Practical - Chromatography

We conducted a chromatography during the lab session. We were tasked to separate a green food colouring into its components by chromatography.

Through conducting the chromatography, I found out that the components that made the green food colouring were blue dye and yellow dye.

Purification and Separation

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(Read up chapter before class as homework)  Determining purity  A pure substance is made up of only 1 substance. (In other words, it is not mixed with other substances)  We use chromatography or check a substance's melting point and boiling point to determine if the substance is pure. Pure solids have exact and fixed melting point. (A pure solid will melt completely at one temperature) Impurities lower the melting point. The greater the amount of impurities, the lower the melting point of the substance. Impurities also cause melting to take place over a range of temperatures. Likewise, pure liquids have exact and fixed boiling point. Impurities cause the boiling point to increase. The greater the amount of impurities, the higher the melting point of the substance. The liquid will also boil over a range of temperatures.  If pressure acting on the liquid is increased, the boiling point of the liquid is raised and vice versa.   CHROMATOGRAPHY   Chromatography is the technique of using a solvent to separate a mixture into its components. (eg. Separate dyes in ink, pigments in plants, amino acids obtained from proteins, to identify poisons or drugs and to detect traces of banned substances in food.)  The chromatography paper with the separated components is called a chromatogram. If there is only 1 spot on the chromatogram, it means that the substance is pure.  The ratio between the distance travelled by the substance and the distance travelled by the solvent is a constant. The ratio is called the Rf value. Rf value = Distance travelled by substance divided by the distance travelled by solvent.  Chromatography can also be used for colourless substance too such as amino acids. We can apply a locating agent to separate and analyse colourless substances. Uses of chromatography: separate the components in a sample, identify the number of components in a sample, identify the components present in a sample, determine if a sample is pure.  Separating a solid from a liquid DECANATING  Used to separate a dense, insoluble solid from a liquid. (eg. pebbles from water) FILTRATION  Used to separate small solid particles which are insoluble from a liquid (eg. sand, clay, dust particles, and precipitates) A solid can be separated from a liquid by filtration as the filter paper acts as a seive. A liquid can pass through the pores of the filter paper but a solid cannot. Upon filtration, the solid that remains on the filter paper is called the residue. The liquid that passes through the filter paper is called filtrate. EVAPORATION TO DRYNESS AND CRYSTALLISATION Used to separate substance that dissolve in water to form solutions from a liquid (eg. common salt) ONLY FOR SUBSTANCES THAT DO NOT DECOMPOSE UPON HEATING  CRYSTALLISATION   For substances that decompose upon strong heating Water is removed by heating of the solution. Heating is stopped at the stage when a hot saturated solution is formed. If the resulting solution is allowed to cool to room temperature, the dissolved solid will be formed as pure crystals.  A clean glass rod is used to test if a solution is saturated. It is dipped into the solution and removed. There will be a small amount of solution on the rod. If small crystals form on the rod as the solution cools, the solution is saturated. The solution is at saturation point/crystallisation point.  Separating solids   DISSOLVING AND FILTRATION      Only when one solid is soluble and the other is insoluble.  Pour some distilled water into the mixture of common table salt and sand. Stir and warm the mixture (dissolve). Next, pour the warm mixture into a filter funnel lined with filter paper. Collect the filtrate in a conical flask. Wash the residue with a little distilled water to remove all the salt solution from it. The residue is sand. Pour the filtrate into an evaporating dish and evaporate the filtrate to dryness. The white solid left in the evaporating dish is salt.  OTHERS  We can use a magnet to separate magnetic materials from non-magnetic materials.  Sublimation - separating a solid that sublimes on heating.  Separating a liquid from a solution  SIMPLE DISTILLATION   A pure solvent (eg. water) can be separated from a solution by simple distillation. The liquid that distils over is called the distillate. The solid that remains in the distillation flask is called the residue.  Note!  The thermometer should be placed beside the side arm of the distillation flask. It should NOT be dipped into the solution. This ensures that the thermometer measures the boiling point of the substance being distilled.  The condenser consists of 2 tubes, an inner tube and an outer water jacket. Cold running water is allowed to enter the water jacket from the bottom of the condenser and leave from the top.  The condenser slopes downwards so that the pure solvent formed can run into the receiver.  If the distillate is volatile, the receiver can be put in a large container filled with ice. This helps to keep the temperature of the distillate low so that it remains in the liquid state.  Uses of simple distillation - 1) recover a solvent from a non-volatile solute, which has a high boiling point. 2) Separate mixture of liquids with different boiling points. (for liquids whose boiling points differ by more than 20°C) Separating liquids  SEPARATING FUNNEL   For immiscible liquids. Liquids that do not dissolve in each other are discribed as immicible. Note! Allow the liquid to separate completely. The denser liquid will be the bottom layer. Open the tap of the funnel to allow the bottom layer to drain into the beaker.  FRACTIONAL DISTILLATION  For misicible liquids  There is a column called fractionating column. It is attached to the round-bottomed flask and the condenser. They are filled with many glass beads that provide a large surface area for vapour to condense on. Some fractionating columns are also filled with plates or a spiral.   The liquid with the lowest boiling point will distil over to the condenser first. The vapour of the liquids with higher boiling points condense along the fractionating column and re-enter the round bottomed flask.

