Noadswood Science, 2012

Tuesday, April 11, 2023

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?

pHpH

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 alkalistronger acid

weak alkali

weakaci

d

strongalka

lineutral

strongacid

1 2 3 4 5 6 7 8 9 1011 1213 14

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

SolubilitySolubility

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

SolubilitySolubility

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 TestsFlame 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 TestsFlame 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 ColoursFlame Test Colours

Flame Test ColoursFlame 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…

ExperimentExperiment

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…

PoisonPoison

Potassium Cyanide

FindingsFindings