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9. melting point

10. balloons float because they are filled with hydrogen, a gas which is less dense than air.

11. specific gravity

12. element

13. mixture

14. sodium and chlorine

15. cutting of paper

16. condensation

17. 1 and 3

18. heat energy is released

tracks, burrows, eggshells, nests, tooth marks, or gastroliths

acid rain does not usually kill trees directly. instead, it is more likely to weaken the trees by damaging their leaves, limiting the nutrients available to them, or poisoning them with toxic substances slowly released from the soil. the main atmospheric pollutants that affect trees are nitrates and sulphates.