Physical Changes Vs Chemical Changes

Chemical Reactions Nuggets

Chemical Reactions Nuggets 
last updated 19th Jan 2014

 Physical and Chemical Changes
  1. Physical reactions/changes doesn’t form new substances and such changes are reversible. (e.g. melting ice, boiling water)
  2. In physical reactions, little or no heat/light is taken in or given out.
  3. Chemical reactions involves chemical changes which forms new substances. (e.g. burning of paper, photosynthesis)
  4. In general, these new substances are different in color, texture when compared to the reactants. Their states(solid/liquid/gas) may also be different. **(do note that in some cases, chemical reactions don’t involve any colour change. for e.g. methane burning in oxygen gas to form carbon dioxide and water vapour. In this reaction, all the reactants and products are colourless. CH4(g) + 2O2(g) —> CO2(g) + 2H2O(g))
  5. In chemical changes, the products usually don’t resemble the reactions. (refer to point 4 for exceptional cases)
  6. We use a word equation or a chemical equation to represent chemical reactions. 
  7. Carbon + Oxygen –> Carbon Dioxide (Word Equation)
  8. C + O2 –> CO2 (Chemical equation)
  9. In a chemical reaction, the total mass of reactants before the reaction is equal to the total mass of the products after the reaction. 
  10. In a chemical reaction, no atoms are created or destroyed. They simply re-arrange among themselves to form new molecules. 
  11. Chemical reactions are usually irreversible but some are reversible. (refer to the list below for such examples)
  12. Chemical reactions can be started by heat, mixing, light, electric current.
  13. Some signs (or evidence) of chemical change are:
    • a gas is produced,
    • the temperature changes,
    • a substance disappears,
    • a solid is formed
    • a colour change occurs,
    • a new odour is produced.
Examples of Physical Changes
  • water turning into ice (freezing)
  • salt dissolves in water (dissolution)
  • evaporation (liquid -> gas)
  • Zinc oxide turns from white to yellow upon heating and turns white again upon cooling
  • magnetising and demagnetising a magnet
  • expansion and contraction of materials under the effect of heat
Examples of Chemical Changes that are reversible

( —><— sign refers to reversible reactions)
  • Sulphur dioxide  + Water   —><—  Sulphurous acid 
  • Ammonia + Water —><— Ammonium hydroxide
  • Iron + Steam —><—  Iron oxide + Water 
  • Calcium Carbonate —><—  Calcium Oxide + Carbon dioxide
  • Ammonium Chloride —><—  Ammonia Gas + Hydrogen Chloride Gas (this reaction is commonly mistake for a sublimation process)

Different Types/Categories of Chemical Reactions 

  1. Combination (Synthesis)
  2. Decomposition (thermal, light)
  3. Single Displacement
  4. Double Displacement (metathesis reaction)
  5. Complete Combustion and Incomplete Combustion
  6. Hydrolysis
  7. Oxidation
  8. Reduction
  9. Neutralisation (Acid-Alkali)
  10. Precipitation
  11. Electrolysis
Types of Chemical Reactions by Heat
  • combination
  • combustion
  • thermal decomposition
  • oxidation (may also happen without heat)
Explanation for the different types of Chemical Reactions


This involves 2 or more substances going through a chemical reaction to form a single new product.

  • Fe + S —> FeS (involving elements)
  • C + O2 —> CO(involving elements)
  • Fe + O2 —> Fe2O(involving elements)
  • 2Na + Cl2 —> 2NaCl (involving elements)
  • PCl3 + Cl2 —> PCl(involving compounds and elements)
In this reaction Fe + S —> FeS , we know this is a chemical change because the product looks different form the reactants. Fe is a silvery metal and S is a yellow solid. But the product is a black solid.

Complete Combustion

Combustion is also known as burning. Combustion is also known as a type combination reaction. In a combustion reaction, oxygen is combined with other reactants in the presence of heat. The end product of combustion reactions are CO2 and H2O.

  • combustion of methane
  • CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)
  • burning of naphthalene
    C10H8 + 12O2 → 10CO2 + 4H2O

  • combustion of ethane
    2C2H6 + 7O2 → 4CO2 + 6H2O
Incomplete Combustion
Combustion reactions can also sometimes be incomplete. In such cases, carbon dioxide (CO2) will not be formed. Instead, carbon monoxide (CO) is formed. CO is a poisonous gas. It is also odourless which makes it hard to be detected. 
Single Displacement

Single displacement reactions has this general pattern. A + BC —> AB + C 
In single displacement reactions, one of the elements in the compound is replaced by another element.

Li(s) + NaCl(aq) > LiCl(aq) + Na(s)

F2(aq) + 2KI(aq) —> I2(aq) + 2KF(aq)

Double Displacement

The general pattern of a double displacement reaction is:

AB + CD —> AD + CB

  • 2KI(aq) + Pb(NO3)2(aq) —> PbI2(s) + 2KNO3(aq)
  • MgCl2(aq) + Ca(OH)2(aq) > Mg(OH)2(s) + CaCl2(aq)


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