In order to identify chemical substances we need a good understanding of how these substances form and what their properties are.

Transcription

1 P & L. Johnson 2012

2 Crack cocaine is a highly addictive and powerful stimulant that is derived from powdered cocaine using a simple conversion involving baking powder, bicarbonate of soda. Crack emerged as a drug of abuse in the mid-1980s. It is abused because it produces an immediate high and because it is easy and inexpensive to produce, rendering it readily available and affordable. During raids on suspected crack houses drug squad officers often look for the chemicals and equipment used to make the crack as often the drug itself has been sold or destroyed. It is illegal to produce crack cocaine and so identifying the chemicals used in its manufacture are just as important as finding the drug, if a suspect is to be convicted. In order to identify chemical substances we need a good understanding of how these substances form and what their properties are. 1. When Elements Combine Page 4 2. Covalent Bonding Page 8 3. Ionic Bonding Page Properties Page Unknown Powders Page 26 P & L. Johnson

3 Nat 4 outcomes O Nat 5 outcomes By the end of this unit you should know the following: What the diatomic elements are and the types of bonds they form. That the valency is the number of unpaired outer electrons. How to calculate the valency of an element from its group number. Covalent bonds form between two non-metals atoms forming a molecule with a fixed number of atoms. Be able to show how covalent bonds form by the overlapping of valence electrons. How the attraction of the positive nuclei for the shared pair of electrons holds the atoms together. Ionic lattices form when metal atoms transfer their outer electrons to the non-metal atom forming oppositely charged ions. Properties of covalent molecules, networks and ionic lattices. The shape of the molecule can be drawn, with symbols representing bonds pointing forwards or backwards. Molecules with four bonds have a tetrahedral shape. Covalent networks have an unlimited number of atoms held by covalent bonds. The five most common are carbon, silicon, boron, silicon oxide and silicon carbide. Covalent molecules have low melting points, due to weak forces of attraction between the molecules, covalent networks have very high melting points and ionic lattices also melt at high temperatures due to the strong bonds that need to be broken. Covalent molecules and networks don t conduct electricity in any state as they have no mobile charged particles. Ionic lattices only conduct as melts or solutions as then they have mobile ions. Ionic compounds tend to be more soluble in aqueous solvents; covalent molecules tend to be more soluble in non-aqueous solvents. Covalent networks are in soluble in any solvent. P & L. Johnson

4 The most unreactive elements are the noble gases. When we look at their electron arrangement we see that they always have a full outer shell. It is assumed that this is what causes them to be so unreactive. All other elements have incomplete outer shells and join together in order to have a complete outer shell. This is known as the Octet Rule of Thumb. When atoms join together they form chemical bonds. Non-metal elements will bond together to form groups of two or more atoms, these are called molecules. When there are just two atoms in a molecule it is called a diatomic molecule. Molymod kits can be used to make models of different molecules. Each coloured ball represents a different element, the balls have a number of holes which represent the number of bonds that the element can form. ACTIVITY 3.1 Molymod Kits Use a Molymod kit to identify which colour each of the elements in the table are represented by and how many bonds they can form. Complete the following table after discussion with your teacher and others: Element Colour Number of Bonds Carbon Black 4 Hydrogen White 1 Oxygen Red 2 Nitrogen Blue 3 Chlorine Green 1 Sulfur Yellow 2 P & L. Johnson

5 ACTIVITY 3.2 Making Molecules Use the Molymod kit to join a hydrogen atom and chlorine atom together using the short grey bond. The molecule formed is called hydrogen chloride and has the chemical formula HCl. The chemical formula tells you the number of atoms of each type of element in the molecule. Continue to use the Molymod kit to make models given in the table below, drawing a picture of your model and working out its chemical formula. Complete the following table after discussion with your teacher and others. Molecule Drawing Chemical Formula Hydrogen oxide H 2 O Nitrogen hydride NH 3 Carbon chloride CCl 4 Carbon dioxide CO 2 Carbon hydride CH 4 P & L. Johnson

