Which one would you use to cook pork in ginger-vinegar, metal saucepan or clay potclay pot?
p.23p.23
The corrosive effect of acids on metals
The corrosive effect of acids on metals
p.23p.23
- Dilute acids react with react with some metals to produce hydrogen.
=>=> The acids are corrosive to these metals.
You have learnt in Unit 9about hydrogen.
What can we use hydrogen for?
Liquid hydrogen is the most common type of liquid fuel for rockets.
However, uncontrolled ignition of hydrogen can lead to explosion.
Liquid hydrogen is the most common type of liquid fuel for rockets.
However, uncontrolled ignition of hydrogen can lead to explosion.
Test for hydrogen
p.23p.23Experiment Centre 10.8Experiment Centre 10.8
1 Light a wooden splint.
2 Remove the stopper of the test-tube containing hydrogen.
3 Quickly put the burning splint at the mouth of the test-tube.4 What happens?
__________________
In the past, hydrogen was used in the airships.
In 1937, the largest airship, Hindenburg ( 興登堡飛船 ), exploded when landing on the ground. 35 people died in that accident.
Nowadays, helium is used in the airships.
What other purposes can hydrogen be used for?
Town gas used in HK contains 50% hydrogen.
The Observatory usesa weather balloon (filled with hydrogen)to collect informationof weather.
Action of dilute acid on metals
p.24p.24Experiment Centre 10.9Experiment Centre 10.9
dilute hydrochlor
ic acid
metal
- The surface of aluminium reacts with oxygen in the air to form a layer of oxide.
can protect the aluminium inside from corrosion by acids
can protect the aluminium inside from corrosion by acids
This tells the difference between aluminium and other commonly used metals towards corrosion.
p.25p.25
- Due to this property, cooking pans made of aluminium gradually replace those made of iron.
For the cooking pan made of aluminium,a layer of material is often added
to protect the oxide the oxide from scratching.
WhyWhy??
If you are going to make a metallic statue metallic statue which can be which can be preserved preserved for a longer timefor a longer time,
which metal would you choose toto make itit?
which metal would you choose toto make itit?
Normally, our average pH
value is about 5.6.
-- When carbon dioxide in the air dissolves in the rainwater,
=>=> increases the acidity of rainwater. The average pH value of rainwater is about 5.6.
=>=> forms carbonic acid
Thus, the normal / unpolluted rainwater is slightly acidic.
Thus, the normal / unpolluted rainwater is slightly acidic.
p.26p.26
Acidic rainwater
The bottom of a cave
Solution formed slowlyflows
downwards
Water in the solution
evaporates. This results in
long pointedlong pointedpiecespieces of solidsolid hanging downhanging down from the roof
of a cave.
The formation of stalactites:The formation of stalactites:
Acidic rainwater corrodes calcium calcium
carbonatecarbonate inlimestone.
-- Some materials like marble, limestone and sandstone contain calcium carbonate.
MarbleMarble
LimestoneLimestone
SandstoneSandstone
=> => Buildings and statues made of these materials can be corroded by acids.
p.26p.26
The HKSAR Legislative Council buildingThe HKSAR Legislative Council buildingis made of materials that containis made of materials that contain
calcium carbonatecalcium carbonate
Tai Mahal Tai Mahal (( 泰姬陵泰姬陵 ) ) in India is made in India is made of of marblemarble
Egyptians used Egyptians used large pieces of large pieces of limestones limestones to to build build pyramidspyramids
Action of dilute acid on marble
p.26p.26Experiment Centre 10.10Experiment Centre 10.10
diluteHCl
lime water
limewater
marble chips
p.27p.27Eggs in Eggs in vinegarvinegarEggs in Eggs in vinegarvinegar
Take the egg out a few days later.What happens to the egg?
Put some vinegar into a glass, immerse a fresh egg in vinegar.
What do you see?
Little ScientistLittle Scientist
stomacstomachhstomacstomachh
Corrosive effect of acids in our bodyCorrosive effect of acids in our bodyCorrosive effect of acids in our bodyCorrosive effect of acids in our body
StomachStomach produces hydrochloric acid that helps break down the food.Hydrochloric acid is highly
corrosive.
There is a mucus lininginside the stomach. It helps prevent helps prevent the stomach from being corroded by the acid.
p.27p.27
- - If the acidic gastric juice flows back to the oesophagus, it will cause damages to the oesophagus.
stomacstomachhstomacstomachh
oesophaguoesophagussoesophaguoesophaguss