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C8.1 The Characteristic Properties of Acids & Bases Bases.notebook
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February 08, 2015
Alkalis
BasesSubstances that can react with acids and neutralise them to make a salt and water are called bases. They are usually metal oxides or metal hydroxides. For example, copper oxide and sodium hydroxide are bases.
Bases that dissolve in water are called alkalis. Copper oxide is not an alkali because it does not dissolve in water. Sodium hydroxide is an alkali because it does dissolve in water.
pH higher than 7
Neutral solutions
Neutral solutions have a pH of 7. They do not change the colour of litmus paper, but they turn universal indicator paper green. Water is neutral.
C8.1 The Characteristic Properties of Acids & Bases
C8.1 The Characteristic Properties of Acids & Bases Bases.notebook
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February 08, 2015
Lime Calcium Hydroxide
Lye Sodium hydroxide
Baking soda Sodium bicarbonate
Antacid Calcium carbonate
Ammonia Ammonia
C8.1 The Characteristic Properties of Acids & Bases
Common Bases
C8.1 The Characteristic Properties of Acids & Bases Bases.notebook
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February 08, 2015
Alkali Conductivity (S/cm pH
sodium hydroxide 20 13.0 strong alkali
potassium hydroxide 15 13.0 strong alkali
ammonia solution 0.5 11.1 weak alkali
Comparing alkalis
When alkalis dissolve in water they produce aqueous hydroxide ions, OH–(aq). For example, looking at sodium hydroxide:NaOH(aq) → Na+(aq) + OH–(aq)Ammonia is slightly different. This is the equation for ammonia in solution:NH3(aq) + H2O(l) → NH4
+(aq) + OH–(aq)only some of the ammonia molecules form ions.
C8.1 The Characteristic Properties of Acids & Bases
C8.1 The Characteristic Properties of Acids & Bases Bases.notebook
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February 08, 2015
Before industrialisation in the 1700s, alkalis were still needed for a number of uses:• Converting oils and fats into soap • Making glass • Neutralising acidic soils • Making chemicals that bind natural dyes to cloth.
Alkalis
Traditional sources of alkalis to enable these processes included burnt wood (potash) and stale urine (ammonia).
Alkalis contain an important property that allows them to neutralise acids. When this happens a salt is made.acid + alkali → salt + water
C8.1 The Characteristic Properties of Acids & Bases
C8.1 The Characteristic Properties of Acids & Bases Bases.notebook
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February 08, 2015
Reactions of alkalisCommon examples of alkalis include soluble hydroxides and carbonates. When each of these alkalis reacts with an acid, there is a difference in the products that are formed.
When an alkaline hydroxide reacts with an acid, salt and water are formed. This is shown by the general equation:Alkaline hydroxide + acid → salt + waterFor exampleSodium hydroxide + hydrochloric acid → sodium chloride + water
When an alkaline carbonate reacts with an acid, salt and water are again formed but there is also a third product, carbon dioxide.Alkaline carbonate + acid → salt + water + carbon dioxideFor exampleCalcium carbonate + sulfuric acid → calcium sulfate + water + carbon dioxide
Note how the name of the salt is found the first part of the name comes from the metal in the alkali used, the second comes from the acid used.
C8.1 The Characteristic Properties of Acids & Bases
C8.1 The Characteristic Properties of Acids & Bases Bases.notebook
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February 08, 2015
Reactive metalsAcids will react with reactive metals, such as magnesium and zinc, to make a salt and hydrogen:
acid + metal → salt + hydrogen
The hydrogen causes bubbling during the reaction, and can be detected using a lighted splint.
For example:Sodium carbonate + hydrochloric acid → sodium chloride + carbon dioxide + waterMg + H2SO4 → MgSO4 + H2
Acids and basesWhen acids react with bases, a salt and water are made:
acid + metal oxide → salt + water acid + metal hydroxide → salt + water
Remember that most bases do not dissolve in water. But if a base can dissolve in water, it is also called an alkali.For example:Sodium hydroxide + hydrochloric acid → sodium chloride + waterNaOH + HCl → NaCl + H2O
Carbonates and hydrogen carbonatesCarbonates and hydrogen carbonates are two other types of base. They also make a salt and water when we neutralise them with acid. But this time we get carbon dioxide gas too.
The reaction fizzes as bubbles of carbon dioxide are given off. These are the general word equations for what happens:
acid + metal carbonate → a salt + water + carbon dioxideacid + metal hydrogen carbonate → a salt + water + carbon dioxide
For example:Sodium carbonate + hydrochloric acid → sodium chloride + carbon dioxide + waterNa2CO3 + 2HCl → 2NaCl + CO2 + H2O
C8.1 The Characteristic Properties of Acids & Bases
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