What is meant by a pure substance?

Classify the following as chemical or physical changes:
$1$. Cutting of trees
$2$. Melting of butter in a pan
$3$. Rusting of almirah
$4$. Boiling of water to form steam
$5$. Passing of electric current through water and the water breaking down into hydrogen and oxygen gases
$6$. Dissolving common salt in water
$7$. Making a fruit salad with raw fruits
$8$. Burning of paper and wood

State whether the following statement is true or false:
Non-metals are lustrous and good conductors of heat and electricity.

Differentiate between an element and a compound (any two points). Give one example each.

$(a)$ Compare metals and non-metals based on their physical properties. (any four points)
$(b)$ What are metalloids? Give two examples.
$(c)$ Identify metals from the following: Boron,sodium,mercury,carbon.

Priya's mother uses iodised salt at home. One day while cooking,Priya asked her mother about iodised salt. She said people who do not use iodised salt suffer from deficiency of iodine which leads to goitre and hypothyroidism.
$(a)$ What are the physical symptoms of goitre and hypothyroidism?
$(b)$ How will you inform others about the importance of iodised salt?
$(c)$ Is iodine a metal or a non-metal?
$(d)$ What are the physical properties of iodine?
$(e)$ What is the difference between iodised salt and normal salt?