Introduction Learning objectives  - To explain the working principles involved in some separation techniques such as filtration, magnetic attraction, simple distillation, crystallization, evaporation, sublimation and the use of separating funnel - To explain how the techniques are used to separate the mixture into its components - To suggest suitable purification techniques, given information about the substances in a mixture. - To apply the various separation techniques in everyday life and industries In nature, almost all substances are in the form of a mixture.  Why is it essential to have pure substances at every instance?               Answer: Mixtures need to be separated into pure substances for characterisation, identification, produce of useful substances such as medicines.  Determine purity by melting and boiling points or to use chromatography. Chromatography  The solvent need not be only water! It can be gas (gas chromatography)! Pure dye only contains one spot while impure dye contains more than one spot Must have a small and concentrated dot so that the individual dyes will not be faint Retention Factor =Rf Unknown substances can be identified by their Rf values Separate and identify compounds in a mixture  ADSORB VS ABSORB  [Watch video on Chromatography and do experiment]  Simple distillation  Liquid (solvent) changes into a gas (boiling). Gas is pure (other substances are left behind) Gas condensed to pure liquid is (called distillate) on cooling Fractional distillation  More efficient than simple distillation Separate miscible liquids with different boiling points and their boiling points are close together. Example of miscible liquids are alcohol and water.   Refer to w/s for compare and contrast of fractional distillation and simple distillation  In the laboratory, the apparatus is essential the same as in simple distillation, except that a fractionating column is introduced between the distillation flask and the condenser. It is this fractionating column which does the actual separation.  The upper part of the column is cooler than the lower portion. Thus only vapours with the same temperature as the upper portion are allowed to pass on to the condenser, whereas vapours with higher boiling points would condense as they enter the upper portion and flow back into the distillation flask. As a result, the liquid with the lowest boiling point will always be distilled over first, followed by the liquid with the next lowest boiling point and so on, until all the fractions in the mixture have distilled over. Use of distillation - Desalination Plant: Obtain Pure Drinking Water Crystallisation Separate a dissolve solid from a solution. This method is commonly used to separate the heat-labile solutes from their solutions.  You need a saturated solution to carry out crystallisation. Saturated solution: solution containing a maximum amount of solute dissolved in a given volume of solvent at a particular temperature. A concentrated solution is not necessarily a saturated solution.

Practical - Comparing Element, Compound and Mixture

Aims of the experiments:

1. To observe and compare the properties of compounds and mixtures made up of the same constituent elements.
2. Thus, infer some common characteristics of mixtures and compounds that can be used to distinguish them.

