everyday uses of acids and alkalis 10.6 everyday uses of acids and alkalis

10.6

Everyday uses of aEveryday uses of a

cids and alkaliscids and alkalis

- Micro-organisms (e.g. bacteria and moulds) act on food and make it go bad.

e.g. the humidity is high

Bacteria grow on fruit

Moulds grow on bread

- Some micro-organisms may also produ

ce toxic substances which cause food

poisoning (食物中毒 ).

Why do Why do micro- organisms decay

food?

Why do Why do micro- organisms decay

food?

Because micro-organisms absorb nutrients from food and decompose and

decay the food

Because micro-organisms absorb nutrients from food and decompose and

decay the food

What is food poisoning?

If we eat food with unnecessary micro-organisms,

we may have diarrhoea, dizzin

ess, vomit and high temperatu

re etc.These are symptoms of food poisoning.

Acids can be used for food preservationAcids can be used for food preservation

- Acids can be used for food preservation

(食物防腐 ).

- Vinegar is commonly used for food preservation.food preservation.

Solution of ethanoic acid (or called acetic acid)

Solution of ethanoic acid (or called acetic acid)

e.g.

Percentage of vinegar needed to preserve fruits

p.37p.37

Cut Cut intointo small piecessmall pieces

Wash Wash under under running waterrunning water

Dry pieces with a cloth or blotting paper

Soak pieces into vinegar and water for observation

Experiment Centre 10.12Experiment Centre 10.12

Slows downSlows down their their activities or even activities or even killskills

themthem

Slows downSlows down their their activities or even activities or even killskills

themthem

- As acidic environment is a poor condition for micro-organisms to grow, acids can be used for food preservation.

- Most micro-organisms cannot live in a low pH environment.

What are the What are the advantages advantages and and disadvantagesdisadvantages of of using acidsusing acids for food for food preservation? preservation?

Effect of pH on browning of fruits

p.39p.39

Fruits likeFruits like apple, banana, pear apple, banana, pear andand peach peach turn turn brownbrown when they are when they are cut cut and and left in the air left in the air for a period of timefor a period of time..

What substances can protect the fruit better?

What substances can protect the fruit better?

You can organize your ideas by filling in the investigation planner.

You can organize your ideas by filling in the investigation planner.


Many Many cleansing agentscleansing agents contain contain acids acids or or alkalis.alkalis.

Acids and alkalis as cleansing agents

Acids and alkalis as cleansing agents

p.41p.41

Toilet cleanser contains hydrochloric

acid

Toilet cleanser contains hydrochloric

acid

Drain cleanser contains sodium

hydroxide

Drain cleanser contains sodium

hydroxide

Glass cleanser contains ammonia

Glass cleanser contains ammonia

Resulting substancesResulting substances can be can be wiped or wiped or

washed awaywashed away with with waterwater easily. easily.

Why can Why can acids and alkalisact asact as cleansing agents? cleansing agents?

Acids Acids react react with with dirtsdirtsAcids Acids react react with with dirtsdirts

Alkalis Alkalis react withreact with greasesgreases

Alkalis Alkalis react withreact with greasesgreases

Word equation:

Neutralization( 中和作用 )Neutralization( 中和作用 ) p.42p.42

- In the process of neutralization, the properties of acids and alkalis cancel out each other and the reaction produces neutral salts and water.

- If the amounts of acids and alkalis are the same, only salts salts and water water remain remain afteafter neutralizationr neutralization..

+ AlkaliAcid + WaterWaterSaltSalt

=> => Chemical reaction takes place between acids and alkalis forming saltssalts and water.

- Salt and water are formed from the nneutralization eutralization of anof an acid acid and anan alkali. alkali. Neutralization Neutralization is ais a chemical reactionchemical reaction..

- Chemical reaction is a process in which new substances are formed.


AlkaliAcid

Add 1 cm3

dilute sodium hydroxide to the acid

Neutralizing a dilute acid with a dilute alkali

p.44p.44

Pour 10 cm3

dilute hydrochloric acid into a beaker

Stir solution with a glass rod and observe colour change


Add 1 or 2 drops of universal indicatorsolution to the acid

- Adding acid into alkali or or adding alkali into acid can cause neutralization. The products formed are salt salt and water.water.

