (Theoretical) Definition Substances have same identifying properties before and after change. bonds are made or broken. electrons are transferred. The chemical entities ** (atoms (free or non-bonded), molecules or ions) present after the change are the same as those present before the change. Can be measured without sample undergoing a chemical change. Substances have new identifying properties after the change. Bonds are broken and formed (atoms are connected in new ways) and/or electrons are transferred. At least some chemical entities present after the change are different than those present before the change. Can only be measured directly 1 if sample undergoes a chemical change. Chemical properties describe the reactions that a substance can undergo and what reactions can be used to produce that substance. * submicroscopic here refers to the atomic/molecular level. ** Chemical entities = atoms (free or non-bonded), ions or molecules that make up a sample. The term free atoms (or non-bonded atoms) refers to neutral atoms in a sample that are not bonded to any other atoms. All samples of matter have atoms so when we are thinking of chemical entities we need to distinguish between free atoms, monatomic ions and bonded atoms. Example: Consider a mixture of the gasses helium and oxygen. Helium is a ble or inert gas which does not form compounds (does not form bonds with other atoms or gain or lose electrons) under most conditions. Therefore helium gas consists of individual free atoms. Oxygen on the other hand is very unstable and tends to react by forming bonds or losing electrons. As a result, oxygen gas is made up of diatomic O 2 molecules. A mixture of oxygen and helium is then composed of O 2 molecules and He atoms. There are no free O atoms. Thus chemical entities present are He atoms and O 2 molecules. More Essential Definitions: Homogenous mixtures in chemistry are strictly defined as mixtures in which the constituent chemical entities are uniformly distributed throughout the mixture. If you could view the sample under a powerful enough microscope, every view would have the same atoms (free atoms), ions and molecules in the same relative ratios distributed randomly throughout the view. A homogenous mixture is also referred to as a solution. Solution homogenous mixture The symbol means is defined as. Heterogeneous mixtures in chemistry are strictly defined as mixtures in which the constituent chemical entities are not uniformly distributed throughout the mixture. If you could view a heterogeneous mixture under a powerful enough microscope, different views show large regions having different compositions of free atoms, ions and molecules. For example, viewing a mixture of sand and water would yield views that were essentially totally water, some that were made of molecules of SiO 2 2 and some views that had both regions of only water molecules and only sand molecules. 1 Chemical properties in a sample can be deduced indirectly from the identification of functional groups which can be done by physical means (e.g.ir spectroscopy). 2 More accurately, sand or silica would be composed of polymeric SiO2 molecules with the generic formula (SiO2)n, where n is a very large and variable integer.

4 Edited 8/19/2014 Identifying Heterogeneous (easy identification) - Many heterogeneous mixtures are visibly heterogeneous: Examples: granite, the sand-water mixture described above, a mixture of salt and pepper. - Some heterogeneous mixtures can be identified by cloudiness or opacity. Examples: smoke (a mixture of air and solid particulates); clouds (water droplets and air); homogenized milk (a mixture of water, dissolved compounds like sugars and salts, small oil droplets and casein micelles 3 which are aggregates of several thousand protein molecule. *** Whenever you see cloudiness or opacity, you should suspect the presence of small droplets or particles. The cloudiness results from light scattering off of the particles. - Some heterogeneous materials can be identified by a knowledge of their makeup or history. Examples: - Styrofoam is a heterogeneous mixture of gas in a solid plastic material (polystyrene). - Clay is a heterogeneous mixture of finely divided mineral (dirt) and water. We know that it is not homogenous because we know that dirt does not dissolve in water (they do not form a homogenous mixture). - Many biological materials (plant or animal tissues or products of plant and animal tissues) such as meat, wood, fat, paper, leather) because those tissues are made up of cells and we know that biological cells have a complex (think heterogeneous) structure. Identifying homogenous mixtures (more comprehensive method) Technical Definition : a sample of matter that can be separated into different substances using only physical means is a mixture. Since physical changes do not change the substances that make up a sample, any sample that can be separated into samples having different identifying properties has to be a mixture. One limitation of this method is that if you cannot separate a sample by physical means you cannot be 100% sure that it is a pure substance (not a mixture) even if you have exhausted all possible known methods of physical separation. Instead we must examine other properties to determine whether or not we have a substance (pure element or compound). Remember, pure chemical (element or compound) substance in chemistry. Instead we use a variety of other properties to identify something as a unique substance: - Elemental composition (compounds have a fix composition) - Physical properties (identifying properties): color, state (solid, liquid or gas) at specific temperature and pressure 4, melting point, boiling point, density, specific heat capacity. Spectroscopic properties: how does it interact with light? Electrical properties. - Chemical properties: how reactive (unstable) is it? What does it react with and what products are formed? What chemical reactions will form the compound? te: mixtures can be separated by chemical means as well, but this cannot be used to identify a mixture. In reality, such a process involves converting one or more substances to different substance(s), separating the products and then chemically reconverting the products back into the original sample constituents. For example sand could be separated from powdered graphite (carbon) by heating the sample to high temperature below, but near the melting point of sand and passing oxygen gas over the sample to convert the graphite to carbon dioxide leaving the sand. If desired, the carbon could be converted back to graphite by decomposing the carbon dioxide. 5 3 To Learn more about casein micelles in milk see Usually a pressure of usually 1 atmosphere (14.7 psi) and room temperature which is 25 o C is used. 5 Reduction of carbon dioxide to graphite carbon via methane by catalytic fixation with membrane reactor Hiroyasu Nishiguchi, Akira Fukunaga, Yumi Miyashita, Tatsumi Ishihara, Yusaku Takita, Studies in Surface Science and Catalysis, Volume 114, 1998, Pages