Part A: Investigating a mixture of elements

Place a spatula of iron filings and a spatula of sulfur powder on a piece of filter paper. Mix them together. Can the iron filings and sulfur be mixed in any proportion to form a mixture?

Observations: Iron filings were stuck onto the filter paper. The elements did not change colour. To form a mixture, the iron filings and sulfur can be mixed in any proportions.

Feel the mixture. Does it feel warm?

Observation: No

Move a magnet under the paper as shown in the diagram. Does the magnet separate the mixture of elements?

Observations. The iron filings are attracted to the magnet. The magnet separates the mixture of elements.

Add the iron and sulfur mixture into a test-tube of water. Stir the mixture and them let it stand for a while. Does water separate the mixture of elements?

Observation: Yes

Do the iron filings and the sulfur act as separate substances or a single substance with water?

Observation: Separate substances

Conclusions: The substance in a mixture can be mixed in any proportion. When a mixture is formed, heat and light are not usually taken in or given out. The mixture can be separated by physical means. The mixture has the properties of its constituent substances.

Part B: Mixture or Compound?

1. Mix one spatula of sulfur and half a spatula of iron filings in a crucible.
2. Cover the crucible with a lid and heat for 10 minutes using strong flame.

                                                                        Residue

The residue was not attracted by the magnet.

When put in water:

Residue did not separate into its components.

Conclusion: The reside formed is a compound. Iron and sulfur have gone through a chemical reaction to produce the residue - Iron (II) Sulfide. Although iron is magnetic, the residue was not attracted to the magnet, showing that it does not possess the properties pf its components. Moreover, the residue could not be separated by physical means (by water).

Practical - Bunsen Burner

Different parts of the bunsen burner

Air holes - To allow air to enter the burner

Barrel - To raise the flame to a suitable height for heating

Base - To support the burner and make it more stable

Collar - To control the amount of air entering the burner

Gas intake tube - To allow the gas for the gas supply to rush into the burner

Gas tap - To control the amount of gas supplied to the burner

Which part of the Bunsen flame is the hottest?

Use a pair of tongs to old a short piece of wire and heat the metal wire at different points of the flame (the middle of the inner blue cone, the tip of the inner blue cone and the middle of the outer non-luminous flame).

Experiment pictures!

                                Heating of the metal wire at the tip of the inner blue cone

                                 Metal wires after heating at different parts of the Bunsen flame

Observations: When the metal wire heated at the tip of the inner blue cone, the metal wire glows the brightest and the end of the wire starts to melt.

Conclusion: The tip of the inner blue cone is the hottest part of the Bunsen flame.

