Question

1. which of the following is a pure substance?

a) air
b) salt (nacl)
c) salad
d) soil

1. which of these is an element?

a) water
b) carbon
c) salt
d) sugar

1. a mixture where the different parts are evenly mixed and you cannot see the different substances is called:

a) pure substance
b) compound
c) homogeneous mixture
d) heterogeneous mixture

Explanation:

1. A pure substance has a uniform and definite composition. Air is a mixture of gases (not pure), salt (NaCl) is a compound (a type of pure substance as it has a fixed composition of Na and Cl), salad is a heterogeneous mixture, soil is a heterogeneous mixture. So, salt (NaCl) is a pure substance.
2. An element is a pure substance that cannot be broken down into simpler substances. Water ($H_2O$) is a compound (combination of H and O), carbon is an element (it cannot be broken down further by chemical means), salt (NaCl) is a compound, sugar ($C_{12}H_{22}O_{11}$) is a compound.
3. A homogeneous mixture has a uniform composition throughout. A heterogeneous mixture has visibly different parts. A compound is a pure substance. A pure substance has a definite composition. A mixture where different parts are evenly mixed (homogeneous) is called a homogeneous mixture.

Answer:

1. B. Salt (NaCl)
2. B. Carbon
3. B. Homogeneous mixture