Activity 3.1 NCERT Class 10 Science, Chapter 3 Metals and Non-metals
Activity 3.1 asks us to check the appearance of some metals and see changes it after rubbing it with sandpaper.
We see these metals look dull.
After rubbing with sandpaper all these material shine.
Metals like iron, copper, magnesium, sodium are reactive metals. These metals react with oxygen and form its oxide on the outer layer. Metal oxides are dull in colour. As a result, the metal loses his lustrous property.
e.g. Iron forms a brown layer which we call rusting of iron; aluminium forms smoke colour aluminium oxide; copper form black and green layer with oxygen and sulphur etc.
When we rub the metals with sandpaper, its outer oxide layer is scrapped out.
You may recall the burning of magnesium in chapter one. here we first clean the magnesium ribbon. It was because magnesium oxide interferes in burning. See: Why should a magnesium ribbon be cleaned before burning?