Pure Substances & Mixtures
What do we call a material made of only one kind of particle with a fixed composition (for example, pure water or pure gold)?
Pure substance (or pure material); made of one kind of particle, fixed composition.
A fluid is a substance that flows and takes the shape of its container; examples: water and air.
What property describes how shiny and how well a material reflects light (common for metals)?
Luster (metallic luster).
When you squeeze a sealed plastic bottle causing water to squirt out, what principle (involving pressure) explains why the water leaves the bottle?
Pascal’s principle — applying pressure to a fluid transmits that pressure throughout, forcing the fluid out.
Is salt water a pure substance or a mixture? Explain briefly why.
Mixture — salt water contains salt and water not chemically combined; composition can vary.
True or false: Both liquids and gases are fluids because they can flow and take the shape of their containers. Explain briefly.
True — both liquids and gases flow and conform to container shape.
Define solubility in simple terms.
Solubility is how much of a substance can dissolve in another substance (usually a solid in a liquid) at a given temperature.
Why do objects sometimes float and sometimes sink when placed in water? Give a simple criterion (mention density).
bjects float if their average density is less than the fluid’s density; sink if more dense than the fluid.
Name two types of mixtures and give one example of each.
Homogeneous mixture (uniform composition; e.g., salt water), heterogeneous mixture (not uniform; e.g., salad, sandy water).
What is viscosity? Which flows more easily — honey or water? Why?
Viscosity is a measure of a fluid’s resistance to flow; honey has higher viscosity, so it flows more slowly than water.
What is density and how can students compare which of two liquids will float on top of the other?
Density = mass per unit volume; compare densities: the liquid with lower density floats on the one with higher density.
Describe one real-world example where changing fluid pressure is useful (for example: hydraulic brakes or a water tower)
Hydraulic systems (like car brakes) use fluid pressure to multiply force; water towers use pressure from elevated water to supply homes.
If a sample contains hydrogen and oxygen chemically bonded in a fixed ratio, is that a pure substance or a mixture? What is it called?
Pure substance; compound (e.g., water, H2O).
Explain what pressure in a fluid means in one short sentence. Then give an everyday example of fluid pressure.
Fluid pressure is the force a fluid exerts per unit area; example: pressure at the bottom of a swimming pool increases with depth (you feel more pressure deeper).
A substance changes from solid to liquid at 0°C. What is this temperature called for that substance? (Name the property.)
Melting point.
Explain Bernoulli’s idea in one or two sentences: how does fluid speed relate to pressure?
faster-moving fluid has lower pressure; slower-moving fluid has higher pressure — this helps explain lift on airplane wings and how a shower curtain can billow inward.
Explain why air is considered a mixture and not a pure substance. Include at least two components of air in your answer.
Air is a mixture because it contains nitrogen (~78%), oxygen (~21%), and other gases like argon and CO2; proportions can vary and components are not chemically bonded.
The pressure in a sealed container of gas increases when temperature increases. Which gas law idea (in simple words) explains this relationship?
In simple words: for a given volume, heating a gas increases the energy of the particles so they push harder and pressure rises (related to Gay-Lussac’s/Charles’ ideas; grade-appropriate wording: heating gas increases pressure if volume is constant).
A teacher gives you two clear liquids that look the same. List three properties or tests you could use to tell them apart (no specialized instruments required).
measure density (mass ÷ volume), test solubility (does one dissolve in water?), test reactivity with vinegar/acid, test boiling point, observe smell, or test conductivity (if safe and available).
A submarine changes its buoyancy to dive and surface. Describe in simple steps how changing mass or volume of water in its ballast tanks allows it to sink or rise.
Submarine uses ballast tanks: to dive, it lets water into tanks to increase overall density and weight; to surface, it pumps water out and fills tanks with air to decrease density so it becomes buoyant