6 Drawing Molecules Drawing pictures of molecules is time consuming and so we can simplify this by using symbols to represent the atoms and straight lines to represent the bonds. We also draw the molecules flat with bonds at 90 0 to one another. This is known as a molecule s structural formula. E.g. H H C H Carbon hydride H DISCUSS After discussion with your teacher and others make sure you can draw the structural formula of different molecules. Complete the following table: Molecule Chemical Formula Structural Formula Hydrogen oxide H 2 O Nitrogen hydride NH 3 Carbon chloride CCl 4 Carbon dioxide CO 2 P & L. Johnson

7 Diatomic Elements There are a number of elements that exist as diatomic molecules. They do this by two of their atoms bonding to together. ACTIVITY 3.3 Diatomic Elements Make a model of a hydrogen molecule by joining two hydrogen atoms together with a single bond. Next make models of the elements in the table below, make sure you join them together so that they have the correct number of bonds. Note whether they have a single, double or triple bond. Complete the following table: Element Chemical Formula Type of Bond Hydrogen H 2 Single Oxygen O 2 Double Nitrogen N 2 Triple Chlorine Cl 2 Single There are 7 diatomic elements: hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine & iodine. Remember them as the magnificent 7 H 2 N 2 O 2 F 2 Cl 2 Br 2 Or BOFINCH Or GENINE (all the elements ending in either -gen or -ine) I 2 P & L. Johnson

8 Electron Arrangement Again In the previous topic on atomic structure you saw that the electrons in an atom moved around the nucleus in fixed energy levels or shells. To keep things simple we used a circle to represent the energy level, but in reality the size and shape of where the electrons move varies. When describing covalent bonding, molecular orbitals are often used, this is the shape of the electron clouds as the atoms begin to bond. The first shell closest to the nucleus is spherical and can hold a maximum of 2 electrons shown here as red dots. The second shell has four clouds in a tetrahedral shape and can contain a maximum of 8 electrons, 2 in each cloud shown here as red dots. The third shell is also tetrahedral shaped but is larger it can also hold a maximum of 8 electrons, 2 in each cloud shown here as red dots. Electrons will fill these orbitals (clouds) singularly before pairing up. The valency of an element is the number of unpaired electrons in the outer shell. In the following example an oxygen atom has an electron arrangement of 2,6. It has 6 electrons in its outer shell, two of which are unpaired so it has a valency of 2. It will also require 2 electrons to have a full outer shell. To the right is a simplified drawing of the outer electrons in oxygen as it begins to combine with another element. P & L. Johnson

9 In the space below draw the shape of the outer shell with their electrons of the elements hydrogen, carbon, nitrogen and chlorine: Using page 6 of your data book complete the following table: Element Hydrogen Carbon Nitrogen Oxygen Fluorine Electron Arrangement Number of electrons needed to fill outer shell 1 2,4 2,5 2,6 2, Valency Element Silicon Phosphorus Sulfur Chlorine Electron Arrangement Number of electrons needed to fill outer shell 2,8,4 2,8,5 2,8,6 2,8, Valency P & L. Johnson

10 When non-metals bond to one another they achieve a complete outer shell by over lapping their clouds that have unpaired (valence) electrons in them. Shown below is the way hydrogen and oxygen combine to form hydrogen oxide (water). The outer shell of the oxygen atom now has 8 electrons and is full. The electron shell of the hydrogen atoms is also full. They have achieved this through the sharing of their valence electrons, this is known as covalent bonding. This only occurs with non-metal elements. DISCUSS After discussion with your teacher and others make sure you can show how covalent bonds occur in simple two element compounds. In the space below show how the following simple two element compounds form through covalent bonding: Nitrogen hydride Sulfur hydride Carbon dioxide Carbon chloride P & L. Johnson

