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Acids and Alkalis.ppt SDCDSdscSDCcCccCASCC
- 1. © Boardworks Ltd2003
- 2. © Boardworks Ltd2003 What is an acid? Here are some facts about acids. • Strong acids are corrosive “eating away” at things like metal, stone and flesh! • Weak acids, like lemon juice and vinegar, taste sour. • Acids turn litmus red. • Acids have a pH below 7. • Acids contain hydrogen (but not all things that contain hydrogen are acids!). • Acids can be neutralised with alkalis.
- 3. © Boardworks Ltd2003 Some common acids Three acids are particularly common in the laboratory . These are strong acids that should be treated with the greatest respect. Acid Formula Salts formed hydrochloric HCl chlorides sulphuric H2SO4 sulphates nitric HNO3 nitrates
- 4. © Boardworks Ltd2003 Indicators: acid or alkali? An indicator is a dye which changes colour according to whether it is in an acidic or alkaline solution. Litmus is available as a liquid that can be added to the solution. For example, litmus is an indicator that is red in acid and blue in alkali. It is also available as strips of paper so that you can add a drop of the solution to the paper.
- 5. © Boardworks Ltd2003 Litmus Test
- 6. © Boardworks Ltd2003 Indicators: how acid or alkali? Litmus will tell you whether a solution is acid or alkali: what it won’t tell you is how how acid or alkali. Universal indicator has a whole range of colours that tell us how strong an acid or alkali is. Strong acid Neutral Strong alkali Weak acid Weak alkali More alkali More acid
- 7. © Boardworks Ltd2003 Indicators: the pH scale This attaches a number called the pH value to each universal indicator colour. This means we can quickly say how acid or alkali a substance is by quoting a single number. 1 2 14 13 12 11 10 9 8 7 6 5 4 3 Strong acid Neutral Strong alkali Weak acid Weak alkali •pH7 is neutral •pH 1 is strongly acid •pH14 is strongly alkali
- 8. © Boardworks Ltd2003 Indicators: the pH scale
- 9. © Boardworks Ltd2003 Will it be acidic, basic or neutral and how strong? 1 2 14 13 12 11 10 9 8 7 6 5 4 3 Substance pH Description of acid / alkali Soda water 6 Car battery acid 1 Soap 8 Washing soda 10 Stomach acid 2 Oven cleaner 14 Vinegar 4 very weak acid very strong acid very weak alkali weak alkali strong acid very strong alkali weak acid Alkali Acid Indicators: the pH scale
- 10. © Boardworks Ltd2003 Bases Bases are substances that neutralise acids. Bases are usually: •Metal hydroxides contain OH •Metal oxides contain O •Metal carbonates contain CO3 The following general word equation describes neutralisations: acid + base a salt + water In the case of carbonates we also get carbon dioxide.
- 11. © Boardworks Ltd2003 Alkalis Bases are substances that neutralise acids. Alkalis are soluble bases. Although both can neutralise acids solubility is important when it comes to the pH of solutions. For example, adding sodium hydroxide to water gives a solution with a pH of about 14. When calcium carbonate is added to water it does not dissolve and so the pH remains close to 7. Even so it can neutralise acid that is added although more slowly than a soluble base might.
- 12. © Boardworks Ltd2003 Neutralisation - Indigestion If we have too much acid in our stomachs, we get indigestion. Acid can move up out of our stomach creating a burning feeling in the chest. We neutralise neutralise the excess acid by taking a tablet containing a base. This is usually a carbonate or an oxide. Strong soluble bases (like sodium hydroxide) would create too alkaline a solution and cannot be used. Alkali Acid 1 2 14 13 12 11 10 9 8 7 6 5 4 3
- 13. © Boardworks Ltd2003 Neutralisation - Stings A bee sting is acidic. One way to treat a basic basic wasp’s sting is with an acid acid : vinegar - ethanoic acid. So one way to treat a an acidic acidic bee sting is to dab on a base base: bicarbonate of soda more properly known as sodium hydrogen carbonate. A wasp sting is alkaline.
