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Lower Secondary Science G2 Chapter 11 : Chemical Changes - Coggle Diagram
Lower Secondary Science G2 Chapter 11 : Chemical Changes
physical change
No new substances are formed
products formed are mostly reversable
propertis of substances do not change
Change of state like melting of ice and boiling of water Peanut grinding to peanut butter Sugar dissolving in water to form sugar solution
chemical change
New substances are formed
products formed are not reversable
Properties of products are different from starting reactants
Rusting Raw egg cooked to fried egg Water undergoing electrolysis to form oxygen and hydrogen Sugar undergoing fermentation to form alcohol
type of reactions
oxidation
A reaction in which a substance reacts with oxygen
Iron + oxygen >iron oxide
combustion
A reaction of a substance with oxygen in the presence of heat One or more substances may be produced.
Carbon + oxygen > carbon dioxide + water
neutralisation
A reaction between acids and alkalis, forming acid and alkali
Acid + alkali → salt + water
thermal decomposition
A reaction in which a substance is broken down into 2 or more simpler substances with heat.
Sugar > carbon + water vapour
electrolysis
A reaction in which a substance is decomposed into 2 or more simpler substance with electric current
Water > hydrogen + oxygen
What about reaction between matter and light?
Decomposition by light
Silver bromide > silver + bromine
Photosynthes
Carbon dioxide + water > glucose + oxygen
ACID
hydrochloric acid
used to remove rust on iron and steel
nitric acid
used in fetilisers
sulfuric acid
found in car batterys
Alkali
sodium hydroxide
used to make soaps and detergents
calcium hydroxide
used in food preparation e.g. found in Chinese century eggs
aqueous ammonia
make fertilisers
Properties of Acids:
Taste: Sour (e.g., lemon juice, vinegar)
pH Value: Less than 7
Litmus Test: Turn blue litmus paper red
Reacts With Metals: Produces hydrogen gas
Reacts With Carbonates: Produces carbon dioxide gas
Corrosive: Can be harmful to skin and materials
Properties of Alkalis
Taste: Bitter and slippery to touch
pH Value: Greater than 7
Litmus Test: Turn red litmus paper blue
Reacts With Acids: Produces salt and water (neutralisation)
Can Be Corrosive: Strong alkalis can burn skin
How to Test for Hydrogen Gas:
Collect the gas in a test tube or bring a lit splint near the mouth of the test tube where hydrogen is being released.
If hydrogen is present, it will burn with a 'pop' sound.
Why This Happens:
Hydrogen is a flammable gas. When it mixes with air and is exposed to a flame, it combusts quickly, producing a small explosion — the popping sound.
How to Test for Carbon Dioxide Gas:
Bubble the gas through limewater (a clear solution of calcium hydroxide).
If carbon dioxide is present, the limewater will turn milky or cloudy.
Why This Happens:
Carbon dioxide reacts with calcium hydroxide in limewater to form calcium carbonate, which is a white, insoluble solid that makes the solution appear milky.