A physical change refers to a change in the form or appearance of a substance without altering its chemical composition.
Among the options provided:
A. Lead is heated and becomes more viscous. - This is a physical change because the lead changes state (from solid to a more fluid state), but its chemical composition remains the same.
B. Sodium metal reacts explosively with oxygen. - This is a chemical change since a new substance (sodium oxide) is formed.
C. Iron combines with oxygen to form rust. - This is also a chemical change, as it results in the formation of a new compound (iron oxide).
D. Calcium is added to acid and hydrogen gas is produced. - This is a chemical change because it involves a reaction that produces new substances.
Therefore, the correct answer is A. Lead is heated and becomes more viscous.
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