Elements vs Compounds
An atom of Krypton
Element
What is the smallest unit of an element?
An atom.
What makes a compound different from an element?
A compound is made of two or more different elements that are chemically bonded together, while an element contains only one type of atom.
A piece of paper is torn into several pieces. Is this a physical or chemical change? Explain.
Physical change. The paper changes in size and shape, but it is still paper and no new substance is formed.
A student dissolves 10 grams of salt in 100 mL of water. What is the solute and what is the solvent?
Salt is the solute because it dissolves. Water is the solvent because it does the dissolving.
A molecule composed of one type of atom.
Element
Where can you find all known elements?
The Periodic Table.
A student says CO₂ is an element because it contains only three atoms. Is the student correct? Explain.
No. CO₂ is a compound because it contains two different elements: carbon and oxygen.
A glass of lemonade with ice in it sits outside in the Texas summer heat. Before long the ice begins to melt.
What type of change has occurred?
Physical Change
Two cups contain the same amount of water. Cup A has 1 spoonful of sugar, and Cup B has 4 spoonfuls. Which solution has the greater concentration?
Cup B because it contains more solute in the same amount of solvent.
A molecule composed of more than one type of atom.
Compound
An unknown substance contains only carbon atoms. Is it an element, compound, or mixture? Explain your reasoning.
It is an element because it contains only one type of atom.
If you combine sodium (Na) and chlorine (Cl) to form sodium chloride (NaCl), does the compound have the same properties as the elements that formed it? Explain.
No. Sodium chloride has different properties from sodium and chlorine because the atoms are chemically bonded and form a new substance.
A bicycle left outside develops rust. Is this a physical or chemical change? What evidence supports your answer?
Answer: Chemical change. A new substance, rust (iron oxide), forms. The color change and formation of a new substance are evidence of a chemical change.
A student drops a whole tablet into a glass of water and another tablet that has been broken into smaller pieces into a second glass of water. Which tablet will dissolve faster, and why?
The broken-up tablet will dissolve faster because breaking it into smaller pieces increases its surface area, allowing more of the tablet to come into contact with the water at the same time.
Accurately describe the 2 substances.
O2 CO2
O2 is and element
CO2 is a compound
A student says that oxygen (O₂) is a compound because it contains two atoms. Is the student correct? Explain.
No. O₂ is still an element because both atoms are oxygen. A compound must contain two or more different elements.
How does the chemical formula H₂O tell you what elements are in water and how many atoms of each are present?
H represents hydrogen and O represents oxygen. The subscript 2 means there are two hydrogen atoms, while the lack of a subscript means there is one oxygen atom.
Ice melts into liquid water and is then frozen again. Is this a physical or chemical change? Explain.
Physical change. The substance remains water (H₂O), and the change can be reversed.
A glass of lemonade with ice in it sits outside in the Texas summer heat. Before long the ice begins to melt. What is happening with the ice to the lemonade?
The lemonade has been diluted by the water from the melting ice.
A molecule with two Phosphorus atoms and five oxygen atoms
Why can’t an element be broken down into simpler substances through a chemical reaction?
An element contains only one type of atom, so chemical reactions cannot break it into simpler substances while still remaining an element.
Glucose is a simple sugar with the molecular formula C6H12O6.Which option correctly identifies the type and number of each atom found in glucose?
Glucose contains 6 atoms of carbon, 12 atoms of hydrogen, and 6 atoms of oxygen.
A candle is burning. Is the melting wax a physical change, a chemical change, or both? Explain.
Both. Melting wax is a physical change because the wax changes state, while burning the wax is a chemical change because new substances are produced.
Erika is making a saltwater solution to remove ice from her driveway. She pours salt into a big bucket of water, but most of it sinks to the bottom.
What is the simplest way to get the salt to dissolve?
Stir the solution with a stick so the salt will dissolve.