* Physical change: Alters the form or appearance of a substance but doesn't change its chemical composition. Examples: melting ice, boiling water, cutting paper.

* Chemical change: Results in the formation of new substances with different chemical properties. Examples: burning wood, rusting iron, baking a cake.

When sodium (Na) reacts with water (H₂O), it undergoes a violent reaction producing sodium hydroxide (NaOH), hydrogen gas (H₂), and heat:

2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g) + heat

Key points:

* New substances are formed: Sodium hydroxide and hydrogen gas are formed, which are different from the starting materials, sodium and water.

* Chemical bonds are broken and formed: The reaction involves breaking the bonds in sodium and water molecules and forming new bonds in sodium hydroxide and hydrogen.

* Energy is released: The reaction is exothermic, meaning it releases heat.

Therefore, the reaction between sodium and water is a chemical change because it results in the formation of new substances and involves a change in chemical composition.