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CGB IGCSE Chemistry

Acids and Alkalis

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Acids and Alkalis




Acids have an excess on H+ ions, the more H+ ions the more acidic the solution.


Acids turn universal indicator yellow, orange or red

Acids turn blue litmus paper red

Acids turn phenolphthalein solution clear


Acids have a pH numbers less than 7


All acids are solutions of pure compounds in water.  Solutions of non-metal oxides are acidic

            SO2 sulphur dioxide

            CO2 carbon dioxide

            P2O5 phosphorus (V) oxide


Some common acids

            Mineral Acids


Hydrochloric Acid HCl strong acid

            Sulphuric Acid H2SO4 strong acid

            Nitric Acid HNO3 strong acid

            Phosphoric Acid H3PO4 strong acid           

            Carbonic Acid H2CO3 weak acid


            Organic Acids


Citric Acid C6H8O7 weak acid

            Ethanoic Acid CH3COOH weak acid

Methanoic Acid HCOOH weak acid

            Lactic Acid CH3CH(OH) weak acid




Acids react with metals to form salts

            A salt is a compound made from acid when a metal takes the place of the hydrogen in the acid.


            Acid + metal Salt + Hydrogen


2HCl(aq) + Zn(s) ZnCl2(aq) + H2(g)

            H2SO4(aq) + Mg(s) MgSO4(aq) + H2(g)

            2HNO3(aq) + 2Na(s) 2NaNO3(aq) + H2(g)

            2H3PO4(aq) + 3Ca(s) Ca3(PO4)2(aq) + 3H2(g)

            3H2CO3(aq) + 2Al(s) Al2(CO3)3(aq) + 3H2(g)


The salt made depends on the acid


            HCl chlorides

            H2SO4 sulphates

            HNO3 nitrates

            H3PO4 phosphates

            H2CO3 carbonates




Alkaline solutions have an excess of OH- ions.  The more OH- ions the more alkaline the solution.


Alkaline solutions turn universal indicator blue, purple and violet

Alkaline solutions turn red litmus paper blue

Alkaline solutions turn phenolphthalein pink

Alkaline solutions turn methyl orange yellow


Alkaline solutions have a pH greater than 7


Metal oxides dissolve in water to form alkaline solutions


            Na2O Sodium oxide

            MgO Magnesium oxide

            CaO Calcium oxide


So what is the difference between alkalis and bases?


            Alkalis are substances that dissolve in water to give a pH greater than 7

            Alkalis turn red litmus paper blue

            Alkalis neutralize acids to form a solution with a pH 7.


            Bases are metal oxides and metal hydroxides which are insoluble in water.

            Bases neutralize acids to form a solution with a pH 7.


            Therefore Alkalis are a subset of a group of chemicals that can neutralize acids.


            Alkalis are soluble bases.


            All alkalis are bases, but not all bases are alkalis.


Common Alkalis


            NaOH sodium hydroxide strong alkali

            KOH potassium hydroxide strong alkali

            Ca(OH)2 calcium hydroxide (limewater) strong alkali

            NH3(aq) or NH3OH ammonia solution or ammonium hydroxide weak alkali


Common Bases


            CaO calcium oxide strong base

            MgO magnesium oxide strong base



Acids react with bases to form salts + water


            This is a neutralization reaction.


            acid + base salt + water


            HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)

            HCl(aq) + KOH(aq) KCl(aq) + H2O(l)

            2HCl(aq) + MgO(aq) MgCl2(aq) + H2O(l)

            2HCl(aq) + CuO(aq) CuCl2(aq) + H2O(l)


            HNO3(aq) + NaOH(aq) NaNO3(aq) + H2O(l)

            HNO3(aq) + KOH(aq) KNO3(aq) + H2O(l)

            2HNO3(aq) + MgO(aq) Mg(NO3)2(aq) + H2O(l)

            2HNO3(aq) + CuO(aq) Cu(NO3)2(aq) + H2O(l)


            H2SO4(aq) + 2NaOH(aq) Na2SO4(aq) + 2H2O(l)

            H2SO4(aq) + 2KOH(aq) K2SO4(aq) + 2H2O(l)

            H2SO4(aq) + MgO(aq) MgSO4(aq) + H2O(l)

            H2SO4(aq) + CuO(aq) CuSO4(aq) + H2O(l)


H3PO4(aq) + 3NaOH(aq) Na3PO4(aq) + 3H2O(l)

            H3PO4(aq) + 3KOH(aq) K3PO4(aq) + 3H2O(l)

            2H3PO4(aq) + 3MgO(aq) Mg3(PO4)2(aq) + 3H2O(l)

            2H3PO4(aq) + 3CuO(aq) Cu3(PO4)2(aq) + 3H2O(l)


Acids react with carbonates to give salt, water and carbon dioxide


            Acid + metal carbonate salt + water + carbon dioxide


            2HCl(aq) + CaCO3(s) CaCl2(aq) + H2O(l) + CO2(g)

            H2SO4(aq) + Na2CO3(s) Na2SO4(aq) + H2O(l) + CO2(g)

General Study Tips

How to outline a chapter/section.
1) Read the chapter/section carefully.  If you don't understand a section or concept write down a question to ask in class the next day.
2) Read the chapter/section objectives at the beginning of the chapter.
3) Write down one objective at the top of a blank piece of paper.
4) Re-read the chapter/section, keeping in mind the objective that you have written.  Any information that pertains to that objective write down underneath your objective.  When you are sure that you have written down all the information on that objective, write down the second objective on a new piece of paper and start the process again.
Tip- write only one objective per piece of paper--save room for class notes and class problems.

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