A substance can be changed by heating, adding other substances, mixing and so on. The changes are either physical of chemical.
Signs of a Chemical Change; new substances are formed meaning the compound has its own set of properties; energy is used during the change, and is irreversible or very hard to reverse. An example of a chemical change is burning Magnesium Ribbon.
When heated the Magnesium reacts with Oxygen and magnesium oxide is formed (a new substance), heat and light - and very very bright light - is given out (energy being used) and once the ribbon is burnt you can’t change back into what it used to be (irreversible). Thechemical equation is 2Mg + O2 = 2MgO.
Other examples are fireworks, cooking of eggs, burning, roasting of marshmallows
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For physical changes; no new substances are formed meaning the mixture has characteristic of both particles and is easily reversible. An example is mixing iron and sulfur particles.
When you mix both of them together you do not get Iron(II) Sulphide but you get a mixture of iron and sulphide (no new substance) and to separate the mixture you just need to drag a magnet over it and it would attract all the iron (easily reversible)
Other examples are boiling water, melting iron, dissolving sugar in water, mixing mud in water.
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