Physical and Chemical Change

Physical changes are alterations in the particle structure of a substance. These changes often cause the physical properties of a substance to change. Some key points regarding physical changes are:

1. No new substances are formed: In a physical change, the composition of a substance doesn't change. For example, ice melting into water is a physical change because it still consists of two hydrogens and one oxygen in both states. 
   
   
2. Usually Reversible: Most physical changes can be reversed. For instance, liquid water can be frozen back into ice or evaporated into steam, then condensed back into liquid water. 
   
   
3. Changes in State: Physical changes often involve a change in the state of matter. This includes freezing, melting, boiling, condensing, sublimating and depositing. 
   
   
4. Change of Appearance: A physical change might alter the shape, size, colour, or phase of a substance, but not its molecular or ionic structure. Crushing a can or tearing a piece of paper are examples of physical changes; the material remains aluminium or paper respectively. 
   
   
5. No Change in Chemical Bonds: In a physical change, the chemical bonds between atoms or ions in a molecule are not broken or formed. Only intermolecular forces, like hydrogen bonds holding water molecules together, might be disrupted or formed.  
   
   

 An example of a physical change is the boiling of water which follows the following formula

$$H_2O(l) \rightleftharpoons H_2O(g)$$

(note: the molecular formula for the compound `H_2O` has not changed) 

Chemical Changes

In contrast, chemical changes, also known as chemical reactions, are processes in which one or more substances are transformed into new substances with different chemical properties. They are characterised by the following:

1. Formation of New Substances: In a chemical change, the original substance or substances are converted into one or more new substances. For example, when iron (`Fe`) reacts with oxygen(`O_2`) , it forms iron oxide (`Fe_2O_3`) (commonly known as rust). 
   
   
2. Irreversible: While some chemical reactions can be reversed under specific conditions, many are essentially irreversible under normal circumstances. For instance, once wood is burned to ash, you cannot turn the ash back into wood. 
   
   
3. Energy Changes: Chemical reactions often involve changes in energy. Energy can either be released (exothermic reactions) or absorbed (endothermic reactions). Lighting a matchstick is one example of an exothermic reaction as it releases energy in the form of heat and light. 
   
   
4. Changes in Chemical Bonds (Intramolecular Forces): In chemical reactions, the bonds between atoms in molecules are broken and new ones are formed. For example, in the combustion of methane `CH_4`, the carbon-hydrogen bonds in methane break, and new bonds form to produce carbon dioxide `CO_2` and water `H_2O`
   
   
5. Indicators of a Chemical Reaction: Several signs can indicate a chemical change, including:

• Colour Change: Like when copper turns green due to oxidation
• Gas Production: Such as the fizz when baking soda and vinegar are mixed
• Formation of a Precipitate: When two clear solutions mix and produce a solid. 
• Change in Temperature: Reactions can either be exothermic (temperature increase) or endothermic (temperature decrease). 
• Odor Production: A new smell might be noticed after a reaction 

Contrary to physical changes, which involve a transformation in a substance's state or form without altering its chemical identity, chemical changes result in the creation of substances with different chemical properties. 

An example of a chemical change is the electrolysis of water whereby water is split into its constituent atoms hydrogen and oxygen according to the following formula:

$$2H_2O(l) \rightleftharpoons 2H_2(g) + O_2(g)$$ 

Another common example of chemical change is the oxidation of iron to form iron rust:

$$4Fe(s) + 3O_2(g) \rightarrow 2Fe_2O_3(s)$$

In summary, if you're evaluating whether a change is chemical or physical, consider if the substance itself has transformed into something new. If it has, then it is likely a chemical change. If it remains essentially the same, just in a different form or state, it's a physical change. 

Next section: Synthesis and Decomposition