Chapter 14: Chemical changes

Different changes


1. Chemical change

• A chemical change that is permanent
• A new substance will be formed
• A chemical change is irreversible
• Chemical reaction* is involved
• Heat/ Light will be given out or taken in 
• Properties of products differ from starting reactants
• E.g. Raw egg becomes a cooked egg

2. Physical change

• A physical change is temporary
• No new substances is formed
• A physical change is reversible
• No chemical reaction* is involved
• No/ little heat or light will be given out of taken in
• Propeties of substances do not differ
• E.g. Liquid ---> Gas (Boiling)

*A Chemical reaction is a process whereby reactants are changed into products and no matter is destroyed or created, only rearrangement of matter as occurred. It is represented in a chemical equation.


Balancing chemical equations
Balancing chemical enables scientist to know the number of atom involved in the chemical reaction. 


How to balance an equation?
Steps:


  1.  Write down the formula of reactants and
     
      N2 + H3 = NH3     


  2. Check the number of atoms on each element on both sides of the Chemical equation


  3.  Balance the equation by placing number in front of the formulae of the substances in the equation.


      N2 + 3H2 = 2NH3


Interaction between matter and heat


1. Changes in state of matter [physical change]
Heat can cause matter to change in state.
For example, heat causes solid to melt and become liquid.


2. Expansion and contraction [physical change]
This change of expansion is more obvious in liquids and gases as compared to solids. As the matter loses heat, it contracts. Thus, these 2 processes are reversible.


3. Combination [chemical change]
Combination only occurs in the presence of some factors(different processes requires different factors) where 2< substances combine to form 1< new substances.

• E.g. Iron fillings + sulfur ---> Iron sulfates (in the presence of heat)
• Iron/steel ---> Rust (in the presence of water and oxygen)

4. Combustion [chemical change]
Combustion is a process at which oxygen and another substance are combined together to form 1< new substances in the presence of heat.

• E.g. Fireworks are created when: Magnesium + oxygen(in the presence of heat) ---> magnesium oxide (a bright flash will be in the process)

5. Thermal decomposition [chemical change]

Thermal decomposition refers to a process whereby a substance is broken down into 2< simpler substances by heat.

• E.g. Calcium carbonate ---> Calcium oxide + carbon dioxide(in presence of heat)
• Copper carbonate ---> copper oxide + carbon dioxide(in presence of heat)

Interactions between Matter and light

1. Photosynthesis [chemical change]

Photosynthesis is a process at which carbon dioxide and water are converted to glucose and oxygen in the presence of sunlight and chlorophyll. This process only occurs in plants.

2. Photography [chemical change]

Thin, plastic films used in photography are coated with chemicals, in particular silver bromide crystals that changes to silver by light. An image will be created as the film is exposed to light.

3. Solar cells 

Solar cells, also known as photo voltaic cells, are made up of silicon or selenium which are capable of converting light energy to electricity.

Interactions between matter and electricity

1. Electricity causes these effects as it passes through matter:

• Heating effect
• production of light
• magnetic effect
• chemical effect

Interactions between matter-mixing substances

1. Physical change

• E.g. Adding of syrup into plain drinking water (no new substances are formed)

2. Chemical change

• E.g. adding acid to carbonate/metal               
• Copper carbonate + Hydrochloric acid ---> Copper chloride + oxygen + Carbon dioxide gas               Copper + Hydrochloric acid ---> Copper chloride + hydrogen gas