Warm-Ups Day 4

Which of the following methods can be used to determine the density of a pure substance?

• Measuring its color

• Calculating mass per unit volume

• Observing its shape

• Testing its solubility in water

Answer explanation

The density of a pure substance is determined by calculating its mass per unit volume. Other methods like measuring color, observing shape, or testing solubility do not provide density information.

When water boils and turns into steam, what type of change is occurring?

• Chemical change

• Physical change

• Both chemical and physical change

• Neither chemical nor physical change

Answer explanation

When water boils, it changes from liquid to gas (steam) without altering its chemical composition. This is a physical change, as the substance remains H2O in both states.

Which of the following is a physical property that can help identify a pure substance?

• Reactivity with acid

• Melting point

• Flammability

• Ability to rust

Answer explanation

The melting point is a physical property that is unique to each pure substance, allowing for identification. In contrast, reactivity with acid, flammability, and ability to rust are chemical properties and not reliable for identifying purity.

A piece of paper is burned, resulting in ash and smoke. What type of change is this?

• Physical change

• Chemical change

• Both chemical and physical change

• Neither chemical nor physical change

Answer explanation

Burning paper transforms it into ash and smoke, indicating a chemical change. The original substance (paper) is altered at the molecular level, creating new substances, which confirms it is a chemical change.

Which of the following is a chemical property of a pure substance?

• Density

• Boiling point

• Reactivity with oxygen

• Solubility

Answer explanation

Reactivity with oxygen is a chemical property, indicating how a substance interacts with oxygen. In contrast, density, boiling point, and solubility are physical properties, describing the state or behavior of a substance without changing its composition.

When salt is dissolved in water, what type of change occurs?

• Chemical change

• Physical change

• Both chemical and physical change

• Neither chemical nor physical change

Answer explanation

When salt dissolves in water, it breaks into ions but does not change its chemical structure. This process is a physical change, as the salt can be recovered by evaporation, demonstrating that no new substances are formed.

Which of the following properties can be used to identify a pure substance?

• Mass

• Volume

• Density

• Shape

Answer explanation

Density is a characteristic property of a pure substance, as it remains constant regardless of the amount present. Mass, volume, and shape can vary, making them less reliable for identification.

A metal reacts with acid to produce hydrogen gas. What type of change is this?

• Physical change

• Chemical change

• Both chemical and physical change

• Neither chemical nor physical change

Answer explanation

The reaction of a metal with acid to produce hydrogen gas involves a change in the chemical composition of the substances involved, indicating a chemical change. Therefore, the correct answer is 'Chemical change'.

Which of the following is a method to identify a pure substance based on its chemical properties?

• Measuring its boiling point

• Observing its color

• Testing its reactivity with acid

• Checking its density

Answer explanation

Testing a substance's reactivity with acid reveals its chemical properties, helping to identify if it is a pure substance. Other methods like boiling point and density can vary in mixtures, making them less definitive.

When a candle burns, what type of change occurs?

• Physical change

• Chemical change

• Both chemical and physical change

• Neither chemical nor physical change

Answer explanation

When a candle burns, wax melts (physical change) and the wax reacts with oxygen to produce heat, light, carbon dioxide, and water (chemical change). Thus, both chemical and physical changes occur during the burning of a candle.