- 14. © Boardworks Ltd2003 Neutralisation – Soil pH Many plants won’t grow well in acid soil and so farmers have to regularly check the pH and adjust it by adding a base. Plants remove compounds from the soil in a way that tends to leave the soil acidic. Calcium carbonate or calcium hydroxide are cheap and so are often used for this purpose.
- 15. © Boardworks Ltd2003 Neutralisation – Soil fertilisers Ammonia is a water soluble gas high in nitrogen and in some countries it is injected directly into moist soil. However, because it is a gas much of it is quickly lost. In the UK ammonia is dissolved in water to give ammonium hydroxide (an alkali) and this is neutralised by reacting it with nitric acid to give a solid nitrogen rich fertiliser. Plants also remove nitrogen compounds from the soil and this is often replaced using fertiliser. water + ammonium nitrate nitric acid + ammonium hydroxide NH4OH + HNO3 NH4NO3 + H2O
- 16. © Boardworks Ltd2003 The gases are “scrubbed”, as much as possible, of these acidic oxides by reacting them with a base before releasing them into the air. Many power stations burn coal containing sulphur. When this burns it produces acidic sulphur oxides which can cause acid rain. Calcium oxide or calcium hydroxide are often used for this purpose. Neutralisation - acid gases
- 17. © Boardworks Ltd2003 Steps have been taken to reduce emissions of acidic sulfur oxides from power stations and nitrogen oxides from cars. Even so the atmosphere still contains enough of them to make the rain from industrial areas quite acidic. Acid rain – living things Trees and lakes are badly affected in many parts of the world including Northern Germany and Scandinavia which suffers from South-West winds from the UK. Acid rain damaged tress
- 18. © Boardworks Ltd2003 Acid rain increases the rate of corrosion of metals. Acid rain – metals and stone The metal above the wheel arch of this car is rusting away It also greatly accelerates the rate of chemical weathering of certain stones used in building such as limestone and marble. (These stones are carbonates. What gas will be given off as they dissolve?) CO2
- 19. © Boardworks Ltd2003 Acid Alkali Carbonate Chloride Hydrogen Indicator Indigestion Litmus Metal Neutralise Nitrate Nitrogen Salt Water • There are sixteen “easy-find” words in the word square. • Write a clue for each word drawing upon what you have studied in this unit of work I Q W R A C I D Q P S S W K G N E U T R A L I S E G A R M Y D P L X S R Q N I T R O G E N I M S U U B K D X O C G W Y I G S C H L O R I D E B Z S R F E W T Y F N S C M E T A L O J S G K D A A U A L K A L I J O T S A R T T L T C U W A T E R I I P O E E F O S D H I M U W O N K G E E U R A C O W U M D N O B E B C R C L Q K D S U J N V J N I T T A T E A J N B B
- 20. © Boardworks Ltd2003 Acids and Alkalis Multiple-Choice Questions
- 21. © Boardworks Ltd2003 A Sodium bicarbonate B Vinegar C Litmus D Salt 1. Which of these is an indicator?
- 22. © Boardworks Ltd2003 A Potassium hydroxide B Sodium chloride C Sodium hydroxide D Sodium 2. Which of these is a salt?
- 23. © Boardworks Ltd2003 A pH 7 B pH 1 C pH 8 D pH 13 3. Which of these is a weak alkali? A substance with:
- 24. © Boardworks Ltd2003 A Water - pH 7 B Lemon - pH 3 C Oven cleaner - pH 14 D Bicarbonate of Soda – pH 9 4. Which of these might you put on a ant sting to neutralise the acid in the sting?
- 25. © Boardworks Ltd2003 A Hydrogen B Sulphur C Oxygen D Nitrogen 5. Which of these do all acids contain?
- 26. © Boardworks Ltd2003 A Silver nitrate B Sodium hydroxide C Calcium hydroxide D Magnesium hydroxide 6. Which of these substances would not neutralise an acid?
Editor's Notes
- #5 TEACHER’S NOTES Results of the litmus test are : alkali acid water