Question

what do these two changes have in common?
dust settling out of the air
filtering air to remove dust and pollen
select all that apply.
both are chemical changes. both are changes of state.
both are only physical changes. both are caused by cooling.

Explanation:

• A chemical change results in the formation of new substances. In dust settling out of the air and filtering air to remove dust and pollen, no new substances are formed. So, they are not chemical changes.
• A change of state (e.g., solid - liquid, liquid - gas) involves a change in the physical state of a pure substance. Here, we are dealing with mixtures (air with dust/pollen) and separation of components of a mixture, not a change of state of a single pure substance.
• A physical change is a change in which the form or appearance of a substance changes, but its chemical composition remains the same. Dust settling out of the air is just a separation (due to gravity) and filtering is a mechanical separation method. In both cases, the chemical composition of dust, pollen, and air components remains unchanged. So, both are physical changes.
• There is no mention of cooling causing these changes. Dust settling can be due to gravity and filtering is a mechanical process.

Answer:

Both are only physical changes.