Elements & Compounds
What is the smallest unit of an element that still has the properties of that element?
An atom.
In the formula H₂O, how many hydrogen atoms are present?
2
Is melting ice a physical change or a chemical change?
Physical change.
Name one factor that can affect how quickly a solid dissolves in water.
Temperature, surface area, or agitation.
Thermal energy naturally moves from a ______ object to a ______ object.
Warmer to cooler.
What does the chemical symbol O represent?
Oxygen.
In CO₂, how many oxygen atoms are present?
2.
In a saltwater solution, what is the salt called?
The solute.
Which will dissolve faster: a whole sugar cube or crushed sugar? Why?
Crushed sugar because it has a greater surface area exposed to the water.
What type of thermal energy transfer occurs when you touch a hot pan?
Conduction.
A student says that water is an element because it is made of only one type of substance. Is the student correct? Explain.
No. Water is a compound because it contains two different elements, hydrogen and oxygen, chemically bonded together.
How many total atoms are represented by the formula H₂SO₄?
7 atoms — 2 hydrogen, 1 sulfur, and 4 oxygen.
A student tears a piece of paper into smaller pieces. Is this a physical or chemical change? Explain.
Physical change because the paper's form or size changes, but no new substance is produced.
Two students put identical sugar cubes into identical cups of water. One student stirs continuously while the other does not. Predict which sugar cube will dissolve faster and explain why.
The stirred sugar cube will dissolve faster because agitation increases the interaction between the water and the solute.
A metal spoon is sitting in a cup of hot soup. After several minutes, the handle becomes warm. What type of thermal energy transfer is occurring?
Conduction.
Compare O₂ and H₂O. How are they different in terms of elements, atoms, and molecules?
O₂ contains only oxygen atoms, so it is an element. H₂O contains hydrogen and oxygen atoms, so it is a compound.
A student says that CaCl₂ contains two calcium atoms and one chlorine atom. What did the student misunderstand?
The subscript applies to the element immediately before it. CaCl₂ contains 1 calcium atom and 2 chlorine atoms.
A student adds more water to a concentrated sugar solution. What happens to the concentration of the solution? Why?
The concentration decreases because the same amount of solute is now spread throughout a larger amount of solvent.
A student wants to test whether temperature affects the rate at which sugar dissolves. What should the student change, and what variables should remain the same?
Change the water temperature. Keep the amount/type of sugar, amount of water, container, and other conditions the same.
Explain the difference between conduction, convection, and radiation.
Conduction transfers thermal energy through direct contact. Convection transfers thermal energy through the movement of fluids such as liquids or gases. Radiation transfers thermal energy through electromagnetic waves and does not require direct contact.
A student claims that CO₂ and O₂ are both elements because both contain oxygen. Use evidence from their chemical formulas to explain why the student is incorrect.
O₂ contains only one type of atom—oxygen—so it is an element. CO₂ contains two different types of atoms—carbon and oxygen—so it is a compound.
You are given the formula Al₂(SO₄)₃. Determine the total number of atoms and explain how you found your answer.
17 total atoms: 2 aluminum, 3 sulfur, and 12 oxygen.
Student A mixes 10 g of salt with 100 mL of water. Student B mixes 10 g of salt with 500 mL of water. Which solution has the greater concentration? Explain your reasoning.
Student A's solution has the greater concentration because the same amount of solute is dissolved in less solvent.
A student wants to make powdered drink mix dissolve as quickly as possible. They have three choices: use cold water without stirring, use warm water while stirring, or use cold water with a whole packet of powder. Which would most likely dissolve the fastest? Use at least two factors to justify your answer.
Warm water while stirring. Higher temperature and agitation both increase the rate of dissolution.
A cold can of soda is placed outside on a hot day. After several minutes, the can becomes warmer. Explain what happens to thermal energy and what will eventually happen if the can remains outside.
Thermal energy moves from the warmer surroundings to the cooler soda. Eventually, the soda and its surroundings will reach thermal equilibrium, meaning they are at the same temperature.