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Understanding Elements, Compounds, and Mixtures

This unit provides an explanation of the differences between elements and compounds, atoms and molecules, and various types of mixtures. It also explores the concepts of emulsions, detergents, and their uses in daily life and industry.

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Understanding Elements, Compounds, and Mixtures

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  1. At the end of this unit you should: 1. Be able to explain the differences between an element and a compound. 2. Be able to explain the differences between an atom and a molecule. 3. Know that there are different types of mixtures and be able to explain their differences. 4. Know that an emulsion is a type of colloid, as well as the uses of some emulsions in daily life and industry. 5. Know what a detergent is and how it is important in daily activities, as well as its environmental effects.

  2. atom ball and stick model colloid compound decanting detergent element emulsion matter mixture molecule Periodic Table of the Elements solutions suspension symbol

  3. LIGHTBULB QUESTION An atom or a molecule can never mean the same thing, but pure elements can be found as either separate atoms or molecules.

  4. Matter: Anything that takes up space and has mass. Element: A substance made up of only one type of atom. Atom: The smallest particle of an element that has the properties of that element.

  5. (a) Copy and complete Table 06.04.02 by writing the missing symbol beside each name. List of Symbols: Cr V Po Si Sn Zn ZrNb Co Cl Mn Mg PbOs

  6. (b) There are three extra symbols in the list. What are the names of these elements? List of Symbols: Cr V Po Si Sn Zn ZrNb Co Cl Mn Mg PbOs

  7. (b) There are three extra symbols in the list. What are the name of these elements? List of Symbols: Cr V Po Si Sn Zn ZrNb Co Cl Mn Mg PbOs V – vanadium; Nb – niobium; Os – osmium.

  8. DEMONSTRATION Demonstration 06.04.01 – Testing sodium and sodium chloride properties Equipment: Water trough, scalpel, clock glass, forceps, wooden splint, cigarette lighter, safety screen, samples of sodium metal, sodium chloride, 400 ml beaker, spatula.

  9. DEMONSTRATION Demonstration 06.04.01 – Testing sodium and sodium chloride properties Instructions: 1. Half-fill a water trough with tap water and place behind a safety screen. Students should be 1.5 m from the screen, and the room should be well ventilated. 2. Using a forceps, remove a cylindrical pellet of sodium from its container. (Note that it is stored under oil.) 3. Place the pellet onto a clock glass, and using the scalpel cut a sliver, no thicker than approximately 3 mm, from the pellet. Note that the colour of the freshly cut metal dull as it oxidises. The forceps may be needed to prevent the pellet from sliding off the clock glass due to the oil.

  10. DEMONSTRATION Demonstration 06.04.01 – Testing sodium and sodium chloride properties Instructions: 4. Return the pellet to the sodium container and secure the lid before proceeding further. 5. Light a splint. 6. Drop the sample into the water down the inside of the water trough. 7. As the reaction becomes more vigorous, the sample should move across the trough. The liberated hydrogen gas can be lit with the lighted splint. The reaction will fizzle out or may finish with a very small ‘pop’ sound. Note: If the sample adhered to the glass wall it may cause a hot spot, which may cause the wall of the trough to crack due to heat shock. Use the forceps to move the sample away from the glass wall if this happens.

  11. DEMONSTRATION Demonstration 06.04.01 – Testing sodium and sodium chloride properties Instructions: 8. Using clean tissue, clean any residue from the clock glass, forceps and scalpel. 9. If the sample is big enough, the sodium may self-ignite as the oxidation reaction will be enough to ignite the liberated hydrogen gas. The reaction should finish in a small ‘pop’ sound. Potassium has a vigorous reaction so tends to self-ignite regardless of sample size. Caution is needed as the reaction nearly always ends in a popping sound. If the sample is big enough this can eject debris. 10. Some sodium chloride can simply be stirred into a beaker of tap water

  12. DEMONSTRATION Demonstration 06.04.01 – Testing sodium and sodium chloride properties Results:

  13. 1. Which one of these substances is an element? Justify your answer. • Only the Group 1 elemental metals react in this way. So although it may be difficult to know which metal is which from this simple test, you do know that it is an element, not a compound.

