Noadswood Science, 2012

Sunday, October 12, 2014  To know how flame tests, the pH scale and solubility helps analytical scientists

 Universal indicator measures the pH of a substance – it is a dye which changes colour depending on whether the substance is an acid or alkali…  If the substance being observed is not a liquid then a solution needs to be made (dissolve solute to distilled water and filter)  What colour does universal indicator turn with an acid / an alkali / a neutral substance? pH

 The strength of an acid or alkali is measured by the pH scale – universal indicator can tell you the pH of a solution as each colour has a separate pH value stronger alkali stronger acid weak alkali weak acid strong alkali neutral strong acid

pH 1 – car battery acid pH 4 – lemon juice; vinegar pH 6 – rain water; coke; citric acid pH 7 – water; salt pH 8 – washing up liquid pH 12 – oven cleaner pH 13 – sodium hydroxide; ammonium hydroxide

 The solubility of a compound eliminates a lot of ionic compounds (e.g. oxides, hydroxides and carbonates are all insoluble in water)  Add a very small amount of the substance to some distilled water in a boiling tube and shake  Allow it to settle and record what the contents looks like ◦ If it is transparent the substance is soluble ◦ If it is cloudy the substance is slightly soluble ◦ If there is no change the substance in insoluble Solubility

 For many chemical tests the substance needs to be in a solution…  Add a spatula of the sample + some distilled water into a boiling tube and shake  Repeat until no more sample can dissolve  Poor the solution through some filter paper to remove the excess solid Solubility

 Flame tests are used to identify metal ions – they can identify which, if any, metals are present in a compound  Why do the different metals give out different colours during a flame test?  As the element is heated electrons are excited to higher energy levels. As these electrons return to their ‘ground state’ specific amounts of energy is released – this energy corresponds to particular wavelengths of light, so producing particular colours of light Flame Tests

 A nichrome wire loop is dipped into dilute hydrochloric acid and then rinsed in distilled water to clean it  The loop is then dipped into the sample and placed in the blue tip of a Bunsen burner on full flame – the colours are then observed… Flame Tests

 Common metals and their colours: - ◦ Barium – pale green ◦ Calcium – yellow-red ◦ Copper – green-blue ◦ Lead – blue flashes ◦ Lithium – red ◦ Sodium – orange ◦ Potassium – lilac ◦ Magnesium – white ◦ Strontium – red Flame Test Colours

 Look at the picture of the crime scene below – in the investigation the cause of death was thought be from poisoning, and the poison itself was made from an exact combination of metal ions… Experiment  Although it looks like a classic case of suicide with a gunshot to the head, on closer inspection the forensic detectives noticed there were traces of powder around the woman’s face…  Your task is to identify this powder…

 The poisons used to commit murders contain an exact combination of metal ions as well having different levels of solubility and measurements on the pH scale  Some well know suspects have connections with different poisons  Follow through the forensic crime scene workbook to try and solve the crime… Poison

 Potassium Cyanide Findings