Unit 4, Lessons 3 and 4

Lesson 3:

Although the interactions between substances are considered chemical reactions, the changes that the substances go through can be either physical or chemical. A physical change implies that the products (what happens after two or more factors are combined) can be altered in order to regain the reactants. A bowl of trail mix is a good example; you can throw together some pretzels, some cracker chips, some raisins (but who likes raisins in their trail mix anyway?), and some M&Ms in a bowl and then pick out each individual food afterward. A chemical change, on the other hand, can't be so easily reversed. If you mix together eggs, oil, cake mix, and throw that sucker in the oven, you most certainly won't get eggs, oil, and cake mix when you take it out. All the ingredients combine to form a new substance with new (and in this case, delicious) properties.

Problems:

2.) Explain how dissolving can be described as either a physical change or a chemical change. Dissolving is a physical change because a solid changes forms and seems to disappear in a liquid. The liquid can be boiled and the solid will be left behind, such as in the case of salt water. However, also using salt water as an example, sodium and chlorine atoms break apart in water and conduct electricity. Technically, this is the product, which has new properties. Solid salt doesn't conduct electricity.

6.) Classify the following two changes as physical or chemical. Explain your reasoning.
A.) CaCO3(s) + H2SO4(aq) --> CaSO4(aq) + CO2(g) + H20(l) Chemical change, because the products are both different substances and in different phases than the reactants.
B.) NaCl(s) --> NaCl(l) This is just a physical change. The product still has its identity; it is still salt, but it's in liquid form. Now it's salt water.

Lesson 4:

The law of conservation of mass states that, in a chemical reaction, or any action for that matter, mass cannot be created or destroyed. It can, however, go somewhere else. In a chemical equation, the mass of the products is the same as the reactants, even if one of the products is a gas, unless the gas is trapped in a sealed-off container. The mass does not change, regardless of the phase change or the factors being combined, because all atoms are accounted for.

Problems:

2.) Explain how the law of conservation of mass applies to garbage. The law of conservation of mass applies to garbage in that garbage sits in landfills for years, and only some of it biodegrades. What doesn't biodegrade still has matter--and mass, because it exists. We recycle that garbage by taking it and making it into usable products, like bottles, recycled paper, whatever have you. The mass of the garbage is not lost, but it changes forms.

7.) What would you have to do to prove that matter is conserved when a piece of paper is burned? The paper would have to be burned in sealed-off container, such as a glass box or something of that sort. That way, once the paper is burned, both its ashes and any gas released from the burning process is still present. Just find out the mass of the box with a scale.