  14. 2. Why can your teacher not safely demonstrate chlorine in the school laboratory? (Hint: See Unit 6.3.) • Chlorine gas is poisonous so cannot be used except in a sealed fume hut.

  15. Molecule: Composed of two or more atoms chemically combined. Compound: Composed of two or more different elements chemically combined.

  16. Using the colour code for different elements, can you draw or make the ball and stick models of these three molecules: • sodium chloride – NaCl

  17. Using the colour code for different elements, can you draw or make the ball and stick models of these three molecules: • ammonia – NH3

  18. Using the colour code for different elements, can you draw or make the ball and stick models of these three molecules: • carbon tetrachloride – CCl4

  19. Mixture: Made of two or more substances mingled together but not chemically combined.

  20. Lay out a table in your copy which lists mixtures and compounds. Construct a table with the headers ‘Mixtures’ and ‘Compounds’.

  21. DEMONSTRATION Demonstration 06.04.02 – Sulfur and iron: mixture and compound Equipment: Four boiling tubes, two measures 10 g sulfur powder, two measures 10 g iron filings, tongs, test tube rack, cork/rubber bung, bar magnet, top-pan balance, weigh boats, brûlée mini-blowtorch (or Bunsen burner).

  22. DEMONSTRATION Demonstration 06.04.02 – Sulfur and iron: mixture and compound Instructions: 1. Find the mean mass of the test tubes. 2. Place 10 g of sulfur powder in each of two test tubes. 3. Place 10 g of iron filings in each of two test tubes. 4. Test each test tube with the bar magnet, by running the magnet from the bottom to the rim of the test tube. 5. Pour one lot of iron filings into one lot of sulfur powder, and stopper with the cork. Shake well to mix. 6. Test the test tube with the bar magnet.

  23. DEMONSTRATION Demonstration 06.04.02 – Sulfur and iron: mixture and compound Instructions: 7. Un-stopper the test tube. 8. Place the test tube in the tongs, and heat the test tube with the mini-blowtorch until the contents glow red-hot. Allow to cool. 9. Test the contents with the magnet. 10. Find the mass of the test tube and contents. Subtract the mean mass of test tubes.

  24. 1. Is the iron sulfur mixture a different substance to the iron sulfur compound? Suggest more than one reason for your answer. • Yes, the iron sulfur mixture is a different substance to the iron sulfur compound. Iron sulfide is not magnetic and is a different colour to the mixture.

  25. 2. Explain whether or not these results agree with the Law of Conservation of Mass. • As no mass is lost in this investigation, it agrees with the Law of Conservation of Mass.

  26. Investigation 06.04.01: Is it a compound or mixture? Equipment: Mortar and pestle, 50 g of iron-fortified breakfast cereal (Special K), zip-lock bag, neodymium magnet, water. Instructions: 1. Place 50 g of fortified breakfast cereal into a zip-lock bag, and seal. 2. Run the magnet over the surface of the bag and record your observations. 3. Empty the breakfast cereal into the mortar. Using the pestle, grind the breakfast cereal to a powder, and place into the zip-lock bag.

  27. Investigation 06.04.01: Is it a compound or mixture? Instructions: 4. Test again with the magnet. 5. Half-fill the bag with water and slosh the cereal in the bag until the contents are soup-like. 6. Allow the bag to sit for thirty minutes, and test again with the magnet. Weblink To see a similar method being used, look up ‘Iron man experiment: Extract the iron from your cereal’: https://www.youtube.com/watch?v=ZIyKe9VE6o8

  28. 1. What evidence can you use to decide whether the iron in the breakfast cereal is part of a compound or a mixture? How do your observations help you make your decision? • Once the cereal was crushed, iron could be extracted and was more easily separated when the cereal was dissolved.

  29. 2. Why was it important to slosh the breakfast cereal about in the bag with water? • To ensure that the water and cereal mixed to dissolve the cereal more thoroughly.

