Physical and Chemical Change

This is part of Year 11 HSC Chemistry course under the topic of Chemical Reactions

• Use modelling to demonstrate:

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 (e.g. solids, liquids, gases). 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. 
   
   

Examples of Physical Changes

An example of a physical change is the boiling or evaporation of water which is represented by the following equation:

  

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

 

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

 

During evaporation of water, energy is absorbed to overcome the attractive intermolecular forces between water molecules including dispersion force, dipole-dipole force and hydrogen bonding. This will increase the average distance between water molecules as well as their average kinetic energy. 

Conversely, when water freezes, the average distance between molecules decreases resulting in stronger intermolecular forces. This in turn releases energy.

 

$$H_2O(l) \rightleftharpoons H_2O(s)$$

 

Both freezing and evaporation of water are physical changes as they do not change the underlying chemical structure or identity of water, only the state of matter changes.

What Are 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, the ash cannot be converted 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 that suggest a change is more likely to be a chemical reaction:

• 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). 
• Odour Production: A new smell might be noticed after a reaction 

 

The presence of above indicators does not always point towards a chemical changes as physical changes can sometimes exhibit these observations. For example, the boiling of water also involves gas production in the form of bubbling, as well as an increase in temperature. 

Examples of Chemical Changes

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:

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

Another example of chemical change is the oxidation of metals such as iron to form rust e.g. 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. 

Similarities and Differences Between Chemical & Physical Changes

Firstly, chemical changes involve the formation or breaking of chemical bonds, resulting in the creation of entirely new substances with different chemical compositions from the original materials. This transformation is often irreversible, fundamentally altering the chemical identity of the substances involved.

Energy changes during chemical changes can manifest as either endothermic or exothermic reactions. Endothermic reactions absorb heat from their surroundings, resulting in a decrease in temperature of the immediate surroundings as energy is consumed in bond-breaking. Conversely, exothermic reactions release heat into their surroundings, often increasing the temperature nearby as energy is released from new bond formations.

On the other hand, physical changes do not alter the fundamental chemical composition of substances but rather involve changes in their physical state or appearance, such as melting, freezing, or dissolving. These changes are typically reversible and do not result in the formation of new substances.

Energy changes during physical changes are primarily characterised by changes in the state of matter (solid, liquid, gas), with heat either absorbed (endothermic) or released (exothermic) depending on the direction of the change (e.g., melting requires heat absorption, while freezing releases heat). While both chemical and physical changes involve energy changes, the amount of energy change that occurs in chemical reaction is generally greater as more energy is absorbed/released during chemical bond breaking and forming. 

 

Next section: Synthesis and Decomposition