Topic 8 - Chemical analysis

Questão	Responda
What is a pure substance?	A substance which only contains one compound or element.
How can you use the melting and boiling points of a substance to see how pure it is?	- a pure substance will melt/boil at a specific temperature (found in data book). - if you then compare this melting/boiling point with the melting/boiling point of your sample, the closer it is to the pure value then the more pure the substance is. - impurities will lower MP and increase the melting range of your substance. - also they will increase the BP and increase the boiling range of your substance.
What is a formulation?	Formulations are useful mixtures with exact amounts of components which contribute to the properties of the formulation so it meets its required function.
Give an example of a formulation and its components?	Paint; pigment solvent binder additives
What is the test for chlorine?	Chlorine bleaches damp litmus paper, turning it white (may turn red briefly).
What is the test for oxygen?	If you put a glowing splint in a tube of oxygen then it will relight the splint.
What is the test for carbon dioxide?	Bubbling carbon dioxide through limewater (calcium hydroxide) causes it to turn cloudy.
What is the test for hydrogen?	Squeaky pop when placing a burning splint next to the open end of a test tube containing hydrogen.
How can you identify ions using a line spectrum?	During the flame tests, as the ions heat up, their electrons move to higher energy levels. When the electrons drop back they emit light energy. No two ions have the same charge and arrangement, they emit different wavelengths of light. The light passes through a spectroscope which can detect different wavelengths of light to produce a line spectrum. The intensity of the spectrum indicates the concentration of the ion. The line spectra can them be used to identify ions and calculate their concentrations.
How could you identify ions in a mixture using flame emission spectroscopy?	You can use reference spectrums of ions to compare to a mixture to see if it contains those ions. Machines can analyse this as they are very fast, sensitive and accurate.
What are anions and cations?	Anions are negative ions. Cations are positive ions.
How can you test for carbonates?	You can test for carbonates by adding a few drops of dilute acid to a test tube containing a substance. Then if there are carbonate ions (CO3^2-) in the substance their reaction would produce CO2. Then bubble this through limewater which will turn cloudy if it is present.
How can you test for sulfates?	You can test for sulfates by adding a few drops dilute hydrochloric acid (HCl) and barium chloride solution (BaCl2). If sulfate ions are present then a white precipitate of barium sulfate will form.
How can you test for hallides?	To test for hallide ions (Cl-,Br-,I-) add a few drops of dilute nitric acid (HNO3) and a few drops of silver nitrate solution (AgNO3). A chloride gives a white precipitate of silver chloride. A bromine gives a cream precipitate of silver bromide. An iodide gives a yellow precipitate of silver iodide.
How do you identify a metal ion using flame tests?	Compounds of some metals produce a characteristic colour when heated in a flame. 1. Rub a clean nichrome or platinum wire loop with fine emery paper and then hold in in a blue Bunsen flame. (flame may colour change but back to blue means clean). 2. Dip the loop into the sample and then put the wire back in the flame and observe the colour. Only works for a sample containing a single metal ion.
What are the flame tests for each positive metal ion (cations)?	Lithium ions, Li+, crimson flame Sodium ions, Na+, yellow flame Potassium ions, K+, lilac flame Calcium ions, Ca2+, orange-red flame Copper ions, Cu2+, green flame
Which metals form coloured precipitates with NaOH?	Many metal hydroxides are insoluble and precipitate out of solution when formed having a characteristic colour. Add a few drops of sodium hydroxide solution to your sample. Calcium, Ca2+, white precipitate Copper, (11)(Cu2+), blue precipitate Iron, (11)(Cu2+), green precipitate Iron, (111)(Fe3+), brown precipitate Aluminium, (Al3+), white but then colourless Magnesium, (Mg2+), white precipitate. E^x+ + xOH- --> E(OH)x
What are the two types of phases in paper chromatography?	Mobile phase - where the molecules can move. This is always a liquid or gas. Stationary phase - where the molecules cant move. This can be a solid or really thick liquid.
Explain paper chromatography?	In paper chromatography the stationary phase is the chromatography paper (filter paper) and the mobile phase is the solvent. The chemicals that spend more time in the mobile phase than the stationary will travel further up through the stationary stage. The components of a mixture will separate out into spots so long as they spend different amounts of time in the mobile phase. The spots may change depending on the solvents. A pure substance will only have one spot.
In paper chromatography what two things does the amount of time the molecules are in each stage depend on?	How soluble they are in the solvent. How attracted they are to the paper. Molecules with a higher solubility in the solvent, and which are less attracted to the paper, will spend more time in the mobile phase and travel further up the paper.
What is the Rf value?	An Rf value is the ratio between the distance travelled by the dissolved substance (the solute) and the distance travelled by the solvent. Rf=distance travelled by the substance/distance travelled by solvent. The further through the stationary phase a substance moves, the larger the Rf value.
How can you use an Rf value to see if a substance is pure?	1. run a pure sample (a reference) alongside the unknown mixture. 2. if the Rf values of the reference match the spots in the mixture then that substance may be present (although haven't proved they are the same).
How is the Rf value affected by the solvent?	If you change the solvent the Rf value will also change. If the Rf value of one of the spots in the mixture in all solvents, then it is likely the reference compound is present in the mixture. If the spot in the mixture and the spot in the reference only have the same Rf value in some solvents, then the reference compound isn't present in the mixture.

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Topic 8 - Chemical analysis

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	Criado por finn squires quase 7 anos atrás