[1NA] Chapter 18: Chemical Changes

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    1. You need to know how to tell if a change is a chemical change or just physical change. Refer to TB page 171.
    2. We need to know how to produce and test for presence of two gases – hydrogen and carbon dioxide.
    3. How to tell if a solution is an acid or alkali?
    4. You need to know the formation of acid rain and relevant chemical formulae.

 

Hydrogen gas

How to produce?

metal + acid –> salt + hydrogen

e.g zinc + sulfuric acid –> zinc sulfate + hydrogen

How to test?

A lighted split will be extinguished with a ‘pop’ sound. This means hydrogen is present.

Carbon dioxide gas

How to produce?

Method 1: Acid reacts with carbonate

carbonate + acid –> salt + water + carbon dioxide

e.g calcium carbonate + sulfuric acid –> calcium sulfate + water + carbon dioxide

Method 2: Thermal decomposition of carbonate

e.g Calcium carbonate —heat—> Calcium oxide + carbon dioxide

*heat should be written directly above the arrow and not on the same line.

Method 3: Combustion

e.g carbon + oxygen –heat–>  carbon dioxide

How to test?

When the gas is bubbled through limewater, white precipitate will form. Carbon dioxide gas is present.

Neutralisation 

Neutralisation is when acid reacts with alkali.

acid + alkali –> salt + water

e.g nitric acid + calcium hydroxide –> calcium nitrate + water

** This is why we brush our teeth every day. Our food is generally acidic and our body also produces acid to break down the food.

Different ways that we can use to test for acid/alkali?

  1. Litmus paper. If blue litmus paper turns red, then the solution is an acid.
  2. pH meter. A substance is considered neutral if it has a pH value of 7. Alkaline substances have pH values more than 7 while acidic substances have pH values of less than 7.
  3. Universal indicator.
  4. Data logger with acid/alkali sensor.

Note: Using universal indicator is better than using litmus paper. Universal indicator provides us with the strength of acid/alkali solution whereas litmus paper can only tell us if the solution is acidic or alkaline.

Some important chemical formula that you need to remember

H2SO4 : Sulfuric acid

CO2 : Carbon dioxide

HNO3 : Nitric acid

SO2 : Sulfur dioxide

SO3 : Sulfur trioxide

NO2 : Nitrogen dioxide

Acid rain

Acid rain is formed due to either sulfur trioxide or oxide of nitrogen dissolves in rainwater.

sulfur trioxide dissolves in rainwater –> sulfuric acid

word equation:  sulfur trioxide + water –> sulfuric acid

oxide of nitrogen dissolves in rainwater –> nitric acid.  

word equation: oxides of nitrogen + water –> nitric acid