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I will explain it in a very simple way.
The surface of the copper is very smooth so thag when you apply the solder to it , it will not stick to it.
To apply solder, the surface need to be rough or it should be tinned so when you apply solder it will melt and stick.
We can apply solder to copper by making the surface of copper wire rough by using a sand paper or any hand file.
Hope my answer helped.
Feel free to comment and do upvote if you found it useful.
Happy Reading!
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