Unit 4: Topic 1 - Introduction to Reactions Flashcards
What is a physical change?
A physical change is a type of change in which the form of matter is altered but the substance is not transformed into a new substance.
Some properties/signs of a physical change:
- No new substances are formed in a physical change, the chemical composition of the object stays the same.
- A physical change is mostly temporary, and it is also usually easily reversible (there are some exceptions to this, so being easily reversible isn’t a requirement for something to be considered a physical change). For example, melting ice can be reversed by just freezing it again.
- Very little energy, if at all(in the form of heat, etc), is absorbed or given off during a physical change.
- Physical changes often deal with a change in color, shape, physical state, and/or size.
What are some examples of physical changes?
Some examples of physical changes include:
- Crumpling a sheet of paper. In this example, only the shape of the paper is being changed, not the chemical composition. This also shows a situation where the change can be easily reversed (by uncrumpling the paper)
- Freezing water into ice. This is a type of phase change. As mentioned before, the chemical composition is not changed during this change (it’s still water, so the chemical formula remains as H2O. In fact, all phase changes are considered physical changes
- Dissolving sugar in water. Although the solution becomes a homogenous mixture and it seems as if the sugar is not there anymore, this is not the case. Sugar has just been broken down into tiny molecules during the dissolution process, and can easily be recovered (reversible) by boiling the water.
What are some common types of physical changes?
Some common types of physical changes:
- Phase changes. This occurs when you alter the temperature or pressure of the object. Although the physical state of the object will be different, the object’s fundamental chemical composition will remain the same. For example, if you freeze water into ice, the final product will still be made up of the same water molecules.
- Mixtures. This includes all heterogeneous mixtures, as well as certain kinds of solutions. Heterogenous mixtures form when you mix together materials where one is not soluble in the other. An example would be mixing sand and water. Although the properties of the mixture may be different from each of its components, each part of the mixture can be easily recovered since the chemical composition is still the same. On the other hand, it’s a lot more tricky to determine for solutions, since whether a physical change has occurred will depend completely on the type of solute you’re dissolving. Generally, you’re only able to safely say that dissolution of covalent compounds like sugar in water can be considered a physical change.
What is a chemical change?
A chemical change is a process by which one or more substances are altered into one more new and different substance, with the new substance(s) no longer having the same chemical composition as the original. Chemical changes are also referred to as chemical reactions.
Some properties of chemical changes:
- When a chemical change occurs, there is also a change in the internal energy of the system, as opposed to little to zero energy change in physical changes. A chemical change that gives off heat/energy is exothermic, while a chemical change that absorbs heat/energy is endothermic.
- In a chemical change, the number and type of atoms remain constant, but the arrangement of those atoms is altered.
How can I recognize a chemical change?
Signs of chemical changes:
- Color change. Examples include the rusting of iron (turning iron into a brown-ish color) and the ripening of fruits
- Formation of a precipitate. A precipitate is an insoluble solid that forms from reacting two ionic salts inside of a solution.
- An absorption or release of energy (often in the form of heat or light). In a chemical reaction, energy is either released (exothermic) or absorbed (endothermic).
- An odor is produced. A common example is the rotting of eggs, which produces an odor that is very different from when the eggs were fresh, indicating a change in the chemical composition of the egg.
- Production of gas or gas bubbles in liquids. The formation of a gas that is not already present in the reactants and also not as a result of a phase change indicates a rearrangement of the atoms in the reactants.
