CBSE Class 7 Science Physics Chemistry Biology Worksheet

PHYSICS

Question 1. Define the terms: a) TIME PERIOD b) OSCILLATION c) AMPLITUDE
Answer: a) Time Period: The time taken by a pendulum or any oscillating body to complete one full oscillation is called its time period. It is measured in seconds.
b) Oscillation: To and fro or back and forth motion of an object from its mean position is called an oscillation.
c) Amplitude: The maximum displacement of an oscillating body from its mean position on either side is called amplitude.
In simple words: Time period is time for one swing, oscillation is one complete round trip, and amplitude is the maximum distance moved from the center.

Exam Tip: Always mention the standard SI unit (seconds for time period, meters for amplitude) to score full marks in definitions.

Question 2. What is the time period of a seconds pendulum?
Answer: The time period of a seconds pendulum is exactly 2 seconds (it takes 1 second to move from one extreme position to the other and 1 second to return).
In simple words: A seconds pendulum takes 2 seconds to complete one full vibration.

Exam Tip: Remember that a "seconds pendulum" does not mean a 1-second time period; it has a time period of 2 seconds.

Question 3. A pendulum completes one oscillation in 1.5 second w hat is the time period of the pendulum?.
Answer: Time taken to complete one oscillation = 1.5 seconds
By definition, the time taken to complete one oscillation is called the time period.
Therefore, the time period of the pendulum = \(1.5\text{ s}\).
In simple words: Since time period is defined as the time for one complete oscillation, it is directly equal to 1.5 seconds.

Exam Tip: Do not confuse the number of oscillations with time period; 1 oscillation in 1.5 seconds directly gives a time period of 1.5 seconds.

Question 4. Name an instrument which is used to produce sound in the laboratory?
Answer: A tuning fork is the standard instrument used in physics laboratories to produce a sound of a single, fixed frequency.
In simple words: A tuning fork is struck against a rubber pad to produce a clear sound.

Exam Tip: Mention both the name of the instrument (tuning fork) and how it is activated (striking on a rubber pad) for complete answers.

Question 5. What are the conditions necessary for hearing sound?
Answer: The three essential conditions necessary for hearing sound are:
1. Presence of a vibrating body (source of sound).
2. Presence of a material medium (solid, liquid, or gas) through which sound can travel.
3. A working organ of hearing (ears) to receive and interpret the sound waves.
In simple words: You need something to make a sound, a medium like air to carry it, and ears to hear it.

Exam Tip: Always list all three conditions (source, medium, receiver) to secure full marks.

Question 6. We cannot hear on moon, why?
Answer: We cannot hear on the moon because sound requires a material medium to propagate. The moon has no atmosphere (vacuum), meaning there is no air or gas molecules present to transmit sound vibrations from one place to another.
In simple words: There is no air on the moon, and sound cannot travel through empty space (vacuum).

Exam Tip: Emphasize the scientific term "vacuum" or "absence of atmosphere/medium" in your explanation.

CHEMISTRY

Question 1. Label the marked parts of the diagram given below.
Answer: Based on the standard laboratory preparation of carbon dioxide setup shown in the diagram:
- Thistle funnel (used for pouring dilute hydrochloric acid)
- Flat-bottomed flask / Woulff flask (reaction vessel containing calcium carbonate)
- Delivery tube (for passing the evolved carbon dioxide gas)
- Gas jar (for collecting the gas)
- Water / upward displacement of air setup
In simple words: The diagram shows acid being added to marble chips in a flask, and the resulting carbon dioxide gas being collected in a gas jar.

Exam Tip: Practice drawing and labeling laboratory apparatus clearly with arrows pointing directly to the correct parts.

Question 2. By which method is the gas collected in the gas jar?
Answer: Carbon dioxide gas is collected by the upward displacement of air (downward delivery). This is because carbon dioxide is denser than air.
In simple words: Since carbon dioxide is heavier than air, it settles down in the gas jar and pushes the air upwards.

Exam Tip: Always justify the collection method by stating the physical property of the gas (its vapor density relative to air).

Question 3. Write the balanced equation for the laboratory preparation of carbon dioxide.
Answer: Calcium carbonate reacts with dilute hydrochloric acid to form calcium chloride, water, and carbon dioxide gas.
Chemical Equation:
\(\text{CaCO}_3(s) + 2\text{HCl}(aq) \rightarrow \text{CaCl}_2(aq) + \text{H}_2\text{O}(l) + \text{CO}_2(g)\)
In simple words: Marble chips mixed with dilute acid produce calcium chloride, water, and carbon dioxide bubbles.

Exam Tip: Ensure physical state symbols ((s), (aq), (l), (g)) are included and the equation is balanced correctly.

BIOLOGY

Question 1. a.Identify the given algae. b. What is obtained from this alga?
Answer: a. The given organism in the image is Volvox (a colonial green alga).
b. Oxygen (through photosynthesis) and various biochemical byproducts, and in broader phycological contexts, specific hydrocolloids or nutrient biomass can be derived from green algae.
In simple words: The picture shows Volvox colonies, which produce oxygen and contribute to aquatic food webs.

Exam Tip: Learn common algal diagrams such as Spirogyra, Chlamydomonas, and Volvox along with their key characteristics.

Question 2. Which fungi is used for the production of citric acid in soft drink industry?
Answer: The fungus Aspergillus niger is widely used for the commercial production of citric acid via fermentation for use in soft drinks and food products.
In simple words: Aspergillus niger is a microscopic fungus that helps manufacture citric acid to give tangy flavors to cold drinks.

Question 3. What happens during the formation of curd?
Answer: During the formation of curd, the bacterium Lactobacillus multiplies in milk and converts lactose (milk sugar) into lactic acid. This lactic acid causes the proteins in milk (casein) to coagulate and curdle, transforming liquid milk into semi-solid curd, while also enhancing its vitamin B12 content.
In simple words: Friendly bacteria turn milk sugar into acid, which thickens the milk into sour curd.

Exam Tip: Always mention the specific bacterium name (*Lactobacillus*) and the chemical change (lactose to lactic acid) for full marks.