GED Science Matter and Chemical Reactions Summary

In a chemical change, a new substance forms. Rust (iron oxide) is a new substance made when iron reacts with oxygen and water. Melting, boiling, and cutting are physical changes because the substance stays the same.

Wrong choices

• Paper being cut in half: Cutting paper changes only its size and shape. It is still paper.
• Water boiling: Boiling is a change of state from liquid to gas. It is still water.
• Ice melting: Melting is a change of state. The ice is still water, so it is a physical change.

1. Density = mass ÷ volume.
2. 60 ÷ 20 = 3 g/cm³.

Check: 3 × 20 = 60 grams.

Wrong choices

• 0.33 g/cm³: 0.33 comes from dividing the volume by the mass. Divide mass by volume.
• 40 g/cm³: 40 comes from subtracting 20 from 60.
• 1,200 g/cm³: 1,200 comes from multiplying 60 by 20.

Condensation happens when a gas cools and turns into a liquid. The cold glass cools the water vapor, and droplets form on the outside of the glass.

Wrong choices

• Melting: Melting is a solid turning into a liquid.
• Evaporation: Evaporation is the opposite: liquid turning into gas.
• Freezing: Freezing is a liquid turning into a solid.

An atom has a nucleus made of protons (positive) and neutrons (no charge). Electrons (negative) move around the nucleus.

Wrong choices

• Proton: A proton has a positive charge.
• Nucleus: The nucleus holds protons and neutrons, so its overall charge is positive.
• Neutron: A neutron has no charge.

An object floats if its density is less than the density of the liquid. Only 0.8 g/cm³ is less than water's 1.0 g/cm³.

Wrong choices

• An object with a density of 2.7 g/cm³: 2.7 is much greater than 1.0, so this object will sink.
• An object with a density of 1.2 g/cm³: 1.2 is greater than 1.0, so this object will sink.
• An object with a density of 11.3 g/cm³: 11.3 is much greater than 1.0, so this object will sink.

1. Left side: O₂ is one molecule with 2 oxygen atoms.
2. Right side: 2H₂O has 2 × 1 = 2 oxygen atoms.

Both sides have 2 oxygen atoms (and 4 hydrogen atoms), so the equation is balanced.

Wrong choices

• 1: The right side has 2 water molecules, each with 1 oxygen atom, so 2 in total.
• 3: 3 does not match either side of the equation.
• 4: 4 is the number of hydrogen atoms on each side, not oxygen atoms.

In a chemical reaction, atoms are rearranged but not created or destroyed. In a closed container, the total mass stays the same: 10 + 50 = 60 grams before and after.

Wrong choices

• 50 grams: 50 grams is only the vinegar. The baking soda's mass is still there.
• 40 grams: 40 grams comes from subtracting 10 from 50.
• Less than 60 grams, because some of the matter became gas: Gas has mass too. In a sealed bag, the gas is still inside, so no mass is lost.

Sodium's symbol, Na, comes from its Latin name, natrium. Table salt is sodium chloride, NaCl. Other common symbols include H (hydrogen), O (oxygen), C (carbon), and Fe (iron).

Wrong choices

• So: So is not the symbol of any element.
• S: S is the symbol for sulfur.
• Sd: Sd is not the symbol of any element.

A solution has two parts. The solvent (usually the larger amount) does the dissolving. The solute is what gets dissolved. In salt water, water is the solvent and salt is the solute.

Wrong choices

• The solute: The solute is the substance being dissolved. Here that is the salt.
• The precipitate: A precipitate is a solid that forms and settles out of a solution.
• The element: Water is a compound of hydrogen and oxygen, not an element.

1. Right side: 2NH₃ has 2 nitrogen atoms and 6 hydrogen atoms.
2. Left side: 3H₂ has 6 hydrogen atoms, which already matches.
3. One N₂ molecule has 2 nitrogen atoms, which matches. So the number is 1.

Wrong choices

• 2: 2N₂ would give 4 nitrogen atoms on the left, but there are only 2 on the right.
• 6: 6N₂ would give 12 nitrogen atoms, far too many.
• 3: 3N₂ would give 6 nitrogen atoms on the left, but there are only 2 on the right.