Matter Lesson 3: Physical and Chemical Changes

Chemical changes result in new substances, but physical changes do not. Which process is an example of a physical change?

• Sodium and water are combined and form sparks.
• Bleaching lightening hair color
• A loaf of bread being sliced into pieces.
• Sodium and chlorine are combined to form table salt.

It is important to understand the differences between chemical and physical changes. Chemical changes result in new substances, but physical changes do not. Which process is an example of a physical change?

• Water turns to steam when boiled over a Bunsen burner.
• Carbon combines with oxygen to form carbon dioxide gas.
• Water breaks down into hydrogen and oxygen gases over time.
• Limestone breaks down into lime and carbon dioxide when heated.

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There are several differences between chemical and physical changes. Which statement is true of a chemical change, but not true of a physical change?

• A chemical change can produce warmth.
• A chemical change can be easily changed back.
• A chemical change can separate a mixture into its components.
• A chemical change can combine substances without altering them.

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A chemical change of matter takes place when one or more substances change into entirely new substances with different properties. Although physical changes affect the appearance of a substance, chemical changes affect its composition. Which process indicates that a chemical change is taking place?

• A lake freezes over into ice.
• A metal bar is rolled into a flat sheet.
• Vinegar bubbles when baking soda is added.
• Sand, water, and salt combine to form a mixture.

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Rita wants to make some toast for breakfast. She puts a slice of bread in the toaster. After 10 minutes, Rita notices that the sides of the bread are black. What caused this chemical change to take place?

• added warmth
• added mass
• added volume
• added particles

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Shanti mixes 39 g of potassium with 39 g of chlorine. She obtains 74 g of potassium chloride and recovers 4 g of unreacted chlorine. Do Shanti’s results confirm the law of conservation of mass?

• No, because no potassium is left over.
• Yes, because the mass of all the reactants is accounted for.
• Yes, because all of the potassium reacted with the chlorine.
• No, because the mass of the potassium chloride is greater than the amount of potassium used.

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What should a scientist do if they want to speed up the chemical reaction?

• Increase the density of the substance
• Decrease the density of the substance
• Increase the temperature of the substance
• Decrease the temperature of the substances

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