11 What holds the Atoms Together? When a covalent bond forms two non-metal atoms share their valence electrons to form complete outer shells. If we look at the simplest molecule, hydrogen, H 2 we can see what holds the atoms together. Like a tug of war the positive proton in each nucleus is attracted to the shared pair of electrons between the two atoms and tries to pull the negatively charged electrons towards itself. It is this force of attraction which holds the two atoms together and forms the bond. The bonds are directional and hold the atoms in particular shapes, depending on the type of atoms bonding. Shapes of Molecules Many two element molecules are linear, like hydrogen chloride and carbon dioxide, but other have different shapes. Carbon hydride has what is called a TETRAHEDRAL shape and nitrogen hydride has a PYRAMID shape. We can draw these molecules by using symbols to represent the bonds pointing out of and into the page. After discussion with your teacher and others show the shape of carbon chloride CCl 4 and phosphorus hydride PH 3 in the space below: P & L. Johnson

12 Molecules and Networks When two or more non-metal atoms join together a covalent bond forms. Most of the time a small number of atoms combine in fixed numbers, this is known as a covalent molecule i.e. hydrogen and oxygen join to form hydrogen oxide, water, with the chemical formula H 2 O. Water will always have the chemical formula H 2 O, with one oxygen atoms joining up with two hydrogen atoms. However in a small number of cases the number of atoms joining together is huge and the number of atoms is not fixed. When this occurs a giant lattice forms containing billions of atoms, this is known as a covalent network. An example of a covalent network is silicon oxide which is what sand is mainly made up of. The ratio of oxygen to silicon atoms is always 2:1 and so silicon oxide has the formula SiO 2. The picture above shows a tiny fragment of the network but you can see that the number of red oxygen atoms is twice that of the grey silicon atoms. The element carbon has two forms that are both covalent networks, diamond and graphite. Silicon and boron are also elements with a covalent network structure. Silicon carbide is another covalent network compound. DISCUSS After discussion with your teacher and others make sure you know the five most common covalent networks structures. Complete the following table: Covalent network elements Carbon Silicon Boron Covalent network compounds Silicon Carbide Silicon Oxide Boron Silicate RESEARCH TASK Using the Internet and other resources to help, produce an advertising pamphlet explaining how real gem quality diamonds can be manufactured in less than a week. P & L. Johnson

13 Quick Test 1 1. Which of the following compounds is likely to be covalently bonded? A. Sodium fluoride B. Potassium chloride C. Silicon tetrachloride D. Barium fluoride 2. Which of the following is not a diatomic molecule? A. CO B. H 2 C. H 2 O D. HCl 3. Which of the following is not made up of molecules? A. HF B. SO 3 C. NH 3 D. CaF 2 4. The shape of a methane molecule, CH 4 is A. linear B. bent C. pyramidal D. tetrahedral 5. Silicon dioxide, SiO 2, has a covalent network structure. This means that A. there is one silicon atom and two oxygen atoms in each molecule B. the structure contains silicon and oxygen atoms in the ratio 1:2 C. there are twice as many oxide ions as there are silicon ions D. the structure contains silicon and oxygen atoms in the ratio 2:1 6. Hydrogen reacts with chlorine to form hydrogen chloride. Draw a diagram to show how the outer electrons are shared in a molecule of hydrogen chloride. P & L. Johnson

14 As well as sharing outer electrons in order to achieve a stable full outer shell, atoms can also transfer their outer electrons. This occurs when metals combine with non-metals, the resulting atoms are charged and are called ions. E.g. sodium and chlorine form sodium ions and chloride ions (note the ending of the non-metal. Ionic bonding occurs between metals and non-metals. Na Cl After discussion with your teacher and others complete the following tables: Number of protons Number of electrons Overall Charge Sodium Atoms Number of protons Number of electrons Overall Charge Chlorine Atoms Na Cl Number of protons Number of electrons Overall Charge Sodium Ions P & L. Johnson Number of protons Number of electrons Overall Charge Chloride Ions _

