Physical changes

1. A physical change alters the state, shape or size of a substance, or mixes or separates substances, without forming a new substance.
2. The particles of the substance stay the same, and only their arrangement, spacing or movement changes.
3. Because no new substance forms, the chemical composition and the chemical properties of the substance stay the same.
4. Common physical changes include:
   1. Changes of state, such as melting, freezing, boiling, evaporation, condensation and sublimation.
   2. Dissolving a solid in a solvent, such as dissolving sugar in water.
   3. Separating a mixture, for example by filtration, crystallisation or distillation.
   4. Changing the shape or size of a solid, such as crushing a crystal or bending a copper wire.
5. Many physical changes are easy to reverse by another physical process, such as freezing water again or evaporating the water from a salt solution.
6. Mass is conserved in a physical change, so $10g$ of ice melts to give exactly $10g$ of liquid water.

Chemical changes

1. A chemical change, also called a chemical reaction, forms one or more new substances.
2. During a chemical change, bonds between the particles of the reactants break and new bonds form, so the atoms are rearranged into new substances.
3. No atoms are created or destroyed, so the total mass of the products equals the total mass of the reactants in a closed container.
4. The products have a different chemical composition from the reactants, so they have different properties.
   1. Sodium is a soft, highly reactive metal and chlorine is a toxic green gas, yet the product of their reaction, sodium chloride, is a white solid that is safe to put on food.
5. Chemical changes are usually difficult to reverse, because getting the original substances back needs one or more further chemical reactions.
6. A chemical change can be summarised by a word equation or a balanced symbol equation, such as $2Mg(\mathrm{s})+{\mathrm{O}}_2(\mathrm{g})\to 2MgO(\mathrm{s})$.

Evidence of a chemical change

1. You cannot see new particles forming, so you use observations as evidence that a new substance has been made.
2. Observations that suggest a chemical change include:
   1. A colour change, such as green copper(II) carbonate turning black when it is heated.
   2. Effervescence, which is fizzing caused by a new gas forming, such as when magnesium is added to dilute hydrochloric acid.
   3. A precipitate, which is an insoluble solid that forms when two solutions are mixed.
   4. A temperature change, because most chemical reactions transfer thermal energy to or from the surroundings.
   5. The release of light or sound, such as a flame or a squeaky pop.
   6. A new smell, such as the sharp smell of sulfur dioxide when sulfur burns.
3. An observation is only evidence, so you must always link it to the formation of a new substance.
4. Some physical changes also produce bubbles, colour changes or temperature changes:
   1. The bubbles in boiling water are water vapour, which is the same substance in a different state.
   2. Dissolving ammonium nitrate in water makes the solution feel cold, but no new substance forms.
   3. Diluting a coloured solution makes it paler, but the dissolved substance is unchanged.
5. To classify a change correctly, look for the cause of each observation, not just the observation itself.

Comparing the two types of change

1. New substance: a physical change forms no new substance, whereas a chemical change forms at least one new substance.
2. Chemical composition: this stays the same in a physical change, whereas the products of a chemical change have a different composition from the reactants.
3. Properties: the chemical properties stay the same in a physical change, whereas the products of a chemical change have new properties.
4. Reversing the change: a physical change is often easy to reverse, whereas a chemical change is usually difficult to reverse.
5. Examples: melting, boiling, dissolving and filtering are physical changes, whereas burning, rusting, neutralisation and cooking an egg are chemical changes.