STAAR Grade 8 Science

1. Question 1 · Organisms and Environments

   A student mixes baking soda and vinegar in two setups. In Setup 1, everything is inside a zipper bag that is sealed before mixing. In Setup 2, the same amounts are mixed in an open cup. Each setup is placed on an electronic balance before and after the bubbling stops.

   Mass Before and After Mixing

   Setup	Mass before (g)	Mass after (g)
   1: Sealed bag	152.4	152.4
   2: Open cup	148.6	146.1

   How much mass did Setup 2 lose during the reaction?
   • A.2.5 g
   • B.3.8 g
   • C.0.0 g
   • D.6.3 g
   Show answer and explanation

   Answer: A. 2.5 g

   Mass before minus mass after is 148.6 g − 146.1 g = 2.5 g. A choice of 3.8 g comes from subtracting the two starting masses (152.4 g − 148.6 g), which compares the setups instead of the before and after of Setup 2.

2. Question 2 · Force, Motion, and Energy

   A student uses a spring scale to pull a 2.0 kg cart across a smooth lab table. She changes the net force in each trial and measures the cart's acceleration with a motion sensor.

   Cart Trials (mass = 2.0 kg)

   Trial	Net force (N)	Acceleration (m/s²)
   1	2	1
   2	4	2
   3	6	3
   4	8	4

   Which conclusion is supported by the data?
   • A.The cart's acceleration increases by 2 m/s² for every 1 N of force.
   • B.When the net force doubles, the cart's acceleration also doubles.
   • C.The cart's acceleration stays the same no matter what force is used.
   • D.When the net force doubles, the cart's acceleration is cut in half.
   Show answer and explanation

   Answer: B. When the net force doubles, the cart's acceleration also doubles.

   From 2 N to 4 N the acceleration goes from 1 m/s² to 2 m/s², and from 4 N to 8 N it goes from 2 m/s² to 4 m/s², so acceleration is directly proportional to net force when mass stays the same. Cutting acceleration in half is what happens when mass doubles, not force.

3. Question 3 · Force, Motion, and Energy

   During a camping trip at Big Bend, a student records four observations around a campfire.

   • Smoke and hot air rise above the flames while cooler air flows in at the bottom.
   • The student's face feels warm even though nothing touches it.
   • A metal roasting stick gets hot near the end the student holds.
   • A rock sitting in the fire's coals becomes hot to the touch.

   Which observation is mainly an example of convection?
   • A.A rock sitting in the fire's coals becomes hot to the touch.
   • B.Smoke and hot air rise above the flames while cooler air flows in.
   • C.A metal roasting stick gets hot near the end the student holds.
   • D.The student's face feels warm even though nothing touches it.
   Show answer and explanation

   Answer: B. Smoke and hot air rise above the flames while cooler air flows in.

   Convection is energy transfer by the movement of a fluid: warm, less dense air rises and cooler, denser air moves in to replace it. The warm face is radiation (infrared waves), which can seem like convection, but no moving air has to reach the face for it to feel warm.

4. Question 4 · Earth and Space

   A student researched five stars and recorded each star's surface temperature and luminosity. Luminosity is the total energy a star gives off compared with the Sun.

   Star Data

   Star	Surface temperature (K)	Luminosity (Sun = 1)
   Sun	5,800	1
   Sirius A	9,900	25
   Betelgeuse	3,500	100,000
   Sirius B	25,000	0.05
   Proxima Centauri	3,000	0.002

   The student plots these five stars on an H-R diagram. Which star will be plotted farthest to the left?
   • A.Sirius B
   • B.Sirius A
   • C.Proxima Centauri
   • D.Betelgeuse
   Show answer and explanation

   Answer: A. Sirius B

   The horizontal axis of an H-R diagram shows surface temperature, with the hottest stars on the left, and Sirius B has the highest temperature (25,000 K). Betelgeuse is the most luminous star, so it would be plotted highest, not farthest left; choosing it mixes up the two axes.

5. Question 5 · Matter and Energy

   Steps in Developing Cell Theory

   Hooke sees tiny boxes in cork and calls them cells

   ▼

   ?

   ▼

   Schleiden: all plants are made of cells

   ▼

   Schwann: all animals are made of cells

   ▼

   Virchow: all cells come from existing cells

   Which discovery belongs in the missing box?
   • A.Darwin explains how species change over time by natural selection.
   • B.A scientist uses an electron microscope to photograph ribosomes.
   • C.Leeuwenhoek uses better lenses to see living cells in pond water.
   • D.Mendel shows that traits pass from parents through inherited factors.
   Show answer and explanation

   Answer: C. Leeuwenhoek uses better lenses to see living cells in pond water.

   After Hooke saw empty cork cells, Leeuwenhoek built stronger lenses and was the first to observe living single-celled organisms. Electron microscopes and the discovery of ribosomes came long after cell theory was proposed, so that step is out of order.