15 Ionic Lattices The ions that form during ionic bonding are charged. They are charged all the way round and so can attract oppositely charged ions in all directions. _ _ _ + _ + _ + _ + _ + _ The ions join together in a giant 3D structure called an ionic lattice, a single grain of sodium chloride could contain billions of sodium and chloride ions, but they will be in a ratio of 1:1. It is possible to work out the charge on an ion by looking at what group it is in, in the Periodic Table. Metals are always positive and have a charge corresponding to their group number. Non-metals are always negative and have a charge corresponding to their valency. Group 4 non-metals tend not to form ions. ACTIVITY 3.4 Ionic Charges Your teacher may let you play this card game. DISCUSS After discussion with your teacher and others make sure you can use your data booklet to workout the charge on the ions of different elements. Complete the following table: Group Element Na Mg Al N O F Charge Element K Ca Ga P S Cl Charge P & L. Johnson

16 ACTIVITY 3.5 Bonding Sentences & Bonding Type Card Games Your teacher may let you play these card games. Complete the following set of notes about the different types of bonding: Covalent Molecular: When two covalent atoms share their unpaired outer (valence) electrons by overlapping their electron clouds a covalent bond forms. The attraction of the two positive nuclei for the pair of shared electrons holds the atoms together. If there are a fixed number of atoms in the structure it is known as a covalent molecule. An example would be water (hydrogen oxide), a drop of water will contain billions of individual water molecules. Covalent Network: When two non-metal atoms share their unpaired outer (valence) electrons by overlapping their electron clouds a covalent bond forms. The attraction of the two positive nuclei for the pair of shared electrons holds the atoms together. If there are an unlimited number of atoms in the structure it is known as a covalent network. An example would be sand (silicon dioxide), a grain of sand will contain billions of individual atoms all held together in a single giant network. Ionic Lattice: When metal and non-metal atoms transfer their unpaired outer ( electrons ) electrons from the metal atom to the non-metal atom a positive metal ion and a negative non-metal ion form. The oppositely charged ions attract each other and form ionic bonds. As the ions are charged they attract each other in all directions and form giant lattices. An example would be salt (sodium chloride), a grain of salt contains billions of ions in a single giant lattice. Covalent Network Ionic Lattice Covalent Molecular P & L. Johnson

17 Quick Test 2 1. Which of the following compounds is likely to contain ionic bonding? A. Sodium chloride B. Sulfur dichloride C. Carbon dioxide D. Phosphorus trifluoride 2. The ions in an ionic crystal are held together by A. the attraction of the two nuclei towards a shared pair of electrons B. the attraction of the two nuclei towards delocalised electrons C. the attraction between positive and negative ions D. electron- electron attractions 3. When an ionic compound forms, A. metal atoms lose electrons to form positive ions and non-metal atoms gain electrons to form negative ions B. metal atoms gain electrons to form negative ions and non-metal atoms lose electrons to form positive ions C. metal atoms lose electrons to form negative ions and non-metal atoms gain electrons to form positive ions D. metal atoms gain electrons to form positive ions and non-metal atoms lose electrons to form negative ions 4. An ion has 16 neutrons,15 protons and 18 electrons. The ion can be represented by A. B. C. D. 5. Which of the following statements about calcium fluoride, CaF 2, is true? A. there is one calcium atom and two fluorine atoms in each molecule B. the structure contains calcium and fluorine atoms in the ratio 2:1 C. there are twice as many negative fluoride ions as there are positive calcium ions D. there are twice as many positive calcium ions as there are negative fluoride ions P & L. Johnson

18 There are number of physical properties that we can look at to help identify the type of bonding present in a substance: Melting point, electrical conductivity and solubility; the different types of bonding will influence these properties. Prediction DISCUSS Think of some examples of substances that are covalent molecules, covalent networks and ionic lattices and then try to think what their properties are. After discussion with your group complete the following table: Bonding Type Melting Point (High, Med or Low) Electrical Conductivity (Yes or No) Solubility in water (Yes or No) Covalent Molecular Low No No Covalent Network High No No Ionic High As solution or melt Yes Melting Point When a substance melts forces of attraction, that hold the substance together as a solid, get broken. Depending on the structure of the solid different forces of attraction are broken. In a covalent network the solid particles consist of a network of atoms held together by strong covalent bonds. In an ionic lattice the solid particles consist of a lattice of ions held together by strong ionic bonds. However in a solid consisting of covalent molecules, the atoms in the molecules are held together by strong covalent bonds but the molecules are held together to form the solid by weak forces of attraction. P & L. Johnson