- When an acid is just neutralized by anan alkali, neutral solution is formed. The pH value should be 7.

Why can’t we see the Why can’t we see the saltsalt in the above experiment?in the above experiment?Why can’t we see the Why can’t we see the saltsalt in the above experiment?in the above experiment?

- When excess alkaliexcess alkali is added into the acid, the pH value increases to above 7.


+ AlkaliAcid Neutral solutionNeutral solution

The products of neutralization

p.46p.46

To find out what are produced in neutralization in the above experiment, the simplest method to use is evaporation.

Measure 10 cm3 dilute hydrochloric acid and 10 cm3 dilute sodium hydroxide and mix solutions

Put neutral solution on a watch glass

Put watch glass over a hot water bath


white solid is formedsteam (salt)


Salts —— a group of chemicalsSalts —— a group of chemicals p.48p.48

=> => Salts are a group of chemicals produced by neutralization.

e.g.e.g.

Hydrochloricacid

Sodiumhydroxide

Sodium chloride

+ waterwater+

Nitricacid

Potassiumhydroxide

Potassium nitrate

+ waterwater+

Sulphuricacid

Magnesiumhydroxide

Magnesium sulphate

+ waterwater+

Carbonicacid

Sodiumhydroxide

Sodium carbonate

+ waterwater+


- We commonly use salts in our daily life.

e.g.

> as table salt

Sodium Sodium chloridechlorideSodium Sodium chloridechloride

> used for production of chlorine and sodium hydroxide

Potassium Potassium nitratenitrate

Potassium Potassium nitratenitrate

> used in matches

Magnesium sMagnesium sulphateulphate

Magnesium sMagnesium sulphateulphate

> used in bath salt

Application of neutralizationApplication of neutralization

- Release of acidic gases from factories or power stations into the air causes acid rain.

- To reduce the release of acidic gases by passing them through alkaline solutions before they are released into the air.

p.42p.42

alkaline solutions

- Other than helping reduce harmful substances, neutralization can also control the pH values of substances.

e.g.

> is an alkali.

Antacid Antacid ((制酸劑制酸劑))

Antacid Antacid ((制酸劑制酸劑))

> can neutralize excess acid in our stomach.

Soda Soda (( 梳打粉梳打粉))

Soda Soda (( 梳打粉梳打粉))> can be used to neutralize

acidic bee stings on the wound.

VinegarVinegar((醋醋 ))VinegarVinegar((醋醋 ))

> can be used to neutralize alkaline wasp stings on the wound.

Lime( 石灰)

Lime( 石灰)

> is added to acidic soil to raise the pH value as plants only grow well at particular pH values.

Mosquito stings leave acidic secretion on the wound.

What kind of substances can we use use to to relieve the painrelieve the pain ??

p.43p.43

Have you ever eaten glutinous rice dumpling preserved in lye? What is the taste of it?

Lye is a natural alkali. It helps neutralize the acidic glutinous rice.

LyeLye is added is added in in making the making the

dumplingdumpling

LyeLye is added is added in in making the making the

dumplingdumpling

Do you add vinegar when eating a bowl of noodles?

Why do we Why do we add add vinegar vinegar

to it to it ??

Why do we Why do we add add vinegar vinegar

to it to it ??

Adding vinegar to a bowl of noodles can reduce the bitter taste by neutralizing the lye.

Lye is added to noodles to make them Lye is added to noodles to make them ‘elastic’ ‘elastic’ to eat. It explains why the to eat. It explains why the noodlesnoodles taste bitter. taste bitter.

Lye is added to noodles to make them Lye is added to noodles to make them ‘elastic’ ‘elastic’ to eat. It explains why the to eat. It explains why the noodlesnoodles taste bitter. taste bitter.

everyday uses of acids and alkalis 10.6 everyday uses of acids and alkalis

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