why do we get red colour when we burned strontium chloride?

When metal salts are heated in a flame, the electrons in the metal absorb the thermal energy and are excited to higher energy states. This is an unstable condition, and the electrons quickly emit the same amount of energy that they absorbed as they return to their more stable ground state. The energy emitted by the metal electrons can be observed as light of various wavelengths, producing a flame with a characteristic color. Each metal has a characteristic color in a flame test.

Strontium electrons emit light energy of approximately 640 to 690 nanometers (nm), which is observed as red light. They also absorb and emit energy of other wavelengths, but the overall color that is seen in the flame test for strontium is red. Metals can be identified by their flame tests, as the electrons of different metals absorb and emit different amounts of energy.

This is the science behind the colors that we see in fireworks. Different metal salts are used to produce the different colors seen in fireworks.

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