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Quizzes > Science

Souring of Milk Chemical Change: Quick Quiz

Quick chemistry quiz to test your knowledge. Instant results on milk turning sour chemical change.

Editorial: Review CompletedCreated By: Arham ZulqarnaenUpdated Aug 23, 2025
Difficulty: Moderate
2-5mins
Learning OutcomesCheat Sheet
Paper art illustration for quiz on solid and liquid transformations, milk souring, physical vs chemical changes on coral background.

This quiz helps you decide if the souring of milk is a chemical change and spot the difference from physical changes. Answer quick, real-life items and get instant feedback, then try our physical vs chemical change quiz, a broader chemical and physical changes quiz, or dive into chemical change questions.

What type of change is milk souring?
Physical change
No change
Nuclear change
Chemical change
Milk souring is a chemical change because new substances (lactic acid) are formed when bacteria metabolize lactose. This process alters the molecular composition irreversibly. Physical changes do not produce new compounds, whereas chemical changes involve breaking and forming bonds. Further information is available on .
Which microorganism is primarily responsible for souring milk?
Aspergillus niger
Lactobacillus
Saccharomyces cerevisiae
Escherichia coli
Lactobacillus species are lactic acid bacteria that ferment lactose into lactic acid, causing milk to sour. Yeasts and molds can spoil milk under other conditions, but Lactobacillus is the main agent in acidic fermentation. This is why dairy fermentations like yogurt rely on Lactobacillus strains. More details at .
Which compound accumulates in milk as it sours, giving it a tart taste?
Citric acid
Lactic acid
Acetic acid
Carbonic acid
Lactic acid builds up when lactose is fermented by lactic acid bacteria, giving sour milk its characteristic tang. Acetic acid is typical in vinegar, while citric acid is found in citrus fruits. Carbonic acid forms in carbonated beverages. For more, see .
Is the denaturation of milk proteins during souring reversible?
Yes, if calcium is removed
Yes, proteins refold when pH returns to neutral
No, denaturation is irreversible in this context
Only at temperatures above 80°C
Protein denaturation during souring unfolds casein micelles irreversibly under acidic conditions, causing permanent coagulation. Returning to neutral pH does not restore the native protein structure. This irreversible change distinguishes chemical denaturation from reversible physical processes. See for more.
How does the pH of milk change as it sours?
It fluctuates randomly
It decreases
It increases
It stays the same
During souring, lactic acid production lowers the pH of milk, making it more acidic. A drop below pH 6.5 initiates coagulation of casein. Physical changes alone would not alter pH significantly. For pH changes in dairy, see .
Which component of milk primarily coagulates when it sours?
Casein proteins
Lactose
Milk fat
Whey proteins
Casein proteins aggregate under acidic conditions when the pH falls, forming curds. Lactose remains dissolved, and whey proteins mostly stay in solution. Milk fat stays dispersed unless mechanical processes separate it. More on casein coagulation at .
Which class of reaction best describes the biochemical process of milk souring?
Oxidation
Fermentation
Hydrolysis
Neutralization
Milk souring is a fermentation reaction where lactic acid bacteria convert lactose into lactic acid under anaerobic conditions. Oxidation involves oxygen, hydrolysis involves water breaking bonds, and neutralization is acid-base reactions. Fermentation is the correct category. More at .
Which indicator would turn red in the presence of sour milk?
Bromothymol blue
Litmus paper
Phenolphthalein
Methyl violet
Red litmus paper indicates acidic environments such as sour milk (pH below 7). Phenolphthalein turns colorless in acid, and bromothymol blue turns yellow. Methyl violet changes in very strong acids. For pH indicators, see .
What laboratory measurement quantifies the total acid content in soured milk?
Somatic cell count
Titratable acidity
Non-protein nitrogen
Freezing point depression
Titratable acidity measures the amount of a base required to neutralize acids in milk, quantifying lactic acid concentration. Somatic cell count relates to milk quality and health, non-protein nitrogen measures other nitrogenous compounds, and freezing point depression gauges adulteration. More at .
Is the fermentation of milk generally exothermic or endothermic?
Exothermic
Neither, it is isothermal
Depends on pH
Endothermic
Fermentation of lactose into lactic acid releases heat, making it an exothermic process. Enzymatic reactions often liberate energy. It is not purely isothermal or dependent solely on pH. Further thermodynamic details at .
After filtering curdled milk, which substance predominantly remains in the filtrate?
Milk fat globules
Casein curds
Lactic acid and whey components
Undigested lactose crystals
The filtrate, known as whey, contains lactic acid, lactose, water-soluble vitamins, and whey proteins. Casein curds are retained by the filter. Milk fat is largely trapped in curds, and lactose is mostly consumed or remains dissolved rather than crystalline. See for more.
How many moles of lactic acid are produced from one mole of lactose via homofermentative lactic acid fermentation?
4 moles
1 mole
2 moles
3 moles
Lactose is a disaccharide of glucose and galactose; each monosaccharide yields 2 moles of lactic acid under homofermentative conditions, totaling 4 moles per lactose. This pathway is key in dairy fermentation. Detailed pathways are discussed at .
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Study Outcomes

  1. Understand Chemical and Physical Changes -

    Grasp the core differences between chemical reactions and physical transformations such as melting and freezing.

  2. Differentiate Chemical vs. Physical Transformations -

    Learn to distinguish examples of chemical change from physical changes by evaluating observable properties and energy shifts.

  3. Identify the Nature of Milk Souring -

    Determine if milk souring is a chemical change by examining evidence like odor, color, and new substance formation.

  4. Analyze Melting and Freezing Examples -

    Break down common melting and freezing processes to understand reversible physical state changes in everyday life.

  5. Evaluate Evidence in States of Matter Transformations -

    Assess experimental clues and select appropriate criteria to confirm chemical reactions or physical processes.

  6. Apply Classification Skills in a Quiz Setting -

    Use your knowledge to successfully complete the physical vs chemical changes quiz and reinforce learning through interactive practice.

Cheat Sheet

  1. Core Definitions of Change -

    Chemical changes yield new substances with different compositions, while physical changes only alter form without changing identity (e.g., melting ice). This distinction is key when you ask "is milk sours a chemical change," because only bond rearrangements count as chemical. (Sources: American Chemical Society, Khan Academy)

  2. Why Milk Souring Is a Chemical Change -

    Souring milk involves bacteria converting lactose into lactic acid, creating new compounds and an irreversible pH shift, so milk souring is a chemical change. This classic example of a chemical change contrasts with physical transformations like freezing. (Sources: University of California Davis, ACS publications)

  3. Mnemonic for Identifying Chemical Changes -

    Remember "BONGO": Bond-breaking, Odor change, New substance, Gas evolution, Order change. Spotting two or more BONGO signs - such as the sour smell and new acid in milk - confirms a chemical change. (Source: Royal Society of Chemistry)

  4. Reversible vs. Irreversible Changes -

    Physical vs chemical changes quiz prep hinges on reversibility: melting and freezing changes are reversible physical changes, whereas milk souring is irreversible. Practice sorting scenarios - like dissolving salt versus burning paper - to sharpen your classification skills. (Sources: LibreTexts, ACS)

  5. States of Matter Transformations -

    Beyond liquid - solid transitions, recall gas→liquid (condensation) and solid→gas (sublimation) are physical changes without new chemicals. Use flashcards or a physical vs chemical changes quiz online to master these states of matter transformations. (Sources: Khan Academy, Purdue University Chemistry)

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