6. Question 6 · Organisms and Environments

   Two investigations are done on an electronic balance. In Investigation 1, a pad of steel wool (iron) is heated in an open dish until it glows and turns dark gray. In Investigation 2, a small candle burns inside a sealed glass jar until the flame goes out.

   Investigation Results

   Investigation	Mass before (g)	Mass after (g)
   1: Steel wool, open dish	8.0	9.6
   2: Candle, sealed jar	312.0	312.0

   What is the most likely reason the mass increased in Investigation 1?
   • A.The flame added its energy to the steel wool as mass.
   • B.Oxygen atoms from the air joined with the iron atoms.
   • C.Heating the steel wool created new iron atoms in it.
   • D.Burned materials always weigh more than they did before.
   Show answer and explanation

   Answer: B. Oxygen atoms from the air joined with the iron atoms.

   When iron burns, it combines with oxygen from the air to form iron oxide, so the product includes atoms that came from outside the dish. Energy does not have measurable mass on a balance, and many burned materials, like wood, lose mass in an open system because gases escape.

7. Question 7 · Force, Motion, and Energy

   Students pull wagons loaded with different amounts of sand across a parking lot. They keep the net force on each wagon at 60 N and record the acceleration. Each trial is done three times, and the results are averaged.

   Wagon Trials (net force = 60 N)

   Wagon and sand mass (kg)	Average acceleration (m/s²)
   10	6
   15	4
   20	3
   30	2

   Which conclusion about mass and acceleration is supported by the data?
   • A.Mass has no effect on how fast the wagon speeds up.
   • B.When the mass doubles, the acceleration also doubles.
   • C.Each extra 5 kg lowers the acceleration by 2 m/s².
   • D.When the mass doubles, the acceleration is cut in half.
   Show answer and explanation

   Answer: D. When the mass doubles, the acceleration is cut in half.

   With the same 60 N net force, going from 10 kg to 20 kg drops the acceleration from 6 m/s² to 3 m/s², and 15 kg to 30 kg drops it from 4 m/s² to 2 m/s², so doubling the mass halves the acceleration. The "drops by 2 m/s² every 5 kg" pattern only fits the first step, not the rest of the table.

8. Question 8 · Force, Motion, and Energy

   Students pour 80 °C water into three covered cups made of different materials. After 20 minutes, they record the water temperature in each cup.

   Water Temperature After 20 Minutes

   Cup material	Starting temperature (°C)	Final temperature (°C)
   Foam	80	68
   Glass	80	55
   Aluminum	80	49

   Which cup slowed the transfer of thermal energy out of the water the most, and by how much did its temperature drop?
   • A.Foam; 12 °C
   • B.Aluminum; 31 °C
   • C.Aluminum; 49 °C
   • D.Foam; 68 °C
   Show answer and explanation

   Answer: A. Foam; 12 °C

   The foam cup had the smallest drop, 80 °C − 68 °C = 12 °C, so foam conducted thermal energy out of the water most slowly. Aluminum had the largest drop (31 °C) because metals are good conductors, and 68 °C is the final temperature, not the change.

9. Question 9 · Earth and Space

   An astronomy club made a table comparing five stars. Luminosity is the total energy a star gives off compared with the Sun.

   Star Comparison

   Star	Surface temperature (K)	Luminosity (Sun = 1)	Color
   Rigel	12,000	100,000	Blue-white
   Arcturus	4,300	170	Orange
   Vega	9,600	40	White
   Procyon B	7,700	0.0005	White
   Barnard's Star	3,100	0.004	Red

   Which statement about Rigel is best supported by the data?
   • A.It is a hot supergiant that would be plotted near the top of the H-R diagram
   • B.It is a white dwarf, since its blue-white color means it is near the end of its life
   • C.It is the closest star to Earth, which is the reason its luminosity is so high
   • D.It is a cool giant that would be plotted near the lower left of the H-R diagram
   Show answer and explanation

   Answer: A. It is a hot supergiant that would be plotted near the top of the H-R diagram

   Rigel is hot (12,000 K) and about 100,000 times as luminous as the Sun, so it is a blue supergiant plotted near the top of the H-R diagram, toward the left. Luminosity is a star's true energy output, so it is not caused by being close to Earth, and Rigel is actually very far away.

10. Question 10 · Matter and Energy

    Two Organisms

    Organism	Number of cells	How new cells form
    Amoeba (pond water)	1	The cell divides in two
    Leopard frog	Trillions	Existing cells divide

    According to cell theory, which statement is true of both organisms?
    • A.The cell is the basic unit of structure and function.
    • B.Their cells can form from nonliving pond material.
    • C.Each of their cells must perform every life function.
    • D.Only their largest cells are counted as living units.
    Show answer and explanation

    Answer: A. The cell is the basic unit of structure and function.

    Cell theory states that all organisms, whether made of one cell or trillions, are made of cells, the basic unit of structure and function. In a frog, different cells do different jobs, so not every cell performs every life function.

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