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A Chemical change:

the tarnishing of silver

A physical change:

the melting of ice

Figura 7.1 A Chemical change produces

new substances; a physical change does _    ,

_not    V study Check

4J Why is theformation oftamish    ) Classify the following changes as physical or Chemical:

considered a Chemical change?    \ a. chopping a carrot

i b. deveioping a Polaroid picture i c. inflating a balloon m

Changes During a Chemical Reaction

Tabie 7.2 Types of Visibłe Evidence of a Chemical Reaction

1.    Change in the color

2.    Formation of a solid (precipitate)

3.    Formation of a gas (bubbles)

4.    Heat (or a flame) produced or heat absorbed

In a Chemical reaction, the original substances change to new substances with different physical properties and different compositions. Ali of the atoms of the original substances are found in the new substances. However, some of the bonds between the atoms in the original substances have been broken and new bonds have formed between different combinations of atoms to give new substances. For example, when you light a gas bumer, the mole-cules of methane gas (CH4) react with oxygen (0₂) in the air to produce C0₂, H₂0, and heat, In another Chemical reaction, a piece of iron (Fe) combines with oxygen (0₂) in the air to produce a new substance, rust (Fe^), which has a reddish-orange color. When an antacid tablet is placed in water, bubbles appear as sodium bicarbonate (NaHCO^) and citric acid (C₆H_B0₇) in the tablet react to form carbon dioxide (C0₂) gas. (See Figurę 7.2.) In each of these Chemical reactions, new properties are visible, which are clues that tell you a Chemical reaction has taken place. Table 7.2 summarizes some types of visible evidence of a Chemical reaction.