3 (b) Chemical versus Physical Change
A physical change
is any change that alters the form or appearance of matter but does not
make any substance in the matter into a different substance. A substance that undergoes a physical change is still the same substance after the change.
Changes of State: Suppose you leave a small puddle of liquid water on the kitchen counter. When you come back two hours later, the puddle is gone. Has the liquid water disappeared? No, a physical change happened. The liquid water changed into water vapor (a gas) and mixed with the air. A change in state, such as from a solid to a liquid or from a liquid to a gas, is an example of a physical change.
Changes in Shape or Form: Is there a physical change when you dissolve a teaspoon of sugar in water? To be sure, you would need to know whether or not the sugar has been changed to a different substance. For example, you know that a sugar solution tastes sweet, just like the undissolved sugar. If you pour the sugar solution into a pan and let the water dry out, the sugar will remain as a crust at the bottom of the pan. The crust may not look exactly like the sugar before you dissolved it, but it’s still sugar. So, dissolving is also a physical change. Other examples of physical changes are bending, crushing, breaking, chopping, and anything else that changes only the shape or form of matter.
Changes of State: Suppose you leave a small puddle of liquid water on the kitchen counter. When you come back two hours later, the puddle is gone. Has the liquid water disappeared? No, a physical change happened. The liquid water changed into water vapor (a gas) and mixed with the air. A change in state, such as from a solid to a liquid or from a liquid to a gas, is an example of a physical change.
Changes in Shape or Form: Is there a physical change when you dissolve a teaspoon of sugar in water? To be sure, you would need to know whether or not the sugar has been changed to a different substance. For example, you know that a sugar solution tastes sweet, just like the undissolved sugar. If you pour the sugar solution into a pan and let the water dry out, the sugar will remain as a crust at the bottom of the pan. The crust may not look exactly like the sugar before you dissolved it, but it’s still sugar. So, dissolving is also a physical change. Other examples of physical changes are bending, crushing, breaking, chopping, and anything else that changes only the shape or form of matter.
A second kind of change occurs when a substance is transformed into a different substance. A change in matter that produces one or more new substances is a chemical change, or a chemical reaction. In some chemical changes, a single substance simply changes to one or more other substances. For example, when hydrogen peroxide is poured on a cut on your skin, it breaks down into water and oxygen gas.
In other chemical changes, two or more substances combine to form different substances. For example, iron metal combines with oxygen from the air to form the substance iron oxide, which you call rust. Unlike a physical change, a chemical change produces new substances with properties different from those of the original substances.
Examples of Chemical Change: The chemical change that occurs when fuels such as natural gas, wood, candle wax, and gasoline burn in air is called combustion. Other processes that result in chemical change include electrolysis, oxidation, and tarnishing. The table in Figure 17 describes each of these kinds of chemical changes.
Conservation of Mass: A candle may seem to “go away” when it is burned, or water may seem to “disappear” when it changes to a gas. However, scientists long ago proved otherwise. The fact that matter is not created or destroyed in any chemical or physical change is called the law of conservation of mass. Remember that mass measures the amount of matter. So, this law is sometimes called the law of conservation of matter. No mass is lost, because during a chemical change, atoms are not lost or gained, only rearranged.
In other chemical changes, two or more substances combine to form different substances. For example, iron metal combines with oxygen from the air to form the substance iron oxide, which you call rust. Unlike a physical change, a chemical change produces new substances with properties different from those of the original substances.
Examples of Chemical Change: The chemical change that occurs when fuels such as natural gas, wood, candle wax, and gasoline burn in air is called combustion. Other processes that result in chemical change include electrolysis, oxidation, and tarnishing. The table in Figure 17 describes each of these kinds of chemical changes.
Conservation of Mass: A candle may seem to “go away” when it is burned, or water may seem to “disappear” when it changes to a gas. However, scientists long ago proved otherwise. The fact that matter is not created or destroyed in any chemical or physical change is called the law of conservation of mass. Remember that mass measures the amount of matter. So, this law is sometimes called the law of conservation of matter. No mass is lost, because during a chemical change, atoms are not lost or gained, only rearranged.