Classification and Elements, Compounds and Mixtures

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Classification  The periodic table is a way of classifying elements.  Elements are arranged in order of increasing proton number  There are 7 rows of elements called periods. There are 8 columns of elements called groups. The elements in the same group have the similar chemical properties. Question(s):   Why is classification important in Chemistry? Elements, Compounds and Mixtures (Read up chapter before class as homework) Element - Pure substance that cannot be split up into 2 or more simpler substances by chemical means or by electricity. Chemical symbols are used to represent elements. For example, Sodium is         represented by the symbol Na and Magnesium is represented by the symbol Mg.   Elements are split (or classified) into 2 main groups - metals and non-metals. Physical properties of metals: 1) Shiny appearance (lustrous); 2) Solid at room temperature and pressure; 3) Sonorous (makes a ringing sound when struck; 4) Malleable (can be hammered into different shapes without breaking); 5) Ductile (can be drawn into wire); 6) High melting point and boiling point (except for mercury, sodium and potassium); 7) Good conductors of heat and electricity.   Physical properties of non-metals: 1) Dull appearance (non-lustrous); 2) Either gas, volatile liquids or solid with low melting points at room temperature and pressure (except for carbon); 3) Brittle if solid (easily broken when hammered); 4) Low melting point and boiling point (except for carbon and silicon); 5) Poor conductors of heat (except for carbon in the form of diamond and graphite); 6) Poor conductor of electricity (except for carbon in the form of graphite)  Atoms - smallest particles of an element that have the same chemical properties of that element.  Atoms can still be split but the particles obtained no longer have the properties of the element.  Not all elements exist as atoms. Only, helium, argon, neon, krypton, xenon and radon exist as atoms. They are called monatomic atoms.  Molecules - A group of 2 or more atoms that are chemically combined.  Molecules formed by the combination of 2 atoms are called diatomic molecules (eg. Oxygen)  Molecules formed by the combination of 3 atoms are called triatomic molecules.  Molecules formed by the combination of 4 or more atoms are called polyatomic molecules.(eg. Sulphur) Compound - Pure substance that contains 2 or more elements chemically combined. (Eg. Common salt made of chlorine and sodium) Rules for naming compounds: 1) Compounds that are made of 2 elements have a name that ends with an -ide; 2) Compounds that contain hydroxide ions is named a hydroxide; 3) Compound that contains a negatively charged polyatomic ion containing oxygen usually ends with an -ate. A compound is made of different elements chemically combined in a fixed ratio. The smallest particle of a compound that can exist independently is a molecule.  Chemical formula states: 1) The type of atoms present in the compound; 2) The ratio of the different atoms present in the compound. Rules for writing chemical formulas: 1) Compounds that have both metallic and non-metallic elements, the symbol of the metallic element is written first; 2) The number of atoms is written as a subscript, to the right of the atom's symbol; 3) It is not necessary to write the subscript '1'; 4)The oxygen atom is usually written the last.  Heat can be used to form compounds and break down compounds (thermal decomposition) Mixtures - formed when 2 substances are added together without chemical bonds being formed.  Differences between mixtures and compounds: 1) Separation: Mixture - can be separated by physical methods, Compounds - can only be separated by chemical means or by electricity; 2) Properties: Mixture - Chemical properties of the mixture is the same to that of its components, Compounds - Physical and chemical properties of a compound are different from its elements; 3) Energy change: Mixtures - no chemical reactions take place, Compounds - Chemical reactions take place, causing energy change; 4) Composition: Mixtures - The component of a mixture can be mixed in any proportion, Compounds - Elements in a compound are always combined in a fixed proportion. Alloy is an example of a mixture. It is a mixture of elements. Hence, alloys have similar chemical properties to those of the elements they contain but have different physical properties.

[Classification activities]  Activity one (in groups of 3, classify these soft toys: Teddy bear  Dog  Rabbit  Giraffe Duck Sheep Monkey  Snake Elephant  Lion Hyena  Turtle Panda Hamster Polar bear Ponyo Totoro Carebears Shrek  Bear Koala Angry Birds Mickey Mouse Doll Kangaroo Dolphin Pooh Eeyore Wombat Puppy Stitch  Penguin Piglet St. Bernard Ninja My group's classification: Activity 2 (Classification of elements, compounds and mixtures) Classification of Matter Matter can be classified into 3 groups, elements, compounds and mixtures.   Elements - Substance that is made up of only one type of atom and cannot be broken down into simpler substances by chemical reactions or by electricity. Chemical reactions (eg. over heating sugar made of hydrogen, carbon and oxygen -> carbon + water) Chemical reactions are not easily reversed. Rusting (iron + oxygen -> oxide of iron) is also a chemical reaction Water (l) -> water (g) is NOT a chemical reaction. Dry ice - Carbon dioxide (s) -> Carbon dioxide (g) is also not a chemical change. Ice melting is also not a chemical reaction. These are physical changes. Distinguishing chemical reaction and physical changes: Check if new substance is formed, if the process can be easily reversed, if heat or light is involved. 116 identified elements (some man-made and some natural) Stable elements (group 0, noble gases) are usually radioactive (harmful)   Our body is made up of 26 important elements such as oxygen, hydrogen, carbon, nitrogen, calcium, phosphorus and other elements.  Chemical symbol - first letter is always capitalised.  Name and chemical symbols are standardised internationally by IUPAC.  Elements can be further classified into 2 main groups, metals and non metals.  Elements can also be classified and arranged according to their chemical properties into groups.  Horizontal row of elements are called periods.  Vertical row of elements are called groups. Elements in the same group show similar chemical properties. Listen to element song. A compound is a substance which consists of 2 or more elements chemically combined together. It is made up of 2 or more types of atoms chemically combined. Examples of compounds - water (hydrogen and oxygen), sodium chloride (sodium and chlorine), sugar (carbon, hydrogen and oxygen)  Compounds are formed by chemical reactions. Many elements combine directly to form compounds. Example: Burning of magnesium in oxygen to form magnesium oxide. Properties of compounds are different from the properties of its constituent elements.  Compounds have fixed compositions (eg. water always contains 11% hydrogen and 89% oxygen by mass) A compound may be broken down into simpler substances by chemical reactions (decomposition). Decomposition is usually carried out using heat (thermal decomposition), electricity (electrolysis) or even bacteria. [Activity compare and contrast compound and elements] A mixture consists of two or more substances which are not joined together chemically.  Similarities of mixture and compounds: both contain 2 or more substances. [Refer to textbook page 68 for the differences between mixtures and compounds]  Alloys are mixtures.

