Q1: What is the thick layer of air surrounding the Earth called?
Ans: Atmosphere

Q2: Which layer of the atmosphere is closest to the Earth's surface?
Ans: Troposphere

Q3: In which layer of the atmosphere do jet airplanes usually fly?
Ans: Stratosphere

Q4: Which layer of the atmosphere is the coldest?
Ans: Mesosphere

Q5: Which layer of the atmosphere is directly above the mesosphere?
Ans: Thermosphere

Q6: What is the outermost layer of the Earth's atmosphere called?
Ans: Exosphere

Q7: What gas makes up about 78% of the Earth's atmosphere?
Ans: Nitrogen

Q8: What gas makes up about 21% of the Earth's atmosphere and is essential for living things?
Ans: Oxygen

Q9: What is the term for substances that make water dirty or unfit for use?
Ans: Impurities

Q10: How can insoluble impurities in water be removed by allowing them to settle?
Ans: Sedimentation

Q11: What process involves passing liquid through a filter paper to remove impurities?
Ans: Filtration

Q12: What is the process of heating water to kill germs and purify it called?
Ans: Boiling

Q13: What is the method of adding chlorine to water to purify it?
Ans: Chlorination

Q14: Which layer of the atmosphere is directly below the exosphere?
Ans: Thermosphere

Q15: What is the term for water that is found in all three states of matter—solid, liquid, and gas?
Ans: Universal solvent