19 Covalent Network Strong covalent bonds Ionic Lattice Strong ionic bonds Covalent Molecular Weak forces of attraction The strength of these forces of attraction affect the melting point. The stronger the force the higher the melting point. Pages 5 & 9 of your data booklet gives the melting points of elements and selected compounds. ACTIVITY 3.6 Melting Points Use pages 5 & 9 of your data booklet to help complete the following table: Substance Bonding Type Melting Point 0 C Carbon Covalent 3825 Phosphorus Covalent 44 Silicon dioxide Covalent 1713 Nitrogen hydride (ammonia) Covalent -78 Hydrogen oxide (water) Covalent 0 Barium chloride Ionic 961 Calcium oxide Ionic 2614 After discussion with your teacher and others complete the following table: Bonding Type Melting Point Force of Attraction Broken Covalent Molecular Low Weak force of attraction Covalent Network Very high Lots of strong covalent bonds Ionic Lattice Very high Lots of strong ionic bonds P & L. Johnson

20 Electrical Conductivity ACTIVITY 3.7 Testing Electrical Conductivity In your group design an electrical circuit that could be used to test the electrical conductivity of different substances, as solids, liquids and solutions. After discussion with your teacher and others draw a diagram of your set up in the space below: After discussion with your group complete the following table: ACTIVITY 3.8 Electrical Conductivity of Ionic Melts Substance Bonding Type Electrical Conductivity Solid Liquid Solution Sugar (C 12 H 22 O 11 ) Water (H 2 O) Sand (SiO 2 ) Graphite (C) Covalent Molecular Covalent Molecular Covalent Network Covalent Network Your teacher may demonstrate to you the electrical conductivity of an ionic melt. P & L. Johnson No No No Yes Slightly Slightly Sodium chloride (NaCl) Ionic No Yes Potassium nitrate (KNO 3 ) Ionic No Yes

21 In order for a substance to be able to conduct electricity it must have mobile charged particles, such as electrons or ions. Covalent molecules do not conduct in any state because they have no mobile electrons. With the exception of graphite, covalent networks do not conduct in any state because the have no mobile electrons. Graphite consists of rings of carbon atoms each with only 3 bonds that leaves each carbon atom having a spare electron that can move around the rings. These are known as delocalised electrons and allow the graphite to conduct electricity. Ionic compounds contain ions, but when they are solids they do not conduct as the ions are held in the lattice and are therefore not mobile. When an ionic compound melts or dissolves to form a solution the ions are mobile and the melt or solution conducts as the ions move towards the oppositely charged electrode. This process is called electrolysis. DISCUSS After discussion with your teacher and others make sure you can explain the electrical conductivity found in different substances. After discussion with your teacher and others complete the following table: Bonding Type Covalent Molecular (Solid, liquid or solution) Covalent Network (e.g. Silicon dioxide) Covalent Network (e.g. graphite) Ionic (Solid) Ionic (Liquid or solution) Electrical conductivity Insulator Insulator Conductor Insulator Conductor Explanation No charged particles able to move, since electrons are shared in bonds. No charged particles able to move, since electrons are shared in bonds. Graphite has one delocalised electron per C, able to move and carry charge. The charged particles (ions) cannot move to carry charge. The ions are able to move and carry charge. P & L. Johnson

