10.4 The corrosive effect of acids

clay pot Which one would you use to cook pork in ginger-vinegar, metal saucepan or clay pot? p.23

clay pot When cooking acidic food, it is better to use clay pot. Do you know why?

The corrosive effect of acids on metals p.23 react with - Dilute acids react with some metals to produce hydrogen. => => The acids are corrosive to these metals.

You have learnt in Unit 9 about 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.

Test for hydrogen p.23 Experiment Centre 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 uses a weather balloon (filled with hydrogen) to collect information of weather.

Action of dilute acid on metals p.24 Experiment Centre 10.9 dilute hydrochloric 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 This tells the difference between aluminium and other commonly used metals towards corrosion. p.25

- Due to this property, cooking pans made of aluminium gradually replace those made of iron. the oxide For the cooking pan made of aluminium, a layer of material is often added to protect the oxide from scratching.

Why? metallic statue which can be preserved for a longer time If you are going to make a metallic statue which can be preserved for a longer time, toit which metal would you choose to make it?

Have you seen stalactites ( 鐘乳石 ) before? Do you know how they are formed? p.26

Normally, our average pH value is about 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. p.26

Acidic rainwater The bottom of a cave Solution formed slowly flows downwards long pointed piecessolid hanging down Water in the solution evaporates. This results in long pointed pieces of solid hanging down from the roof of a cave. The formation of stalactites: calcium carbonate Acidic rainwater corrodes calcium carbonate in limestone.

- - Some materials like marble, limestone and sandstone contain calcium carbonate. Marble Limestone Sandstone => => Buildings and statues made of these materials can be corroded by acids. p.26

The HKSAR Legislative Council building is made of materials that contain calcium carbonate

Tai Mahal ( 泰姬陵 ) in India is made of marble Egyptians used large pieces of limestones to build pyramids

Bath tub and floor made of marble

Action of dilute acid on marble p.26 Experiment Centre dilute HCl lime water marble chips

p.27 Eggs in vinegar 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 Scientist

stomachstomach Corrosive effect of acids in our body Stomach Stomach produces hydrochloric acid that helps break down the food. Hydrochloric acid is highly corrosive. helps prevent There is a mucus lining inside the stomach. It helps prevent the stomach from being corroded by the acid. p.27

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

Acids remain in our mouth will corrode the enamel of our teeth, causing tooth decay. p.27