0620 · 6.1Physical and chemical changes

Physical changes and substance identity

0:00An ice cube melts, and a sheet of paper burns. Both look different afterwards, but only one of them forms anything new. Melted ice is still water, whereas burning paper produces new substances. By the end of this video, you will be able to classify changes by considering what happens to the substances involved. We need one starting idea about matter. Substances are made from particles, such as atoms, molecules or ions.

0:24Water contains water molecules, whether it is solid ice, liquid water or water vapour. Changing the arrangement or movement of those molecules does not create a different substance. A physical change is a change in which no new substance is formed. The material might change its state, shape or size, but its chemical identity remains the same. Physical changes are usually easily reversed. However, the absence of a new substance is the most important feature.

0:52When ice melts, its water molecules gain freedom to move past one another. The regular solid arrangement becomes the less ordered arrangement of a liquid. Those molecules remain water molecules throughout the change. Cooling can make the liquid freeze again, so melting is a physical change that is easily reversed. Boiling water is also a physical change. The liquid becomes water vapour, but no new substance forms. Bubbles appear because gaseous water is forming within the liquid.

1:20This example already shows why seeing bubbles is not enough to prove that a chemical reaction has occurred. Cutting paper is another physical change because the pieces are still made from the original material. Crushing a solid changes the size of its pieces rather than its chemical identity. These examples may not be easily reversed. Therefore, difficulty reversing a change cannot, by itself, prove that the change is chemical. Dissolving sodium chloride in water is a physical change.

1:47The salt becomes distributed through the water, forming a solution, but it has not turned into a new substance. Its disappearance from view does not mean it has been destroyed. Evaporating the water allows the salt to be recovered as a solid.

Chemical changes and reaction evidence

2:01A chemical change is a change in which one or more new substances are formed. We also call this a chemical reaction. Atoms become rearranged into different combinations, producing substances with different chemical properties. Chemical changes are usually not easily reversed, although reversibility alone does not define them. When magnesium burns, it reacts with oxygen to form magnesium oxide. The product is a different substance from either starting material. It does not simply become magnesium again when it cools.

2:30Forming magnesium oxide is what makes this chemical, not the rise in temperature or the new appearance. Rusting is another chemical change because iron reacts with oxygen and water to form new substances. Melting iron, by contrast, would leave the substance chemically unchanged. Both processes alter how the material looks, but only rusting changes its chemical identity. The useful distinction is between changing how a substance looks and changing what that substance is.

2:58Chemical reactions often involve an energy change, a colour change, gas production or formation of a precipitate. A precipitate is an insoluble solid that forms when solutions react. These observations can provide evidence that new substances have formed. They are clues to investigate, not a replacement for the definition. A temperature change alone does not establish that a reaction has happened. Heating ice can melt it without producing anything new.

3:24Similarly, boiling produces bubbles through a change of state rather than a chemical reaction. Always connect an observation to the substances present before and after the process.

Classifying changes in exam questions

3:34A burning candle gives us a worked example containing both types of change. Near the flame, solid wax melts into liquid wax. First, compare the substance before and after melting. It remains wax, so no new substance forms and this part is a physical change. Some wax then vaporises, and the vapour burns in oxygen. Burning produces new substances, including carbon dioxide and water. That formation of new products makes combustion a chemical change.

4:01A single object can therefore undergo physical and chemical changes at the same time. Classify these three examples before hearing the explanations. Consider ice freezing, a glass shattering and magnesium burning. For each example, decide whether a new substance forms. Pause the video and give a reason for each classification, rather than relying only on whether it looks reversible. Freezing is physical, since water becomes ice without forming a new substance.

4:29The fragments of a shattered glass are still glass, so that change is also physical. Magnesium oxide forms when magnesium burns, which makes burning chemical. These examples show why substance identity is more reliable than changes in shape or ease of reversal. An examination question may ask you to state whether each change is physical or chemical. The answers are that melting ice is physical and burning magnesium is chemical.

4:54If asked to explain a classification, connect your answer directly to whether a new substance forms. For a question asking you to describe the difference, give clear contrasting points. In a physical change no new substance is formed, whereas one or more new substances are formed in a chemical change. In addition, physical changes are usually easily reversed, and chemical changes usually are not. Avoid writing that every physical change is reversible or that every chemical change is irreversible.

5:22Those absolute claims ignore exceptions, such as shattered glass being difficult to restore. Also, saying that something looks different is not a sufficient explanation. Identify whether its chemical identity remains unchanged or new substances have formed. The central distinction is what happens to the substances, not simply what you can see. Physical changes alter properties such as state, shape or size without forming anything new. Chemical changes produce new substances through a reaction.

5:51Use reversibility and observations as supporting clues, but make substance identity your deciding test.