22 Electrolysis When an ionic solution or melt conducts electricity, the ions move towards the oppositely charged electrode. The negatively charged non-metal ions lose their extra electrons to the positive (anode) electrode, turning back into neutral atoms. The positively charged metal ions gain electrons from the negative (cathode) electrode and turn back into neutral atoms. ACTIVITY 3.8a Electrolysis of copper(ii) chloride solution Set up the following circuit: Carbon electrode Copper(II) chloride solution DISCUSS After discussion with your teacher and others make sure you can explain how the products form at each electrode during the electrolysis of copper(ii) chloride. After discussion with your teacher and others complete the above diagram. Ion-electron 1/2 equations can be used to show what happens at each electrode: Positive (anode) 2Cl - (aq) Cl 2(g) + 2e - Negative (cathode) Cu 2+ (aq) + 2e - Cu (s) After discussion with your teacher and others try to write ion-electron 1/2 equations for the electrolysis of a lead(ii) iodide melt. Positive (anode) 2I - (l) I 2(g) + 2e - Negative (cathode) Pb 2+ (l) + 2e - Pb (l) P & L. Johnson

23 Solubility When a solid dissolves the forces of attraction that hold the particles together are broken. New forces of attraction between the particles and the solvent molecules occur. ACTIVITY 3.9 Testing Solubility In your group design a fair test for testing the solubility of various substances listed below, using an aqueous solvent like water and a non-aqueous solvent like petroleum ether: Sodium chloride, potassium nitrate, calcium carbonate, sand (silicon dioxide), wax (small grains), sugar (C 12 H 22 O 11 ) and sulfur (S 8 ). After discussion with your group complete the following table: Substance Bonding Type With solubility there are many exceptions to any rules but in general: Ionic compounds tend to be more soluble in aqueous solvents Covalent molecules tend to be more soluble in non-aqueous solvents Covalent networks are insoluble in any solvent. Page 8 in your data booklet gives the solubility of various ionic compounds in water. P & L. Johnson Water (aqueous) Solubility Petroleum ether (Non-aqueous) Sodium chloride (NaCl) Ionic Soluble Insoluble Potassium nitrate (KNO 3 ) Ionic Soluble Insoluble Calcium carbonate (CaCO 3 ) Ionic Insoluble Insoluble Sand (SiO 2 ) Covalent Insoluble Insoluble Wax (C 40 H 82 ) Covalent Insoluble Soluble Sugar (C 12 H 22 O 11 ) Covalent Soluble Soluble Sulfur (S 8 ) Covalent Insoluble Insoluble

24 ACTIVITY 3.10 Properties Sentences Game Your teacher may let you play this card game. DISCUSS After discussion with your teacher and others make sure you can explain the properties of different types of substances. Complete the following set of notes about the properties for the different types of bonding: Covalent molecules conduct electricity in any state because they have no electrons or ions that are free to move. They usually exist as gases, liquids or low melting point solids because as they melt only weak forces of attraction are being broken. Examples are carbon dioxide, petrol & sugar. Covalent molecules tend to dissolve in covalent solvents like carbon tetrachloride. Covalent networks, with the exception of graphite, do not conduct electricity in any state because they have no electrons or ions that are free to move. They only exist as high melting point solids because as they melt you have to break very strong covalent bonds. An example would be sand (silicon dioxide) which is insoluble in any solvent. Ionic lattices only conduct as a solution or melt because then they contain mobile ions that are free to move. They generally exist as high melting point solids because when they melt you have to break strong ionic bonds. An example would be salt (sodium chloride) which is soluble in water, a polar solvent. RESEARCH TASK Using the Internet and other resources to help, produce a Bonding for Dummies P & L. Johnson

25 Quick Test 3 1. Which of the following substances is a non-conductor of electricity? A. graphite B. iron C. sulfur D. magnesium 2. Which of the following substances is a conductor of electricity? A. Solid sodium chloride B. Paraffin wax C. Sucrose D. Molten lead(ii) iodide 3. Which of the following substances is a non-conductor of electricity? A. Copper(II) sulfate solution B. Solid lithium chloride C. Potassium bromide solution D. Molten sodium iodide 4. Which of the following substances has the lowest melting point? A. Copper(II) sulfate B. Silicon dioxide C. Silver chloride D. Phosphorus chloride 5. Which of the following substances has the highest melting point? A. Copper(II) sulfate B. Silicon dioxide C. Silver chloride D. Phosphorus chloride 6. Which of the following substances is soluble in an aqueous solvent? A. Copper(II) sulfate B. Silicon dioxide C. Silver chloride D. Carbon chloride P & L. Johnson

