When we talk about matter and how it changes, it's important to know the difference between physical and chemical changes. Both types of changes affect matter, but they do it in different ways.

1. What Are They?

• Physical Changes: These changes only change how something looks or feels. They don’t change what the substance actually is. Examples are melting, freezing, dissolving, and chopping.

• Chemical Changes: These changes create new substances. This happens when the original substances undergo a reaction that changes their structure at a tiny level.

2. What Changes?

• Physical Changes:

  • These changes affect how a substance looks or feels, like its size, shape, state (solid, liquid, or gas), and texture.
  • The original substance stays the same. The tiny parts, called molecules, do not change.
  • Some common examples are:
    • Melting Ice: Ice turns into water at 0°C, but it's still H₂O.
    • Dissolving Salt in Water: You can get the salt back by evaporating the water; its make-up doesn’t change.

• Chemical Changes:

  • These changes result in new substances that are different from what you started with.
  • Examples include:
    • Burning Wood: This produces ash, carbon dioxide (CO₂), and water (H₂O), all of which have different identities than wood.
    • Rusting Iron: When iron meets oxygen, it forms rust (Fe₂O₃), which is completely different from the original iron.

3. Energy Changes

• Physical Changes:
  • These often involve heat, like absorbing or releasing it, but the molecules stay the same. For instance, when ice melts, it takes in heat but doesn’t change into something new.
• Chemical Changes:
  • These usually involve big changes in energy. They can either release energy (exothermic) or take in energy (endothermic). For example, when something burns (like in a fire), it releases energy as heat and light.

4. Can They Go Back?

• Physical Changes: Many physical changes can be reversed. For example, you can freeze water, melt it, evaporate it, or turn it back into a liquid without changing what water is.

• Chemical Changes: These usually can’t go back to the way they were under normal conditions. Once something like wood burns, you can’t make it turn back into wood again.

5. Observations in Experiments

In labs, about 65% of physical changes can be reversed. But only around 15% of chemical changes can. Also, mixtures (which involve physical changes) make up about 75% of what we see every day, while pure chemical compounds (involving chemical changes) make up about 25%.

Conclusion

In short, physical changes mostly change how things look or feel without changing what they are. On the other hand, chemical changes turn things into new substances. Knowing these differences is important for understanding chemistry and how it relates to our daily lives.