Chemical Changes

Physical & Chemical Changes

  • Changes in matter take place around us all the time. These changes involve living and non-living matter.

    • E.g. Some changes like: Burning paper → fast. Rusting → slow.


Physical Change

(E.g. Evaporation, boiling, sublimation)

Chemical Change

Is the change reversible? 

Usually reversible, when no new substances are formed

Usually, irreversible changes where new substances are formed. 

Are new chemical products formed?

No

Yes – Compound properties are different from their constituent elements. 

Does the change involve a release or absorption of energy?

Yes – Some changes occur when heating or cooling is done. Friction may also lead to an energy change/ 

Yes – Always. Interaction between matter and energy can take place in the form of heat or light (and electricity)

Does the change directly affect the chemical bonds of the substances involved? 

No

Yes


E.g. of 

Physical changes: 

  1. Expansion of a metal when heated (expansion and contraction)

  2. Sublimation of dry ice into carbon dioxide vapour (change of physical states)

  3. Melting of ice

  4. Metal rod turns red when heated

  5. Heating + Magnetic effects of electricity

Chemical changes: 

  1. Formation of water = H2O

  2. Aerobic respiration = C6H12O6 + O2 → CO2 + 2H2O + energy

  3. Formation of salt (sodium chloride) = 2Na + Cl2 → 2NaCl

  4. Photosynthesis = CO2 + 2H2O → C6H12O6 + O2  

  5. Combustion/Burning? = Wood + O2 → Burning → CO2 + H2O

  6. Thermal decomposition = CaCO3 → Heat → CaO + CO2 

  7. Electrolysis (Electricity)


Representing Chemical Changes

→ Word equations may be used to represent chemical reactions, showing reactants, products and sometimes the conditions allowing the chemical reaction to happen.

* use → for equations


Acids and Bases

  • Acids = Substances that release hydrogen ions (H+) when dissolved in water.

  • Bases = Any substance that reacts with an acid to form salt and water only.

  • Alkali is a soluble base that dissolves in water to produce hydroxide ions (OH-).


Acids

  • E.g. Hydrochloric acid HCl, Nitric acid HNO3, Sulfuric acid H2SO4


Bases

  • Metal oxide or metal hydroxides

    • e.g. sodium oxide, sodium hydroxide, iron oxide, iron hydroxide

  • Alkali is a soluble base.

    • Soapy, bitter

      • e.g. Antacids, cleaning soap, detergent

Reactions !

★ Acid + Base → Salt + Water

  • Bases are metal oxides and metal hydroxides

    • E.g. Sodium oxide, Magnesium oxide, potassium oxide, iron oxide, calcium oxide, sodium hydroxide, magnesium hydroxide 

    • Alkalis are soluble bases.


  • Acids react with bases (including alkalis) to produce a salt and water only.

  • This reaction is called neutralisation because a pH-neutral solution is formed.


E.g. 

  • sulfuric acid + copper (II) oxide → copper (II) sulfate + water

  • nitric acid + sodium hydroxide →  sodium nitrate + water

  • Hydrochloric acid + magnesium hydroxide → magnesium chloride + water

  • hydrochloric acid + barium oxide →  barium chloride + water

  • sulfuric acid + potassium hydroxide  → pottasium sulfate + water


→ Salt is a compound consisting of metal positive ions and non metal negative ions. 

E.g. sodium chloride, magnesium sulfate, potassium nitrate, iron chloride


(first metal ion → cation ,, second non-metal ion → (anion))


Test for neutralisation

  • Universal Indicator Solution & Universal Indicator Paper will turn green.

  • Methyl orange indicator turns orange.


★ Acid + Metal → Salt + Hydrogen gas

  • Some metals are too unreactive and do not react with dilute acids. 

    • E.g. copper, silver, gold

      • Hydrochloric acid + copper → no reaction

E.g.

  • hydrochloric acid + iron → iron chloride + hydrogen gas

  • nitric acid + zinc → zinc nitrate + hydrogen gas

  • sulfuric acid + potassium → potassium sulfate + hydrogen gas


Due to the production of hydrogen gas, effervescence is observed.

Test for hydrogen gas

  • Insert a lighted splint into the test tube.

  • If the lighted splint extinguishes with a pop sound, hydrogen is present.


★ Acid + Metal Carbonate → Salt + water + carbon dioxide

  • All metal carbonates react with acids.

  • Due to the production of carbon dioxide gas, effervescence is observed. 


E.g. 

  • hydrochloric acid + sodium carbonate → sodium chloride + water + carbon dioxide

  • sulfuric acid + magnesium carbonate →   magnesium sulfate + water + carbon dioxide

  • nitric acid + zinc carbonate →  zinc nitrate water + carbon dioxide

  • hydrochloric acid + calcium carbonate →  calcium chloride + water + carbon dioxide

  • hydrochloric acid + copper carbonate →  copper chloride + water + carbon dioxide


Test for carbon dioxide gas

  • Pass the gas through limewater (calcium hydroxide)

  • If white precipitate is formed in the limewater, carbon dioxide gas is present


Note:

sulfuric acid → sulfate salts

nitric acid → nitrate salts

hydrochloric acid → chloride salts