5 Edited 8/19/2014 Ten Signs of Chemical Change From 1. Bubbles of gas appear. Gas-producing reactions run to completion when the gas can leave the reaction mixture. Gaseous products appear as bubbles only after the reaction mixture has become saturated with the gas. If there are no rough surfaces or dust particles to nucleate the bubbles, the reaction may produce a supersaturated gas solution. Bubbles can also be produced when the liquid boils, or when air dissolved in the liquid comes out of solution as the liquid is warmed. 2. A precipitate forms. When mixing a pair of soluble reactants in solution, the sudden appearance of a solid that 'rains down' (precipitates) into the bottom of the container is a sign that a reaction has occurred. Sometimes the precipitate particles are too small to settle out; in this case, look for a cloudy solution. If the amount of precipitate is very small, you can sometimes detect fine particles of precipitate suspended in the solution by shining a very bright light through the liquid. If you can see the beam of light in the solution, fine suspended particles are present. For reactions that occur in a melt, don't confuse precipitation with simple freezing. 3. A color change occurs. Every compound absorbs a characteristic set of colors of light. This absorption spectrum is a chemical fingerprint for detecting the presence of that compound. When the compound is altered in a chemical reaction, the fingerprint will change- and so the color of the reacting mixture may change, as the reaction progresses. (For a more detailed explanation of why color change accompanies some chemical reactions, see Water to Wine). It is possible to have a color change without a chemical change, however, because a compound's absorption spectrum isn't the only thing that affects its color. For example, heating zinc oxide changes it from white to yellow but no real chemical change occurs. The color change is caused by holes and other defects that are created in the zinc oxide lattice as the compound is heated. 4. The temperature changes. It takes energy to break chemical bonds. And energy is released when new chemical bonds form. When the reaction involves more bond-breaking than bond making, the energy required is often absorbed from the surroundings, making them cooler. When there is more bond-making than bond-breaking, the excess energy is released, making the surroundings hotter. 5. Light is emitted. Sometimes energy is released by bond-forming reactions in the form of light. This occurs in most combustion reactions. Living things that glow in the dark- such as fireflies, funguses, and deep sea creatures- produce light without heat, using chemical reactions. 6. A change in volume occurs. Density is a characteristic of a compound, and if new compounds are produced as othe compounds are consumed in the reaction, the change in density can cause the reacting mixture to expand or contract as the reaction proceeds. Sometimes this volume change can be large and very rapid- and an explosion occurs! Of course, volume increases and decreases will also accompany temperature and pressure changes, so these must be held constant if volume is being monitored to see if a reaction is occurring. 7. A change in electrical conductivity occurs. Some reactions produce or consume ions (charged particles) in a solution. Changes in the character and concentrations of the ions will cause the reacting mixture's ability to conduct electricity to change. 8. A change in melting point or boiling point occurs. The melting or boiling point is characteristic of a compound; when the composition of a mixture changes, the melting point and boiling point also change. 9. A change in smell or taste occurs. Since many chemical reactions have poisonous reactants or products, this method of detecting chemical change isn't recommended! 10. A change in any distinctive chemical or physical property occurs. Pick a property that uniquely characterizes one of the compounds involved in the suspected reaction, and monitor it. If the property really distinguishes that compound from all the others, you'll see it change when a reaction occurs.

Lab 7: Metathesis Reactions. Pre-lab: Pages Post-Lab: Pages 164

Lab 7: Metathesis Reactions. Pre-lab: Pages Post-Lab: Pages 164 Lab 7: Metathesis Reactions Pre-lab: Pages 159-160 Post-Lab: Pages 164 Lab Objectives Careful observation and detailed description of chemical reactions in solution Inferring from observation and from