Answer:
A mixture of two or more substances that are evenly mixed throughout.
Explanation:
This is because a solution shouldn't be chemically bonded, that's called a compound.
1st and 4th options are suitable answers, as these 2 changes are not exactly physical changes as it cant return back to original form and as well as its not cooling, so I feel its 1st and 4th options
Urea Formaldehyde Foam Insulation is the thermal insulation material that is no longer used because of formaldehyde gas emission
Urea Formaldehyde Foam Insulation is mixture of urea formaldehyde resin, foaming acid agent, and a propellant used in the 1970s for insulating purposes.
It was specifically designed to insulate the areas of the building that were hard to access. Unlike regular process, it was sprayed into the walls by using holes.
It is no longer used because it caused a raise in the formaldehyde gas in the residence. This gas caused irritations to the people living in the building. Residents started to report health issues like respiratory problems, and eyes, nose, throat irritation.
If you need to learn about the formaldehyde gas, click here.
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