Question

7 analyze as the bright sun shines upon the water, the water slowly disappears. the same sunlight gives energy to the surrounding plants to convert water and carbon dioxide into sugar and oxygen gas. which change is physical and which is chemical?
8 compare relate the statement “you can’t get something for nothing” to the law of conservation of mass.
9 infer sharpening a pencil leaves behind pencil shavings. is sharpening a pencil a physical change or a chemical change? explain.
lesson 3 physical and chemical changes 45

Explanation:

7

Step1: Define physical and chemical change

A physical change is a change in the state or form of a substance without changing its chemical composition. A chemical change results in the formation of new substances with different chemical properties.

Step2: Analyze water - disappearance

When water slowly disappears due to sunlight, it is changing from the liquid state to the gaseous state (water - vapor). The chemical composition of water ($H_2O$) remains the same. So, this is a physical change.

Step3: Analyze plant - process

When plants convert water and carbon dioxide into sugar and oxygen gas using sunlight, new substances (sugar and oxygen) with different chemical properties are formed. So, this is a chemical change.

Step1: State the law of conservation of mass

The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction or a physical transformation. In any process, the total mass of the reactants is equal to the total mass of the products.

Step2: Relate the statement

The statement "You can't get something for nothing" implies that there is a cost or input required to obtain an output. In the context of the law of conservation of mass, it means that to have a certain amount of products in a chemical or physical process, you must start with an equivalent amount of reactants (in terms of mass). There is no creation of mass out of thin - air; you need to have the initial mass present to end up with the final mass.

Step1: Recall the definitions

A physical change is a change in the physical properties (such as shape, size, or state) of a substance without changing its chemical composition. A chemical change involves the formation of new substances with different chemical properties.

Step2: Analyze pencil - sharpening

When you sharpen a pencil, you are simply changing the shape and size of the pencil by removing some of the graphite and wood material in the form of pencil shavings. The chemical composition of the graphite (carbon) and the wood remains the same. No new substances are formed.

Answer:

The disappearance of water is a physical change. The conversion of water and carbon dioxide into sugar and oxygen by plants is a chemical change.

8