  30. Rewrite Table 06.04.05. Complete it using your own knowledge and with the help of your fellow students and your teacher.

  31. Suspension: A mixture where insoluble solids will sink to the bottom of the solvent if left still. Colloid: A mixture in which insoluble particles spread evenly through a mixture and will cause a light beam to scatter. Emulsion: A mixture in which two liquids do not mix but one spreads evenly through the other in droplets.

  32. Detergent: A substance that causes oil to spread evenly in water-based solutions.

  33. Investigation 06.04.02: Can water and oil mix? Equipment: Test tube rack, two test tubes, two corks (rubber bungs), tap water, washing-up liquid, cooking oil, Pasteur pipette. Instructions: 1. Half-fill each test tube with water and add an equal number of drops of cooking oil to the surface of the water in each test tube. 2. Cork one of the test tubes and shake vigorously. Then allow to settle. 3. Add one small drop of washing-up liquid to the other test tube, cork and shake vigorously. 4. Add detergent drop by drop, shaking between each addition until a cloudiness can be seen throughout the liquid.

  34. 1. How can you make sure you are carrying out a fair test? • Equal amounts of water and oil should be used in each test tube. Each shaking should last equally long and be equally vigorous.

  35. 2. If the oil you used was mixed with other liquids, would you get the same results? Justify your answer. • It would depend on whether the other liquid was similar or dissimilar to water. The more like water the liquid is, the more detergent will be needed for dispersion.

  36. 3. Was there a colloid in this investigation? How can you prove your answer? • The emulsion formed in the second test tube is a colloid and can be proven by shining a laser pointer through the test tube.

  37. Copy and Complete In this unit I learned that an element is made up of one type of atom. A molecule is made up of two or more atoms chemically combined. A compound is made up of two or more different elements chemically combined. Two or more substances blendedtogether but not chemically combined is called a mixture. Particles sink to the bottom if it is a suspension type of mixture. If insoluble solute particles are spread evenly through the solvent it is called a colloidtype of mixture.

  38. 1. Write down the correct symbols from the periodic table for these elements. Which of these elements are gases? Which of these normally exist as pure molecules? Chlorine – Cl Lead – Pb Oxygen – O Molybdenum – Mb Thallium – Th Helium – He Chlorine, oxygen and helium are all gases and exist naturally as molecules of two atoms (a dimer).

  39. 2. Marie Curie showed the existence of the element radium and she produced 0.1 g of the compound radium chloride in 1902 by processing tonnes of pitchblende ore, obtained from mines in Bohemia. Explain the underlined terms. An element is a substance made up of only one type of atom. A compound is made up of two or more different elements chemically combined.

  40. 3. Robert Boyle introduced the word ‘element’ into the language of chemistry. Copy and complete Table 06.04.07, identifying each of the substances listed by ticking the box for element, mixture or compound.

  41. 4. The apparatus shown in Fig. 06.04.12 was used to strongly heat 2.4 g of magnesium in a crucible. The lid of the crucible was left slightly off during the heating. A white powder with a mass of 4.0 g was produced. (i) Identify the compound in this experiment. Magnesium oxide.

  42. 4. The apparatus shown in Fig. 06.04.12 was used to strongly heat 2.4 g of magnesium in a crucible. The lid of the crucible was left slightly off during the heating. A white powder with a mass of 4.0 g was produced. (ii) List two ways in which it is different to the element used in the experiment. It is a white powder, which does not have a shiny surface.

  43. 4. The apparatus shown in Fig. 06.04.12 was used to strongly heat 2.4 g of magnesium in a crucible. The lid of the crucible was left slightly off during the heating. A white powder with a mass of 4.0 g was produced. (iii) Name the mixture that was present in the experiment. Air.

  44. 5. Copy and complete Table 06.04.08. Decide which substances are colloids by ticking the True or False box for each substance.

  45. 6. Pumice is a type of rock formed when volcanoes erupt. Why can pumice be classified as a colloid? Pumice can be classified as a colloid as air bubbles disperse through a lava flow before the lava hardens but the air never truly mixes through the lava.

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