The original substance cannot be obtained easily again. Chemical reactions must be distinguished from physical changes. In this change the original substance gives rise to one or more substances with different properties. Thus we can say that a chemical change is a permanent change or irreversible change. What does it mean in terms of chemical bonds, at a tiny microscopic level of atoms and molecules? Well, the chemical bonds break and new bonds are formed in the new chemicals. The chemicals are combining to give new chemicals. Students will be able to explain that cooling water can change it to ice and that heating ice can change it back to liquid water. Chemical changes occur when a chemical reaction causes bonds between atoms. In all the above examples, wherever there is a chemical change, a common thing is happening to the chemicals. Examples of chemical changes include: Burning Rusting Tarnishing. The chemicals in the firecracker burn to give bright colored sparks and flashes. When a firecracker is lighted – is it a physical change? No, surely not. When switched OFF, the filament cools to regain its original form. When a light bulb is emitting light by electricity – is it a physical change or a chemical change? When switch is ON, the tungsten filament is heated up by an electrical current (or electrical energy).
On the other hand when a candle is burning, the melting wax is a physical change but the heat and light that we get is due to chemical changes or reaction of the wax molecules with oxygen molecules in the air. When salt or sugar is dissolved in water, it is a physical change, too. When an ice cream melts, what do you think is happening? The cold ice cream changes physically from a solid to a liquid form.
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