Density is the ratio of mass to volume and volume is based on the space between molecules. When temperature rises, molecules move faster and their spacing increases. The mass stays the same but its volume expands. So density goes down with temperature. Air is an example: that is how hot air balloon rises in air.
in general as temperature increases, matter will expand; meaning its volume increases. as density is inversely proportional to volume, density decreases with temperature. it is true 4 air - hot air is less dense n rises up.
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