10.410.4

The corrosiveThe corrosive

effect of acidseffect of acids

Which one would you use to cook pork in ginger-vinegar, metal saucepan or clay potclay pot?

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When cooking acidic food, it is better to use clay potclay pot.

Do you know why?

The corrosive effect of acids on metals

The corrosive effect of acids on metals

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- 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.

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- 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?

Have you seen stalactites ( 鐘乳石 ) before?Do you know how they are formed?

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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.

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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.

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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

Bath tub Bath tub and and floorfloor made ofmade of

marblemarble

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.

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- - If the acidic gastric juice flows back to the oesophagus, it will cause damages to the oesophagus.

stomacstomachhstomacstomachh

oesophaguoesophagussoesophaguoesophaguss

Acids remain in our

mouth will corrode the

enamel

of our teeth, causing

tooth decay.

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