Which of the following is always true when a substance undergoes a physical change ?
a. the composition changes.
b. a change in color occurs.
c. no new substance is formed.
d. its specific gravity remains the same.

c. no new substance is formed.

Explanation:

A physical change is defined as a change in which there is alteration in shape, size etc. No new substance gets formed in these reactions.

Example: Melting of ice

A chemical change is defined as a change in which a change in chemical composition takes place. A new substance is formed in these reactions.  Chemical reactions are usually characterized by change in color, heat change, precipitate formation and evolution of bubbles.

Example: Oxidation of magnesium

the correct answer is answer a. a mixture

no other options make sense

hope this

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  - sprinkles

answer: the enthalpy of the overall chemical reaction is +87.88 kj or +88 kj.

according to hess's law, the overall enthalpy of a reaction is the difference between the sum of enthalpy values of all the products and the sum of enthalpy values of all the reactants.

δh(reaction) = ∑nδh(products) - ∑nδh(reactants)

δh(reaction) = [δh +δh] - [δh]

δh(reaction) = [1(-287.02) + 1(0) ] - [1(-374.9)] kj = +87.88 kj

here, δh (pcl3) = -287.02 kj, δh(cl2) = 0, δh(pcl5) = -374.9 kj

therefore, enthalpy of the overall reaction is +87.88 kj or +88 kj.