The Air And Oxygen

Question 1

How do you investigate the percentage of oxygen in the air (using a tea light)?

Answer 1

1. Light a tea light and float the tea light on the surface of the water in a basin
2. Place a graduated cylinder over the burning tea light
3. Note what happens as the night light burns. The water rises up inside the graduated cylinder. The water is pushed up by the pressure of the atmosphere
4. Allow the apparatus to cool. Measure the volume of gas left in the graduated cylinder
5. By subtraction, calculate the percentage of oxygen in the air (roughly one fifth)

Question 2

How do you measure the percentage of oxygen in air (using gas syringes)?

Answer 2

1. Set up 2 gas syringes on separate retort stands connected with silica glass tubing with copper turnings inside over a Bunsen burner
2. Fill one syringe with 100 cm3 of air while leaving the other syringe with 0 cm3
3. Gently pass the 100 cm3 of air from the occupied syringe to the empty one. The brown copper begins to turn grey black by reacting with the oxygen to form copper oxide.
4. Repeat part 3 several times
5. Allow the apparatus to cool and push all the gas into the formerly empty syringe and measure the volume of air in it.
6. Compare to the initial volume of air and then divide the decrease in volume of air (21 cm3) by the initial volume of air (100 cm3) multiply the answer by 100 to get 21%, the percentage of oxygen in air

Question 3

How do you show that carbon dioxide is present in air?

Answer 3

1. Set up tubing in a two holed rubber stopper through a gas-washing bottle containing limewater
2. Allow air through one tube and connect the other to a vacuum pump.
3. Turn on the pump and allow air to be drawn through for half an hour.
4. The limewater should turn milky

Question 4

How do you show that water vapour is present in air?

Answer 4

1. Put a mixture of salt and crushed ice in a test tube with a rubber stopper, held on a retort stand.
2. Observe the droplets of liquid that condense on the outside of the tube and collect this liquid and test with anhydrous copper sulfate
3. The anhydrous copper sulfate should turn white to blue showing the liquid condensed is water showing water vapour is present in air

Question 5

What is the word equation for decomposing hydrogen peroxide?

Answer 5

Hydrogen peroxide —-> Water + oxygen

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Question 6

What is the chemical equation for the preparation of oxygen?

Answer 6

2H² O² —> 2H² O + O²

Question 7

What is a catalyst?

Answer 7

A substance that changes the speed of a chemical reaction but is not used up in the reaction itself

Question 8

How do you get prepare a sample of oxygen?

Answer 8

1. Set up hydrogen peroxide in a dropping funnel dropping onto manganese dioxide (catalyst) in a Büchner flask connected to tubing that goes into a water basin and into a beehive shelf with a gas jar over it.
2. The gas (oxygen) should displace the water in the gas jar
3. Collect samples of the gas in gas jars

Question 9

What are 5 ways of examining the properties of oxygen?

Answer 9

1. Place moist red and blue pieces of litmus into a jar of oxygen. No colour change will occur showing oxygen is neutral
2. Place a splint with just an ember on it into a jar of oxygen. The glowing splint is re-kindled and bursts into flames showing oxygen supports combustion
3. Heat carbon on a deflagrating spoon and when the charcoal is glowing place it in a jar of oxygen. The carbon continues to burn. Add limewater to jar. It turns milky. Carbon dioxide has been formed when carbon was burned in the presence of oxygen
4. Add water to a gas sample of carbon dioxide. Put blue litmus paper in the jar. It turns red showing the carbon dioxide dissolves in water to form carbonic acid.
5. Burn magnesium metal in the air. Put some on a deflagrating spoon, light the magnesium and put it into a jar of oxygen. The magnesium burns more vigorously and a white powder called magnesium oxide is formed. Add water to the jar and red litmus, it turns blue showing magnesium hydroxide (milk of magnesia) is basic

Question 10

Give 3 uses of oxygen

Answer 10

1. Breathing apparatus (mountain climbers, scuba divers, people who have difficulty breathing)
2. Burning and welding
3. Space rockets