Atomic Structure

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Self-study chapter What I learnt -   Atoms are made of 3 different particles – protons, neutrons and electrons. They are also called sub-atomic particles.  (taken from: agssciencetransition.wikispaces.com) Protons: ✓Carries one positive electric charge ✓ Relative mass of 1 ✓ Represented by symbol, p Neutrons: ✓ Carries no electric charge ✓ Relative mass of 1 ✓ Represented by symbol, n Electrons: ✓Carries one negative electric       charge ✓ Relative mass of 1/1840 (often negligible) ✓ Represented by symbol, e All atoms are electrically neutral as they contain an equal number of protons and electrons. The proton number is the number of protons in an atom. The proton number is also known as atomic number. It is represented by symbol, z. The proton number can also tell the number of electrons in an atom. The nucleon number is the total number of protons and neutrons in an atom. The nucleon number is also called mass number. For example: 24 Mg 12 Mg – chemical symbol for Magnesium   24 – Nucleon number = mass number = number of protons and neutrons 12 – Proton number = atomic number = number of protons/electrons Isotopes are atoms of the same element with the same number of protons but different number of neutrons. Therefore, isotopes have the same proton number but different nucleon number. For example: 17 Cl        35      17 Cl 37   Some elements do not have isotopes (eg. fluorine) Isotopes have the same chemical properties but slightly different physical properties.  Radioisotopes are isotopes that emit high-energy radiation. They are radioactive. The radiation emitted by radioisotopes is dangerous as it can damage living cells and can cause cancer. The electron arrangement determines the atom’s chemical properties. Electrons move around the nucleus of an atom in electron shells. The way electrons are arranged in an atom is called electronic structure or electronic configuration.  Valence shell is the shell that is the furthest from the nucleus. The chemical properties of an element depend on the number of valence electrons. Question(s) after reading: How and who found that an atom was the smallest particle of an element that posses the chemical properties of the element?

1) History of the model of an atom: People involved: Thales of Miletus (600 B.C.), Greek philosopher, Democritus (460 B.C), Aristotle, John Dalton (1800s), Ernest Rutherford (1911) Roles: Democritus – named the smallest particle of an element as an atom. Aristotle – disagreed with Democritus and dismissed the idea of atoms. (there was a contradiction of opinions) John Dalton – Performed experiments to prove that atoms did exist. Ernest Rutherford – Conducted experiments to find out more about the structure of atoms. They managed to find out about the arrangement of electrons in atoms. ^ Rutherford’s experiment (assisted by Geiger and Marsden) (taken from: http://www.daviddarling.info/ encyclopedia/R/Rutherfords_experiment_ and_atomic_model.html) 2) The first electron shell must be filled up first. It can hold a maximum of 2 electrons. The second and subsequent shells can hold a maximum of 8 electrons.