الأسبوع 4 · الدرس الثاني «تفاعلات الاحتراق والتلوث البيئي» (٢): لون اللهب والكشف عن نواتج الاحتراق وخواصها والسحابة السوداء وعوادم السيارات والصناعة والمحول الحفزي وأجهزة غسل الغازات · 3 دروس
1
Blue flame, yellow flame and what comes out
Science Prep 3 Term 1, Unit 1: pp. 20–24
GoalRead the colour of a gas flame, name the products of burning natural gas and say how mass is kept in combustion.
New words
water vapour · water as a gas; it turns back into droplets when cooledcondense · to change from a gas to droplets of liquid when cooledconservation of mass · in a reaction the total mass of the products equals the total mass of the reactants
Explanation
A gas stove shows how well the gas is burning. A blue flame means complete combustion: carbon dioxide and water vapour form, there is a lot of heat and less gas is used. A yellow or orange flame means incomplete combustion: toxic carbon monoxide and soot form, the pots turn black and the heating is weaker. Then the burner needs cleaning or more air, and a grown-up deals with it. The products of burning natural gas can be detected. In the laboratory, water vapour cooled by ice condenses to droplets, which turn blue cobalt chloride paper pink, and carbon dioxide turns limewater milky. We only read about these tests, because they need a flame and chemicals. A safe idea of the first test: breathe on a cold mirror and droplets appear, because the water vapour in your breath condenses. This is breathing, not burning. In every combustion, mass is conserved: weigh everything before and after, and the two totals agree, because atoms are only rearranged.
Flame
Combustion
Main products
Heat
blue
complete
CO₂ and water vapour
more
yellow or orange
incomplete
CO and soot
less
Worked examples
Propane burns completely: 44 g of propane with 160 g of oxygen give 132 g of carbon dioxide and 72 g of water. Before: 44 + 160 = 204 g. After: 132 + 72 = 204 g. The masses match.
Breathing on a cold spoon makes it cloudy with tiny droplets. Water vapour in the warm breath meets the cold surface and condenses. The same change happens when the water vapour from burning gas is cooled, so the droplets show that water was among the products.
Hana sees black marks on the bottom of a pot and a yellow flame under it. That means incomplete combustion with soot. A blue flame would mean more heat and complete burning. She tells her father, who cleans the burner.
Class activity
Cold mirror, with a grown-up. Breathe on a cold mirror, spoon or glass and watch the droplets appear and vanish. Say why: this is water vapour condensing, and it is breathing, not burning. Then draw a stove with a blue flame and one with a yellow flame, and write under each the products and the amount of heat. Do not light anything.
Practice
1
What does a blue flame on a gas stove tell you?
Complete combustion: carbon dioxide and water vapour, with much heat.
2
In the laboratory, which product of burning gas is detected with blue cobalt chloride paper, and what does the paper do?
Water vapour; the paper turns pink.
3
In a burning test, 8 g of methane use 32 g of oxygen and give 22 g of carbon dioxide. How much water forms?
8 + 32 = 40 g before, so water = 40 − 22 = 18 g.
4
Why is breathing on a cold mirror not a combustion test?
Nothing is burning: it only shows that exhaled air holds water vapour.
