Everyday uses of acids and alkalis 10.6 Everyday uses of acids 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 produce toxic substances which cause food poisoning ( 食物中毒 ).

Why do Why do micro- organisms decay 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, dizziness, vomit and high temperature etc. These are symptoms of food poisoning.

Acids can be used for food preservation - Acids can be used for food preservation ( 食物防腐 ). - Vinegar is commonly used for food preservation. Solution of ethanoic acid (or called acetic acid) e.g.

Percentage of vinegar needed to preserve fruits p.37 Cut into small pieces Wash under running water Dry pieces with a cloth or blotting paper Soak pieces into vinegar and water for observation Experiment Centre 10.12

Slows down their activities or even kills them - 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 advantages and disadvantages of using acids for food preservation?

Effect of pH on browning of fruits p.39 Fruits like apple, banana, pear and peach turn brown when they are cut and left in the air for a period of time. What substances can protect the fruit better? You can organize your ideas by filling in the investigation planner. Experiment Centre 10.13

Many cleansing agents contain acids or alkalis. Acids and alkalis as cleansing agents p.41

Toilet cleanser contains hydrochloric acid Drain cleanser contains sodium hydroxide Glass cleanser contains ammonia

Resulting substances can be wiped or washed away with water easily. Why can Why can acids and alkalis act as cleansing agents? Acids react with dirts Alkalis react with greases

Word equation: Neutralization ( 中和作用 ) p.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 s ss salts and w ww water remain after neutralization. + AlkaliAcid +WaterSalt => salts => Chemical reaction takes place between acids and alkalis forming salts and water.

neutralization of an acid an alkali. Neutralization is a chemical reaction. - Salt and water are formed from the neutralization of an acid and an alkali. Neutralization is a chemical reaction. - Chemical reaction is a process in which new substances are formed. + AlkaliAcid +WaterSalt AlkaliAcid

Add 1 cm 3 dilute sodium hydroxide to the acid Neutralizing a dilute acid with a dilute alkali p.44 Pour 10 cm 3 dilute hydrochloric acid into a beaker Stir solution with a glass rod and observe colour change Experiment Centre Add 1 or 2 drops of universal indicator solution to the acid

or salt water. - Adding acid into alkali or adding alkali into acid can cause neutralization. The products formed are salt and water. an - When an acid is just neutralized by an alkali, neutral solution is formed. The pH value should be 7. Why can’t we see the salt in the above experiment? excess alkali - When excess alkali is added into the acid, the pH value increases to above 7. + AlkaliAcid +WaterSalt + AlkaliAcid Neutral solution

The products of neutralization p.46 To find out what are produced in neutralization in the above experiment, the simplest method to use is evaporation. Measure 10 cm 3 dilute hydrochloric acid and 10 cm 3 dilute sodium hydroxide and mix solutions Put neutral solution on a watch glass Put watch glass over a hot water bath Experiment Centre 10.15

white solid is formed steam (salt) + AlkaliAcid +WaterSalt

Salts —— a group of chemicals p.48 => => Salts are a group of chemicals produced by neutralization. e.g. Hydrochloric acid Sodium hydroxide Sodium chloride + water + Nitric acid Potassium hydroxide Potassium nitrate + water + Sulphuric acid Magnesium hydroxide Magnesium sulphate + water + Carbonic acid Sodium hydroxide Sodium carbonate + water + + AlkaliAcid + WaterSalt

- We commonly use salts in our daily life. e.g. > as table salt Sodium chloride > used for production of chlorine and sodium hydroxide

Potassium nitrate > used in matches Magnesium sulphate > used in bath salt

Application 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.42 alkaline solutions

- Other than helping reduce harmful substances, neutralization can also control the pH values of substances. e.g. > is an alkali. Antacid ( 制酸劑 ) > can neutralize excess acid in our stomach.

Soda ( 梳打粉 ) > can be used to neutralize acidic bee stings on the wound.

Vinegar ( 醋 ) > can be used to neutralize alkaline wasp stings on the wound.

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 to relieve the pain ?

p.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. Lye is added in making the dumpling

Do you add vinegar when eating a bowl of noodles? Why do we add vinegar 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 ‘elastic’ to eat. It explains why the noodles taste bitter.