26 The hydrochloride (HCl) salt of cocaine is by far the most commonly encountered. This is the white powder that you see actors sniffing in TV shows and films. This form of cocaine is very soluble in water and hence the moist membrane in your nose, where it is absorbed into your bloodstream. It cannot however be smoked as the powder burns rather than vaporising. Crack cocaine became popular when it was discovered that by reacting the cocaine salt with bicarbonate of soda (baking powder) it was possible to create a substance that could be smoked and this gave an even quicker and more intense high, though it was quite short lived. When a drugs raid takes place all substances found in the den need to be analysed. Chemicals used in the manufacture of crack cocaine can be used as evidence to help convict somebody of drug dealing. Bicarbonate of soda (NaHCO 3 ) is a white powder and is the main reagent in the manufacture of crack. In order to make the sale of crack and cocaine more profitable dealers will often cut the drug with other cheaper chemicals that look similar, the presence of these in large quantities can also be used as evidence to help convict a dealer. Compounds commonly used to cut crack and cocaine are calcium carbonate (CaCO 3 ) or chalk, salt (NaCl) and starch ([C 6 H 10 O 5 ] n ). White Powders! During a drugs raid on a crack house a number of white powders were found. You will be given samples of each of these to test and to try and identify them. It is believed that the powders could be bicarbonate of soda, calcium carbonate, salt and starch. Bicarbonate of soda Salt P & L. Johnson Calcium carbonate Starch

27 After discussion with your group complete the following table: Substance Bicarbonate of soda (NaHCO 3 ) Calcium carbonate (CaCO 3 ) Salt (NaCl) Starch ([C 6 H 10 O 5 ] n ) Ionic Ionic Ionic Bonding Type Covalent Predicted Properties High melting point, soluble in water, conductor as solution, insulator as solid. High melting point, insoluble in water, conductor as solution, insulator as solid. High melting point, soluble in water, conductor as solution, insulator as solid. Low melting point, insoluble in water, insulator as a solid or liquid. ACTIVITY 3.11 Testing Unknown Substances In your group design a set of experiments that will help you identify what the four powders are. Then after discussion with your teacher carry them out. After discussion with your group complete the following table: Powder (melt point) A (>800 0 C) B (<250 0 C) C (>800 0 C) D (>800 0 C) Substance Sodium Chloride Starch Calcium Carbonate Bicarbonate of Soda Reason High melting point, conducts when dissolved, doesn t react with acid. Low melting point, doesn t dissolve. High melting point, doesn t dissolve in water, fizzes on contact with acid. High melting point, conducts when dissolved, fizzes on contact with acid. P & L. Johnson

28 Quick Test 4 1 Compound X has a melting point of C and does not conduct electricity when molten. a. State the type of bonding and structure which exists in compound X. Covalent Network a. Explain your answer. No charged particles, but high melting point. 2 Melting Point / 0 C Boiling Point / 0 C Sodium chloride Carbon tetrachloride From the above information, a pupil deduced that ionic bonding must be stronger than covalent bonding. Explain whether or not you agree with this conclusion. No. Covalent bonds aren t broken when melting a molecular substance like carbon tetrachloride. 3. A pupil investigated the properties of three compounds. The results are shown in the following table: Explain whether the structure in A, B and C is covalent molecular, covalent Substance Melting Point Electrical Conduction Answer A High Does not conduct in any state Covalent Network no charged particles, high MPt B Low Does not conduct in any state Covalent Molecular no charged particles, low MPt C High Conducts in solution and as melt Ionic charges can move when melted or dissolved network or ionic. 4. Titanium is used in the making of supersonic aircraft and space vehicles. To extract titanium from its ore, it is first converted into titanium chloride, which is a liquid at room temperature. a. From its name what type of bonding should titanium chloride have? Ionic b. From its melting point what type of bonding must it have. Covalent Molecular P & L. Johnson