Chemical Analysis (review)

1. Gases are often made in a chemical reaction. Scientists may use indicator tests to determine the gas present. Match the name of the gas with its test.

Answer: glowing splint is re-lighted (oxygen)  ;  damp blue litmus paper turns red then bleaches white (chlorine)  ;  limewater turns from colourless to cloudy (carbon dioxide)  ;  a lighted splint causes a squeaky pop sound (hydrogen)

2. The table below shows the data that a student collected on three different substances: A, B and C.

a) Give the state of substance A at 0°C.

Answer: liquid

b) State and explain which substance(s) are pure.

Answer: A and B are pure substances as each changes state (melts and boils) at a specific temperature.

c) Justify why the data in the table above is not enough to determine whether substance C is a formulation.

Answer: A formulation is a mixture that has been designed to be a useful product. The data in the table show that C changes state (melts and boils) over a range and so it is a mixture. Without knowing the name or use of substance C we cannot decide if it is a mixture that has been created for a specific purpose.

d) A sample of substance C was put into a blue Bunsen flame and the flame turned crimson red. Give the formula of the ion that caused this.

Answer: Li+ (lithium)

3. An unknown blue chemical was analysed by a student. When acid was added to the sample, bubbles were formed.

a) Describe an experiment to collect the gas, and then determine that the gas is carbon dioxide.

Answer: Mix a sample of the blue chemical with an acid in a test tube. Put a bung with a delivery tube on the end of the test tube and allow the gas to blow through limewater. If the limewater turns from colourless to cloudy then the gas is carbon dioxide.

b) The unknown blue chemical was added to sodium hydroxide solution. It made a blue precipitate. Write an ionic equation for this reaction.

Answer: Cu2+ + 2OH  →  Cu(OH)2

c) Use the information in the question above and your own knowledge to outline how to complete a flame test on the unknown chemical and the likely result.

Answer: Get a clean, dry piece of nichrome wire (or damp splint). Put it into the sample for testing. Turn the air hole on the collar of the Bunsen flame to give a blue/roaring/ heating flame. Put the sample into the blue flame and observe the colour. As this chemical contains copper ions the expected flame colour would be green.

d) Use the information from all the questions above to give the formula of the unknown compound.

Answer: CuCO3