Low pH values mean high acidity (or low alkalinity). 7. High pH values mean high alkalinity (or low acidity). 8. Substances with pH values < 7 :acidic; pH values = 7 :neutral; pH values > 7:alkaline. 10. 3 Acids and Corrosion 1. Dilute acids react with some metals and produce hydrogen. 2. Test for hydrogen: Put a burning splint near the mouth of a test tube containing the gas. If the gas is hydrogen, it burns with a 'pop' sound. 3. The rate of reaction of some metals with dilute hydrochloric acid or dilute sulphuric acid: maznesium > zinc> iron > copper (no reaction) . Carbonates react with acids and give carbon dioxide. 5. Marble and limestone mainly contain calcium carbonate. They are used as building materials and they can be corroded by acids. 10. 4 Safety Related to the Use of Acids and Alkalis 1. Strong acids / alkalis are corrosive. Weak acids / alkalis are generally less corrosive. 2. Dilute acids / alkalis contain a low percentage of acids / alkalis. Concentrated acids / alkalis contain a high percentage of acids / alkalis. 3. Concentrated strong acids and alkalis are highly corrosive. 4. Safety measure in handling strong acids and alkalis |Reason | |(a) Stick the hazard warning label ‘corrosive ‘ on the surface|To warn users about the corrosive property of the chemicals. | |of the container. | | |(b) Wear safety spectacles. |To prevent the acids and alkalis from getting into the eyes. | |(c) Put on protective gloves. |To prevent our skin from contact with the acids or alkalis. | |(d) Wear laboratory coat. |To prevent damage of clothing and body. |(e) Work in a fume cupboard. |To prevent vapours of the acids or alkalis from irritating our| | |eyes and respiratory system. | 5. To dilute a concentrated acid or alkali, always add it slowly to a large amount of water with stirring. Never pour water into a concentrated acid or alkali. 6. The first step in treating acid or alkali spillage on our body is to wash with plenty of water. 10. 5 Acid Rain 1. Clean or normal rain has a pH value of 5. 6. 2. Acid rain has a pH value lower than 5. 6. 3.
Practice test: answer keyAcid rain is mainly caused by the acidic pollutant gases sulphur dioxide and nitrogen oxides. 4. The main sources of sulphur dioxide and nitrogen oxides in the air are motor vehicles, power stations and factories. 5. Acid rain may ? corrode structures made of metals, marble and limestone, ? kill fish and plants in lakes and rivers, and ? slow down plant growth and even kill plants. 6. The government, industries and citizens should work together to reduce the release of sulphur dioxide and nitrogen oxides, which cause acid rain. 10. 6 Neutralisation 1. An alkali and an acid can neutralise each other. . Neutralisation occurs when an alkali is mixed with an acid until the resulting solution becomes neutral (pH = 7). 3. When an acid is neutralised with an alkali, a salt and water are formed. The word equation for neutralisation is: Alkali + Acid (Salt + Water 4. Antacids are weak alkalis used to neutralise excess acid in the stomach. 5. Weak acids, such as ethanoic acid in vinegar, can be used to neutralise the alkaline stings of wasps. 6. Weak alkalis, such as baking soda (sodium hydrogencarbonate), can be used to neutralise the acidic stings or bites of bees, ants and mosquitoes. . Weak alkalis, such as slaked lime (calcium hydroxide) and powdered limestone, can be added to acidic soil to raise the pH for plant growth. 8. Acids in industrial wastes can be neutralised by adding alkalis such as sodium hydroxide, while alkalis can be neutralised by adding acids such as sulphuric acid. 10. 7 Daily Uses of Acids and Alkalis 1. Acids and alkalis can be used in cleaners to remove stains and grease. 2. Acids such as ethanoic acid can be used to preserve food. 3. Acids can be used to prevent the browning of fruits. ----------------------- [pic] [pic]
